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

[Kees Cook <keescook@chromium.org>]

On Thu, Sep 23, 2021 at 06:34:08PM -0700, Vito Caputo 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.)
> > 
> 
> And you have the syscall # onhand when performing the syscall entry,
> no?
> 
> The point is, if the alternative is to always get 0 from
> /proc/PID/wchan when a process is sitting in ioctl(), I'd be perfectly
> happy to get back sys_ioctl.  I'm under the impression there's quite a
> bit of vendor-specific flexibility here in terms of how precise WCHAN
> is.

Oh, yeah, if you're happy with syscall-level granularity, that'd be
totally fine by me too.

> If it's possible to preserve the old WCHAN precision I'm all for it.
> But if we've become so paranoid about leaking anything about the
> kernel to userspace that this is untenable, even if it just spits out
> the syscall being performed that has value.

I'd like to find a middle ground -- wchan has always seemed like a info
leak, even with only symbols. And it doesn't help that walking the stack
from outside the current task is difficult. :)

-Kees

-- 
Kees Cook