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From: Jann Horn <jannh@google.com>
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>,
	"Michael Weiß" <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
Subject: Re: [PATCH] proc: Disable /proc/$pid/wchan
Date: Fri, 24 Sep 2021 02:08:45 +0200	[thread overview]
Message-ID: <CAG48ez0Rtv5kqHWw368Ym3GkKodPA+JETOAN+=c2KPa3opENSA@mail.gmail.com> (raw)
In-Reply-To: <20210923234917.pqrxwoq7yqnvfpwu@shells.gnugeneration.com>

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?)

  reply	other threads:[~2021-09-24  0:09 UTC|newest]

Thread overview: 26+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-09-23 23:31 Kees Cook
2021-09-23 23:38 ` Kees Cook
2021-09-24 13:31   ` Peter Zijlstra
2021-09-23 23:49 ` Vito Caputo
2021-09-24  0:08   ` Jann Horn [this message]
2021-09-24  0:22     ` Vito Caputo
2021-09-24  1:16       ` Kees Cook
2021-09-24  1:34         ` Vito Caputo
2021-09-24  1:42           ` Kees Cook
2021-09-24 13:54         ` Mark Rutland
2021-09-24 14:26           ` Kees Cook
2021-09-27  9:03             ` Mark Rutland
2021-09-27 18:07               ` Kees Cook
2021-09-27 20:50                 ` Josh Poimboeuf
2021-09-29 18:54                   ` Kees Cook
2021-09-29 19:00                     ` Mark Brown
2021-09-29 19:26                       ` Kees Cook
2021-09-29 19:40                     ` Peter Zijlstra
2021-09-29 21:10                       ` Josh Poimboeuf
2021-09-27  9:16           ` David Laight
2021-09-29  8:48           ` Peter Zijlstra
2021-09-24  2:13 ` Andrew Morton
2021-09-24  6:04   ` Kees Cook
2021-09-24 13:29 ` Peter Zijlstra
2021-09-30 18:05 ` Stephen Brennan
2021-09-30 18:12   ` Kees Cook

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