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From:   Jann Horn <jannh@google.com>
Date:   Fri, 24 Sep 2021 02:08:45 +0200
Message-ID: <CAG48ez0Rtv5kqHWw368Ym3GkKodPA+JETOAN+=c2KPa3opENSA@mail.gmail.com>
Subject: Re: [PATCH] proc: Disable /proc/$pid/wchan
To:     Vito Caputo <vcaputo@pengaru.com>
Cc:     Kees Cook <keescook@chromium.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Josh Poimboeuf <jpoimboe@redhat.com>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        "H. Peter Anvin" <hpa@zytor.com>, Jens Axboe <axboe@kernel.dk>,
        Mark Rutland <mark.rutland@arm.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Stefan Metzmacher <metze@samba.org>,
        Andy Lutomirski <luto@kernel.org>,
        Lai Jiangshan <laijs@linux.alibaba.com>,
        Christian Brauner <christian.brauner@ubuntu.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        "Kenta.Tada@sony.com" <Kenta.Tada@sony.com>,
        Daniel Bristot de Oliveira <bristot@redhat.com>,
        =?UTF-8?Q?Michael_Wei=C3=9F?= <michael.weiss@aisec.fraunhofer.de>,
        Anand K Mistry <amistry@google.com>,
        Alexey Gladkov <legion@kernel.org>,
        Michal Hocko <mhocko@suse.com>, Helge Deller <deller@gmx.de>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Andrea Righi <andrea.righi@canonical.com>,
        Ohhoon Kwon <ohoono.kwon@samsung.com>,
        Kalesh Singh <kaleshsingh@google.com>,
        YiFei Zhu <yifeifz2@illinois.edu>,
        "Eric W. Biederman" <ebiederm@xmission.com>,
        linux-kernel@vger.kernel.org, x86@kernel.org,
        linux-fsdevel@vger.kernel.org, linux-hardening@vger.kernel.org
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On Fri, Sep 24, 2021 at 1:59 AM Vito Caputo <vcaputo@pengaru.com> wrote:
> On Thu, Sep 23, 2021 at 04:31:05PM -0700, Kees Cook wrote:
> > The /proc/$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@redhat.com>
> > Cc: Vito Caputo <vcaputo@pengaru.com>
> > Signed-off-by: Kees Cook <keescook@chromium.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?)