Re: [PATCH] io_uring: don't audit the capability check in io_uring_create()

[Ondrej Mosnacek]

On Tue, Jul 18, 2023 at 3:24 PM Jeff Moyer <jmoyer@xxxxxxxxxx> wrote:
>
> Hi, Ondrej,
>
> Ondrej Mosnacek <omosnace@xxxxxxxxxx> writes:
>
> > The check being unconditional may lead to unwanted denials reported by
> > LSMs when a process has the capability granted by DAC, but denied by an
> > LSM. In the case of SELinux such denials are a problem, since they can't
> > be effectively filtered out via the policy and when not silenced, they
> > produce noise that may hide a true problem or an attack.
> >
> > Since not having the capability merely means that the created io_uring
> > context will be accounted against the current user's RLIMIT_MEMLOCK
> > limit, we can disable auditing of denials for this check by using
> > ns_capable_noaudit() instead of capable().
>
> Could you add a comment, or add some documentation to
> ns_capable_noaudit() about when it should be used?  It wasn't apparent
> to me, at least, before this explanation.

This has been requested before, so I finally forced myself to look
into it and only now I realized that there is a subtle difference
between the has_capability and capable helpers. As the docstrings say,
the former doesn't set the PF_SUPERPRIV on the task when the check
succeeds, while the latter does. The problem is that I don't know what
the exact implications are and thus I'm not able to document which
helper should be used in what situation... It is possible some of the
existing call sites use the wrong helper in the noaudit case (possibly
including ones that I added/suggested).

The comment at its declaration says "Used super-user privileges" and
it seems to be used only to propagate into the ASU flag in task
accounting information. But in the case of capability checks that do
not fail the syscall it is not easy to tell if "super-user privileges"
were "used" or not (or, rather, whether the task should be accounted
as such or not after a successful check).

If anyone is reading this and has a better understanding of the
PF_SUPERPRIV flag semantics, I'd be thankful for a clarification so
that we can sort out this mess :)

--
Ondrej Mosnacek
Senior Software Engineer, Linux Security - SELinux kernel
Red Hat, Inc.