[PATCH 2/3] proc: query LSMs for introspective mem access (if PROC_MEM_FORCE_ALWAYS)

[Jann Horn]

On Tue, Aug 25, 2026 at 3:42 PM Lorenzo Stoakes (ARM) <ljs at kernel.org> wrote:
> On Mon, Aug 24, 2026 at 07:43:02PM +0200, Jann Horn wrote:
> > On Mon, Aug 24, 2026 at 7:33 PM Lorenzo Stoakes (ARM) <ljs at kernel.org> wrote:
> > > On Mon, Aug 24, 2026 at 07:06:04PM +0200, Jann Horn wrote:
> > > > On Fri, Aug 21, 2026 at 8:52 PM Lorenzo Stoakes (ARM) <ljs at kernel.org> wrote:
> > > > > The sharing a /proc/mem fd seems like that's a pretty dumb thing to do in
> > > > > general :) but I guess you have to protect against that.
> > > >
> > > > Yeah, it's a kinda weird thing to do...
> > >
> > > Yup :)) but I guess we have to account for people doing weird stuff...
> > >
> > > In this case (I do mention it in a reply elsewhere I think) it does seem
> > > like perhaps you should separately check for current->mm != mm of (what was
> > > originally /proc/self/mm)?
> >
> > I wouldn't want to do it for this access check, since that could lead
> > to "confused deputy" problems.
>
> I guess if it got the decision wrong somehow that'd be a problem? Or wrongly
> OK'd it on one level but then that led to the fd being passed on assumption it
> was OK to do it or something?

The problematic scenario would be something like:

1. process A opens fd1=open("/proc/self/mem",O_RDWR)
2. process A does lseek(fd1, <address of libc>, SEEK_SET)
3. A sends fd1 to privileged daemon B as a "log output" FD
4. privileged daemon B write()s into fd1

In this scenario, daemon B is just trying to write log output into a
file descriptor. If we checked the current credentials on write(), we
might enable FOLL_FORCE just because daemon B is generally permitted
to use ptrace.

This illustrates why, in general, the "ambient privilege" that a
process has must not influence write() access decisions.

For a similar historical example, see
https://project-zero.issues.chromium.org/issues/42450869 where we used
to do capability checks in the old expand_downwards() logic, which
could be reached by writing into /proc/$pid/mem. That made it possible
for an unprivileged process to map virtual address 0 by providing
/proc/$pid/mem as stderr to a setuid root binary.