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Date:   Thu, 30 Sep 2021 11:12:30 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Stephen Brennan <stephen.s.brennan@...cle.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Vito Caputo <vcaputo@...garu.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 WeiƟ 
        <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 Thu, Sep 30, 2021 at 11:05:35AM -0700, Stephen Brennan wrote:
> On 9/23/21 4:31 PM, 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.
> 
> Hi all,
> 
> It looks like there's already been pushback on this idea, but I wanted
> to add another voice from a frequent user of /proc/$pid/wchan (via PS).
> Much of my job involves diagnosing kernel issues and performance issues
> on stable kernels, frequently on production systems where I can't do
> anything too invasive. wchan is incredibly useful for these situations,
> so much so that we store regular snapshots of ps output, and we expand
> the size of the WCHAN column to fit more data (e.g. ps -e -o
> pid,wchan=WCHAN-WIDE-COLUMN). Disabling wchan would remove a critical
> tool for me and my team.

Thanks for speaking up! Yes, we've moved to fixing wchan correctly as
it's clear it's still very much in use. :) Current thread is here:
https://lore.kernel.org/lkml/20210929220218.691419-1-keescook@chromium.org/

-- 
Kees Cook

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