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

From: Vito Caputo <vcaputo@pengaru.com>
To: Kees Cook <keescook@chromium.org>
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, 23 Sep 2021 16:49:17 -0700	[thread overview]
Message-ID: <20210923234917.pqrxwoq7yqnvfpwu@shells.gnugeneration.com> (raw)
In-Reply-To: <20210923233105.4045080-1-keescook@chromium.org>

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.

I don't see how providing the symbol name of a given task's kernel
function, especially if shallow near the user->kernel entrypoint, is a
worrisome information leak.  Just make sure it's not failing open with
addresses like my original report documented seems to happen
spuriously as-is w/kallsyms.

When I worked full-time as a sysadmin WCHAN's were regularly the first
thing to look at in `ps -o stat,wchan | grep D` when things were
falling over.  e.g.:

```
root@shells:/root# ps -o stat,wchan | grep D
D    io_schedule
```

Furthermore this is a well documented on dead trees and understood
*nix/posix system observation technique.  Even the POSIX ps(1) man
page documents it:

https://pubs.opengroup.org/onlinepubs/9699919799/utilities/ps.html

Frankly I'm a bit mortified that I have to write this email.

Today I'm hoping to test Josh's patch @
https://lore.kernel.org/all/20210831083625.59554-1-zhengqi.arch@bytedance.com/

Thanks,
Vito Caputo