Since Linux 6.9, LUKS suspend stopped wiping disk-encryption keys from memory

While it is certainly an interesting bug, I kinda feel that the title is click bait? Because this `cryptsetup luksSuspend` from what I understood is not really officially supported but an extension done in Debian, so if anything this regression only affected Debian? I am not sure if you can blame the kernel for something that is not supported or even widely tested.

I still find this impressive, and it is nice that we now have a test (NixOSTests BTW are awesome, I agree with OP) to avoid this regression from coming back. But from the title it seems to be a widespread issue, not something that affects only one Distro.

I don't see any other way? When you sleep (suspend to RAM), everything is stored in RAM and is encrypted but the master key is present in kernel memory (if I recall correctly).

However, if you hibernate (suspend to disk) the entire contents of RAM (including the master key) is written/encrypted to disk and the RAM is cleared.

When you wake the machine up you have to re-enter the passphrase to decrypt the master key to re-load disk contents back to memory.

I don't think this bothers me.

The only reason that I do the disk encryption is so that I don't have to worry about people going through my laptop to steal tax documents and/or credit card stuff when I sell the laptop. I of course also wipe the laptop too, but I figure that if the data is encrypted at the drive level then there's very little risk of anyone being able to use some kind of forensics tool and recover data.

This is one of those regressions that's easy to miss because everything still "works." Security bugs often don't announce themselves.

I don't have to re-enter my boot password after Sleep, so obviously the encryption key is still in memory.

I am far from a security expert, but from the number of "we missed a single line C check across files during refactoring" critical security bugs discovered on a regular basis these days, the whole premise of a "giant secure open source C codebase" seems questionable. It is not specific to C of course, but invariants are arguably even harder to enforce and track consistently (esp under changes to code) in C. Unsure if FP with invariants encoded in types is a practically feasible scalable solution either. Model checking? [LLM] fuzzing? Fewer primitives with clear boundaries? Is that how seLinux was "checked"?

On my laptop with Fedora I just configured Linux to hibernate to disk after 15 minutes of suspend. Powering memory off ensures that bugs like this Debian-specific would not matter.

Plus what Debian extension to Linux tooling does although nice in theory, but in practice if one really worries about cold-boot attacks, then all keys and important documents has to be wiped out from memory, not only LUKS keys.

So hibernating is really the only proper way to protect against cold boot.

on the subject of encryption keys and memory there is something you can do:

- if your CPU supports it, enable memory encryption.

- if your TPM module supports this look for MemoryOverwriteRequestControl & MemoryOverwriteRequestControlLock (/sys/firmware/efi/efivars/) and toggle them. make sure that your computer always reboots and never powers off. memory will always be wiped on boot.

It's because of vulnerabilities like this that I enable Intel's "total memory encryption" feature. No plaintext leaves the CPU package. DIMM swap attacks become useless. Moreover, it's basically free: the cryptography happens directly in the memory controller, in hardware, inline with the bus transactions the CPU is doing anyway.

> Except that, for more than two years, the encryption key remained resident in memory across suspend, leaving it there for the taking by anyone who seized the still-powered laptop.

I don't get it. Obviously, the laptop is locked when it resumes, how is that key "for the taking by anyone"? I'm not saying it is impossible to read out RAM from a locked laptop, but surely not by "anyone".

[flagged]

Definitely not a symptom of Linux being a hodgepodge of code thrown together from a thousand different sources and no one person could tell you how it all fits.