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@xxxxxxxxxxxx> 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. -- Kees Cook
