Re: [PATCH 4/5] kprobes: Do not expose probe addresses to non-CAP_SYSLOG

[Linus Torvalds <torvalds@linux-foundation.org>]

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.

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.

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
default.

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.

              Linus