Domesticating applications, OpenBSD style
lwn.net
lwn.net
Otherwise, kudos on LWN for reporting this.
That is fine and dandy, but when someone criticizes Theo's work in the same scornful manner, it's suddenly 'narrow-minded', 'willfully ignorant' and part of a 'vendetta'.
Pot meet kettle.
While spender is wrong for multiple reasons. Such as that /var is mounted nosuid on OpenBSD by default so their hypothetical "httpd chmod'ing to setuid" doesn't work ("nix" points this out to them but isn't aware of OpenBSD's defaults in this area). The other reason is they are decrying the whole system because of a fairly weak attack scenario (requires a local user account) and it's not like that attack isn't possible now, so while tame() doesn't protect against that specific thing it protects against others but spender is just saying nope, it doesn't protect against this specific scenario therefore it's useless.
Also if you keep reading spender's commenters they go on to claim OpenBSD's devs are delusional plagiarists so "vendetta" does seem appropriate.
While Spender's criticism isn't usually very diplomatic, he's often right.
Here's an example, this is QEMU's seccomp whitelist: http://git.qemu.org/?p=qemu.git;a=blob_plain;f=qemu-seccomp....
As you can see, it's mostly worthless since QEMU has to do lots of potentially-dangerous operations by design (even reading/writing to raw devices). A proper mandatory access control framework actually restricts which device files it can access, for example - as implemented in the AppArmor/SELinux sVirt drivers. Theo's approach is similarly flawed.
tame() isn't a capabilities-based approach. Don't confuse POSIX capabilities with actual capability-based security. The former hijacked an existing term to refer to a different thing.
It's pretty obviously a limited API, but it also makes privilege dropping absolutely trivial. Which is the point.
Attacks on Theo are the opposite of interesting but that spender guy lashes out at his follow commenters:
Oh sorry, I forgot LWN readers require me to do all their homework for them
And so on.
I'm not sure what is going on when a guy who is apparently a professional pulls a from my parents basement in Wyoming I stab at thee! but it's not atypical of LWN comments.
On the other hand, there is plenty of useful stuff there sometimes. You take the bad with the good.
While 'Oh sorry, I forgot LWN readers require me to do all their homework for them' comes across as a petty remark (though hardly as hateful as you try to paint it), I've read a lot worse from for example Theo who is also 'apparently a professional', and of course from countless other 'apparently professionals', in countless places beyond LWN, like right here on Hacker News (yes, really!).
Vezzy-fnord totally disregarded the tone in which Theo characterised existing solutions while painting this spender guy as some bully driven by narrow-mindedness because he came from a 'Linux bubble' and I assume therefore could not accept the brilliance of Theo's solution or something like that.
To me it seems like this spender guy is providing strong criticism against this solution from a practical perspective, and vezzy-fnord chose to paint him as a 'narrow-minded' and 'wilfully-ignorant' through his tone (again totally disregarding Theo's same tone) in order to sweep the technical criticism under the rug (don't listen to him, he is on a 'vendetta'!) .
Vezzy-fnord was commenting on the quality of LWN comments.
It's as if you do not understand that Theo did not comment on that LWN page...
>It's as if you do not understand that Theo did not comment on that LWN page...
The comments in question where in large responses to Theo's claim, which in turn was part of the article on which the comments are based.
Vezzy-fnord used this to paint LWN comment section as being populated by narrow-minded, Linux-centric people with vendetta's, totally ignoring that the comments by 'spender' were perfectly in line with the comments typically uttered by Theo, whose solution was under discussion.
And as to further underline how this was all bs from start to finish on Vezzy-fnord's part, the guy in question (spender) who was making the aggressive criticism is not some Linux zealot, he is criticizing Linux security solutions in exactly the same manner.
On one hand, a simple opt-in way for applications to give away the ability to do operations is something I've wanted for sandboxing for a long time. From an API perspective, tame() looks great.
However, the kernel-side implementation is way too ugly. Hardcoding pathnames needed to use things like DNS, NIS, etc is just way too inflexible and fragile.
I think the kernel should just support a set of fs/network/etc restrictions that can be grown but not shrunk by the process. i.e. basically what seccomp-bpf gives you on linux. What we really need is user-friendly libraries to make using seccomp-bpf as easy as using tame()!
In this case though, the problem with growing the permission set is you then need rules to define the sandbox "size" outside the process, then tools to administer it, then the policies to be deployed with the program.
If you're using tame which is restrictive, based on how they do privsep in OpenBSD, you're forking tasks off from a control process and then removing privileges. So the program defines the policy internally, very granularly and correctly.
How you'd do that with seccomp-bpf with forked child processes etc would be vastly more complex. Consider OpenSMTPd for example which consists (I can't remember the exact separations): master process, smtp listener, queue, writer. These are all forked from the master process and talk to each other with pipes. How would you mandate the call limitations efficiently if this was externalised? Also how would you assure that a misapplied external policy doesn't compromise your system.
The user of tame() quite sensibly wants to specify a simple filter based on what things they might need to do in the future. i.e. "I need to be able to loop up details about local users". At the system-call level this implies a number of things should be allowed i.e. "can open /etc/group for read-only access, can read NIS configuration files, ..."
In the OpenBSD implementation of tame() all of these rules are hardcoded into the kernel. Using LDAP instead of NIS? Well, you'll have to patch and recompile I guess.
Since ultimately the userland process is opting-in to the restrictions there's no security reason that it can't specify the rules to the kernel. (Assuming here that care is taken so it can't DoS the kernel by pushing down a billion-entry list or something)
Again, what OpenBSD gets right here is the API -- it should be as simple as possible for a process to say "I need to look up users, use DNS, and read files; block all else" Making that a simple call like tame() is a great step forward. I hope the seccomp-bpf people do something similar.
However, tame() should be a library function, not a syscall. The kernel shouldn't know or care what "I need to use DNS" means, it should just be told what files I can open and what sockets ops I'm allowed to do.
The OpenBSD implementation is simultaneously too rigid for some things (since things like plugable nsswitch.conf modules can't work at all) and too loose to support other useful ones (per-application policies like "block any attempt to open a file for writing unless it's an append to $HOME/.ssh/known_hosts")
In UNIX there is a very long tradition of not baking policies like this into the kernel, and for good reason.
Is this perhaps an artifact of the OpenBSD development model, where programs are assumed to work tightly coupled to the kernel?
Similar arguments apply to host resolution, but these days it's almost always just /etc/hosts and DNS so less of an issue.
I don't think I've ever met a casual sysadmin who groks SELinux, and I've never met a user who understands what it's all about.
Something that does 90% of the job that is used 50% of the time, is better than something that does 99% of the job and is used 1% of the time.
I'm slightly bitter that I've become a bit of an SELinux expert over the years. I shouldn't have to be one, hell the tech shouldn't even need to exist in 2015.
Granted, creating a really secure policy takes effort, but even just having it enabled with simple policies can help mitigate the impact of a bunch of vulnerabilities, even if it won't make you invulnerable.
1. enable permissive mode
2. test application
3. check audit logs for any complaints
4. if no complaints, you're done. enable enforcing mode and test again.
5. otherwise, evaluate the complaints and fix the issues, either by tuning fcontexts (often, a simple path equivalency is enough if you're installing things on nondefault paths, as is common.) or by creating a custom policy module (audit2allow helps), then go to 2.
It's work you should do anyway. Securing an environment is part of setting it up.
o Run Application
o Get weird error.
o Google the error, see someone mentioning "This is because of SElinux"
o Google how to "Disable SELinux"
I'm not saying that's what they should be doing, just saying it's what I've observed. What's nice about tame is - there is nothing to enable/disable, it's just part of software.
> What's nice about tame is - there is nothing to enable/disable, it's just part of software.
And what's nice about SELinux (if you learn it) is it applies to the system and everything running on it whether or not it's been written for it. You can even put it in a logging only, and use those logs to generate the needed policies. SELinux will tell you what is going on.
What you point out as nice about tame() means that nothing will use it except what is in the core of OpenBSD.
It's a good tool for dropping privileges from a process, but it doesn't even attempt to solve the same problem that SELinux does.