Really? Is that evidenced by Lennart's response to this, which stated "The ability to hose a system is certainly reason enought to make sure it's well protected and only writable to root."[1]? I think it implies the opposite.
Being able to brick hardware through a very oftenly used action (unliking a filesystem entry) throws us back into the times where one could damage display devices beyone repair by feeding them scan frequencies outside their operational range or by destroying hard disks by smashing the heads into a parking position outside of the mechanical range. We left those days behind us some 20 years ago: Devices got smart enough to detect potentially dangerous inputs and execute failsafe behaviour. It's just reasonable to expect this from system firmware.
When talking about (U)EFI variables we're not talking about firmware updates, which are kind of a special action (and even for firmware updates its unacceptible that a corrupted update bricks a system¹). Manipulating (U)EFI variables is considered a perfectly normal day-to-day operation and the OS should not have to care about sanity checks and validity at boot time. (U)EFI is the owner and interpreter of these variables, so it is absolutely reasonable to expect the firmware to have safeguards and failsafe values in place.
IMHO (U)EFI is a big mess, a bloated mishap of system boostrap loader. And I'm totally against trying to workaround all the br0kenness in higher levels. The more and often systems brick due to the very fundamentals of (U)EFI being so misguided, the sooner we'll move on to something that's not verengineered.
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¹: Just to make the point: When we developed the bootstrap loader for our swept laser product we implemented several safeguards to make it unbrickable. It's perfectly fine to cut the power or reset the device in the middle of a firmware upgrade. It safely recovers from that. Heck, the firmware in flash memory could become damaged by cosmic radiation, the bootloader would detect it and reinstall it from a backup copy in secondary storage.
A sane linux distro will mount it ro and switch to rw whenever need. Defaulting to rw efivars is, excuse the language stupid.
I've done a fair share of efi debugging even removing some of the variables that the kernel will now protect you from breaking.
If the issue is that users should be able to remount efivars as rw whenever needed then that should be addressed, not prevent you from doing stuff to it because there is a rogue init system doing crazy stuff.
EDIT: BTW, i don't think systemd does anything besides write to the various Boot* variables, but I may be wrong. I don't see why that can't be addressed with a remount. If you replace the boot.efi you still have to remount the efi partition anyway.
While Matthew may be right that there is an issue that needs to be addressed, but in one of his tweets he basically says the kernel should fix it because tooling isn't and bioses suck. Well, maybe tooling should be forced to fix it.
or from the issue:
Matthew-Jemielity commented 24 days ago
What needs efivars mounted at all anyway? So far I've seen:
grub
systemctl --firmware-setup reboot
efibootmgr
Since those likely need superuser, couldn't they handle (un)mounting it themselves?
@annejan
annejan commented 23 days ago
As long as distribution that are aimed at consumers remount it ro and on updating kernels wrap grub with remount this is a complete non-issue.No, it's not the same. Mounting `/boot` rw by default does not put your system in the danger of getting damaged beyond repair. If you hose the boot partition you can always start a recovery system (live Linux or similar) to repair the damage.f
But if deleting efivars renders a system inoperable on a firmware level you're essentially SOL, save for rewriting the contents of the system firmware flash using an external programmer and a clean image. That is an absolutely inacceptable situation. The year is 2016 and hosing a firmware by writing malformed values into the firmware API is, simply put, a software vulnerability that allows to permanently DoS a system. As such this is a security issue that must be fixed at where the security issue happens. And in case of efivars the issue is that certain input is not properly validated and/or sanitized. If a system firmware can not properly start with certain variables being unset or removed or set to invalid valued, its should be a implementation requirement to validate input on such variables before executing the change.
> Defaulting to rw efivars is, excuse the language stupid.
It probably it. But it's not the responsibility of the OS to sanitize values that are not intended for being used by the OS. efivars are intended to be used by (U)EFI and hence it's the (U)EFI implementation's task to properly sanitize access to them.
Essentially we're talking Bobby Tables here, just with a different API.
If the implementation is bad (which it is), that doesn't excuse reckless userland software that refuses to acknowledge flaws in the firmware.
Saying that Lennart is wrong has become a rather popular sport, let's not go overboard to say it even when he's right by all accounts.
A virus should not be able to destroy the system BIOS. The problem with efivars is illuminating a vulnerability, not a feature for "doing stuff". I would expect to see this used in the wild if left unfixed, especially the next time we hear about a remote vulnerability that permit arbitrary memory execution.
It's perfectly reasonable to ro mount efivars. But doing so must not be the workaround to fix an security issue (and yes, this is a security issue) in (certain implementations of)(U)EFI. Just to make clear why this is an security issue: Security rests of three pillars:
- availability - confidentiality - authenticity
Rendering a system unusable (DoS-ing it) it an attack on availability. And speaking of security, if the goal is sabotage and causing large financial damages, then being able to permanently brick a system in case of a privilege escalation (there's nothing stopping UID=0 from remounting rw efivars) is pretty bad; and no, the implemented fixes in the efivars kernel code don't help, because an attacker can still mount a custom kernel module which will talk to the respective efivars code directly circumventing sanity checks (or directly talk to (U)EFI without using the efivars code).
There's a certain rich irony that Lennart is being flogged for following the Unix tradition (root can do anything, including blow up their monitor with bad X configs); that his detractors are suggesting userland tools manage their hardware (normally the job of the kernel in a Unix system); that systemd ought to be expanded to manage hardware (after years of complaining it's too big), presumably by adding whitelisting capabilities and a database of known-good/known-bad UEFI implementations.
I guess at least it demonstrates how utterly unhinged some people become when his name is attached to anything.
He earned it with his hard work with the Linux community.
OpenRC is what got closest to be the replacement SysV-Init, just by its qualities and market share. But other, nicer systems exist, but they never found wide adoption because the main distributions didn't let go of SysV.
Now Lennart claims all the bragging rights, but people who used modern init systems before it was cool know better (there are a lot of Lennart opponents among those, BTW).
> But beyond that: root can do anything really.
... so running "rm -rf /" as root should brick your motherboard because it's the responsibility of the motherboard manufacturer to protect against this. That's all fine and dandy in an idealized world, but in the "real world" there are going to be motherboard manufacturers that play fast and loose with these things.
It's the responsibility of the OS to make sure it doesn't ruin firmware.
The standard practice for PCs has always been that firmware configuration settings can be cleared (through a jumper or by pulling the battery) to reset the system to its factory state, forcing it to fall back to its conservative and safe defaults. Some systems have apparently forgotten to have defaults. Their firmware is already broken and afflicted with a major bug even if you avoid triggering it in this particular manner.
Again, this is the OS's fault.
rm /sys/efivar/MyVariable
var obj = {MyVariable: 1};
obj.MyVariable = null;
var obj = {MyVariable: 1};
delete obj.MyVariable;
My understanding is that firmware made to spec would not brick over the the EFI variables getting wiped. The motherboards that are encountering these issues are running firmware that is cutting corners. Unfortunately cutting corners and ignore specs is nothing new for hardware manufacturers who are more concerned with the manufacture of the physical devices and usually put only the minimum amount of effort into getting any sort of software components (firmware, OS device drivers, etc) running.
> ... so running "rm -rf /" as root should brick your motherboard because it's the responsibility of the motherboard manufacturer to protect against this.
You are assuming "only writable by root" means "root can write without any restrictions" which is a fairly uncharitable reading, and requires assumptions about his intent which are not evident.
I could make a statement such as "cars in the united states can only be legally driven by people of an appropriate age" and you could assume I meant that's all that needs to apply and start calling out my statement as wrong, or you could assume I was aware of the additional requirement of a driver's license, or you could ask me to clarify my point of view. I just don't believe the first option is conducive to useful discussion, nor do I think it's appropriate to use assumed information in a negative way towards a third party.
Edit: s/disparage/use assumed information in a negative way/ for lack of better phrasing coming to mind. The statement wasn't really disparaging, just an uncharitable interpretation, so I don't want to overstate that.
I don't think that's a useful perspective here. This is not a feature - nothing should ever want to permanently brick a motherboard, there's no use case for that. There's no benefit to allowing root to do this. The OS is supposed to abstract the hardware in a way that it can be safely operated.
The fact is that 'rm -rf /' is a common mistake, both at the command line (i.e. manually typing) and in scripts that don't adequately protect against things like missing variables. E.g.:
MY_DIR=
rm -rf "$MY_DIR/"
[1] I realize that boot loaders like grub are everywhere and probably need to write to efivarfs, but that's still a data point of 1. Would it be that difficult for grub and it's related scripts to upgrade to remount the filesystem readwrite when it needs to perform an operation? I'm sure it's only been a couple of years since efivarfs functionality was even added to grub.
To my mind there are two places to fix this, in the kernel for a real mitigation technique that helps "solve" the problem, and in the distros for quick fixes and hacks, or backports of the kernel fix, as necessary. Systemd pushing a fix of their own 1) only affects distros using systemd, while this problem affects all recent distros that use efivarfs), 2) probably won't get picked up by distros immediately excapt as a backported fix anyway, as I doubt most of them push new versions of something as integral as systemd every time a new version is out, at least not with a lot of testing, and 3) would not have been a good fix, and would have required the utilities that still needed access to actually remount a filesystem.
1: Triggering this problem is not as easy as what you (and I) wrote in most instances, as / has special consideration in rm, and generally requires the "--no-preserve-root" flag.
Not necessarily. Leaving it "wide open" for anything to accidentally write to it all of the time vs. just mounting it readwrite when you actually need to write to it are two different risk profiles.
The kernel fix doesn't have such drawback.
Put another way, systemd mounting efivarfs read-only doesn't necessarily prevent you from bricking your machine by deleting files within that file system, it protects only those using systemd. The problem stems from the nature of efivarfs, and that it was implemented as a file system and not some other interface. Making it default to something more sane is a better (but by no means the best) solution, and that ideally would happen within the source, which is the kernel.
Really, I think the two appropriate places for this fix are the kernel, and if that's not expected to be rolled out soon, as a patch to systemd to mount it read-only by the distro shipping systemd. Systemd shipping the fix would only really help the small group of people that install systemd from source during the short window from 11 days ago until the next kernel is available (or that choose to run an older kernel), and makes the whole efivarfs situation a bit more confusing by leaving it read-only and immutable for the future.
I read this as Lennart saying that when root issues an 'rm' in efivarfs, the variable should be removed even if this renders the motherboard unusable without physical repairs. What's your interpretation?
I've edited to fix my terrible punctuation, and to make it clear that 'it' refers to 'systemd', and to add a link to MJG's response on Twitter. I can edit further if you have a way to make it clearer.
It should be mounted rw because existing userspace expects it to be rw.
> It shouldn't be mounted by default.
It should be mounted by default because it's information that's relevant to various pieces of userspace.
> It shouldn't even be a bloody filesystem.
With hindsight, it should absolutely not have been a filesystem. There's very little metadata associated with EFI variables and the convenience of exposing this in a way that can be handled using read and write is huge, but real-world firmware turns out to be fragile enough that this was a mistake. But, in the absence of a time machine, there's very little I can do to fix that now.
Can you explain this a bit? I'm not familiar with the particulars of firmware, but I'm having a hard time imagining why any userspace program would expect a firmware partition to be writable. Even if there are any, certainly I'd have a hard time believing that they would need it to be mounted and writable all the time.
As for why userspace couldn't remount it itself - it could. It doesn't. Changing the behaviour of systemd without changing the behaviour of the rest of userspace would result in userspace being broken, and making that kind of incompatible change is annoying - especially when fixing it in the kernel allows us to avoid that breakage.
Best example of why you need to have it RW: if uefi is in fast boot, the only way to actually enter uefi is to boot an OS and have the OS set the uefi variables so that it goes into the firmware screen on next restart. This is (on Linux) done by changing something in that virtual filesystem.
> certainly I'd have a hard time believing that they would need it to be mounted and writable all the time.
You could probably remount it, but that also means that you all the sudden have concurrency problems with that operation. So not really ideal.
The kernel fix prevents that, using mount flags alone only restrict the vulnerability but it doesn't make it go away.