linux-kernel - Re: [PATCH] proc: Disable /proc/$pid/wchan

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Date:   Fri, 24 Sep 2021 02:08:45 +0200
From:   Jann Horn <jannh@...gle.com>
To:     Vito Caputo <vcaputo@...garu.com>
Cc:     Kees Cook <keescook@...omium.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>, Jens Axboe <axboe@...nel.dk>,
        Mark Rutland <mark.rutland@....com>,
        Peter Zijlstra <peterz@...radead.org>,
        Stefan Metzmacher <metze@...ba.org>,
        Andy Lutomirski <luto@...nel.org>,
        Lai Jiangshan <laijs@...ux.alibaba.com>,
        Christian Brauner <christian.brauner@...ntu.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "Kenta.Tada@...y.com" <Kenta.Tada@...y.com>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Michael Weiß <michael.weiss@...ec.fraunhofer.de>,
        Anand K Mistry <amistry@...gle.com>,
        Alexey Gladkov <legion@...nel.org>,
        Michal Hocko <mhocko@...e.com>, Helge Deller <deller@....de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Andrea Righi <andrea.righi@...onical.com>,
        Ohhoon Kwon <ohoono.kwon@...sung.com>,
        Kalesh Singh <kaleshsingh@...gle.com>,
        YiFei Zhu <yifeifz2@...inois.edu>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        linux-kernel@...r.kernel.org, x86@...nel.org,
        linux-fsdevel@...r.kernel.org, linux-hardening@...r.kernel.org
Subject: Re: [PATCH] proc: Disable /proc/$pid/wchan

On Fri, Sep 24, 2021 at 1:59 AM Vito Caputo <vcaputo@...garu.com> 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@...hat.com>
> > Cc: Vito Caputo <vcaputo@...garu.com>
> > Signed-off-by: Kees Cook <keescook@...omium.org>
> <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?)