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

[Josh Poimboeuf]

On Mon, Sep 27, 2021 at 11:07:27AM -0700, Kees Cook wrote:
> On Mon, Sep 27, 2021 at 10:03:51AM +0100, Mark Rutland wrote:
> > On Fri, Sep 24, 2021 at 07:26:22AM -0700, Kees Cook wrote:
> > > On Fri, Sep 24, 2021 at 02:54:24PM +0100, Mark Rutland wrote:
> > > > On Thu, Sep 23, 2021 at 06:16:16PM -0700, Kees Cook wrote:
> > > > > On Thu, Sep 23, 2021 at 05:22:30PM -0700, Vito Caputo wrote:
> > > > > > Instead of unwinding stacks maybe the kernel should be sticking an
> > > > > > entrypoint address in the current task struct for get_wchan() to
> > > > > > access, whenever userspace enters the kernel?
> > > > > 
> > > > > wchan is supposed to show where the kernel is at the instant the
> > > > > get_wchan() happens. (i.e. recording it at syscall entry would just
> > > > > always show syscall entry.)
> > > > 
> > > > It's supposed to show where a blocked task is blocked; the "wait
> > > > channel".
> > > > 
> > > > I'd wanted to remove get_wchan since it requires cross-task stack
> > > > walking, which is generally painful.
> > > 
> > > Right -- this is the "fragile" part I'm worried about.
> 
> I'd like to clarify this concern first -- is the proposed fix actually
> fragile? Because I think we'd be better off just restoring behavior than
> trying to invent new behavior...
> 
> i.e. Josh, Jann, do you see any issues with Qi Zheng's fix here:
> https://lore.kernel.org/all/20210924062006.231699-4-keescook@xxxxxxxxxxxx/

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.

-- 
Josh