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

[Vito Caputo]

On Fri, Sep 24, 2021 at 02:08:45AM +0200, Jann Horn wrote:
> On Fri, Sep 24, 2021 at 1:59 AM Vito Caputo <vcaputo@xxxxxxxxxxx> wrote:
> > On Thu, Sep 23, 2021 at 04:31:05PM -0700, Kees Cook wrote:
> > > The /proc/$pid/wchan file has been broken by default on x86_64 for 4
> > > years now[1]. As this remains a potential leak of either kernel
> > > addresses (when symbolization fails) or limited observation of kernel
> > > function progress, just remove the contents for good.
> > >
> > > Unconditionally set the contents to "0" and also mark the wchan
> > > field in /proc/$pid/stat with 0.
> > >
> > > This leaves kernel/sched/fair.c as the only user of get_wchan(). But
> > > again, since this was broken for 4 years, was this profiling logic
> > > actually doing anything useful?
> > >
> > > [1] https://lore.kernel.org/lkml/20210922001537.4ktg3r2ky3b3r6yp@treble/
> > >
> > > Cc: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
> > > Cc: Vito Caputo <vcaputo@xxxxxxxxxxx>
> > > Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
> > <snip>
> >
> >
> > Please don't deliberately break WCHANs wholesale.  This is a very
> > useful tool for sysadmins to get a vague sense of where processes are
> > spending time in the kernel on production systems without affecting
> > performance or having to restart things under instrumentation.
> 
> Wouldn't /proc/$pid/stack be more useful for that anyway? As long as
> you have root privileges, you can read that to get the entire stack,
> not just a single method name.
> 
> (By the way, I guess that might be an alternative to ripping wchan out
> completely - require CAP_SYS_ADMIN like for /proc/$pid/stack?)

WCHAN is a first-class concept of the OS.  As a result we have
long-standing useful tools exposing them in far more organized,
documented, and discoverable ways than poking around linux-specific
/proc files at the shell.  Even `top` can show WCHAN in a column
alongside everything else it exposes, complete with sorting etc, and
I've already demonstrated the support in `ps`.

I also think it's worth preserving the ability for regular users to
observe the WCHAN of their own processes.  It's unclear to me why this
is such a worry.  If the WCHAN as-implemented is granular enough to
expose too much kernel inner workings, then it should be watered down
to be more vague.  Even if it just said "ioctl" when a process was
blocked in D state through making an ioctl() it would still be much
more useful than saying nothing at all.  Can't regular users see this
much about their own processes via strace/gdb anyways?

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?

Regards,
Vito Caputo