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

From: Kees Cook <keescook@chromium.org>
To: Stephen Brennan <stephen.s.brennan@oracle.com>
Cc: "Thomas Gleixner" <tglx@linutronix.de>,
	"Josh Poimboeuf" <jpoimboe@redhat.com>,
	"Vito Caputo" <vcaputo@pengaru.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: Thu, 30 Sep 2021 11:12:30 -0700	[thread overview]
Message-ID: <202109301109.4E172219@keescook> (raw)
In-Reply-To: <c283c978-2563-06b9-4c21-59bedceda9ea@oracle.com>

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