Re: [PATCH] proc: Disable /proc/$pid/wchan

[Kees Cook]

On Wed, Sep 29, 2021 at 08:00:42PM +0100, Mark Brown wrote:
> On Wed, Sep 29, 2021 at 11:54:55AM -0700, Kees Cook wrote:
> > On Mon, Sep 27, 2021 at 01:50:56PM -0700, Josh Poimboeuf wrote:
> 
> > > Even with that patch, it doesn't lock the task's runqueue before reading
> > > the stack, so there's still the possibility of the task running on
> > > another CPU and the unwinder going off the rails a bit, which might be
> > > used by an attacker in creative ways similar to the /proc/<pid>/stack
> > > vulnerability Jann mentioned earlier.
> 
> > Since I think we're considering get_wchan() to be slow-path, can we just
> > lock the runqueue and use arch_stack_walk_reliable()?
> 
> Unfortunately arch_stack_walk_reliable() is only available for powerpc,
> s390 and x86 currently - work is in progress to implement it for arm64
> as well but it's not there yet.

Strictly speaking, we're only trying to fix this for x86+ORC. The other
architectures (or non-ORC x86) already have their own non-ORC unwinders
behind get_wchan(). They may have similar weaknesses (which should
certainly be fixed), I think the first step here is to restore wchan
under x86+ORC.

-- 
Kees Cook