On Thu, Jul 02, 2020 at 06:00:17PM -0700, Linus Torvalds wrote:
On Thu, Jul 2, 2020 at 4:26 PM Kees Cook keescook@chromium.org wrote:
The kprobe show() functions were using "current"'s creds instead of the file opener's creds for kallsyms visibility. Fix to use seq_file->file->f_cred.
Side note: I have a distinct - but despite that possibly quite incorrect - memory that I've discussed with somebody several years ago about making "current_cred()" simply warn in any IO context.
IOW, we could have read and write just increment/decrement a per-thread counter, and have current_cred() do a WARN_ON_ONCE() if it's called with that counter incremented.
That does sound familiar. I can't find a thread on it, but my search abilities are poor. :) So an increment/decrement in all the IO-related syscalls, or were you thinking of some other location?
The issue of ioctl's is a bit less obvious - there are reasons to argue those should also use open-time credentials, but on the other hand I think it's reasonable to pass a file descriptor to a suid app in order for that app to do things that the normal user cannot.
But read/write are dangerous because of the "it's so easy to fool suid apps to read/write stdin/stdout".
So pread/pwrite/ioctl/splice etc are things that suid applications very much do on purpose to affect a file descriptor. But plain read/write are things that might be accidental and used by attack vectors.
So probably just start with read/write and tighten it over time, if we find other clear places, leaving ioctl/pread/pwrite/splice alone.
If somebody is interested in looking into things like that, it might be a good idea to have kernel threads with that counter incremented by
Just throwing this idea out in case somebody wants to try it. It's not just "current_cred", of course. It's all the current_cred_xxx() users too. But it may be that there are a ton of false positives because maybe some code on purpose ends up doing things like just *comparing* current_cred with file->f_cred and then that would warn too.
Yeah ... and I think the kthread test should answer that question.