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Date: Fri, 24 Sep 2021 02:08:45 +0200 From: Jann Horn <jannh@...gle.com> To: Vito Caputo <vcaputo@...garu.com> Cc: Kees Cook <keescook@...omium.org>, Thomas Gleixner <tglx@...utronix.de>, Josh Poimboeuf <jpoimboe@...hat.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 Weiß <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/$pid/wchan On Fri, Sep 24, 2021 at 1:59 AM Vito Caputo <vcaputo@...garu.com> wrote: > On Thu, Sep 23, 2021 at 04:31:05PM -0700, Kees Cook wrote: > > The /proc/$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/$pid/stat with 0. > > > > This leaves kernel/sched/fair.c as the only user of get_wchan(). But > > again, since this was broken for 4 years, was this profiling logic > > actually doing anything useful? > > > > [1] https://lore.kernel.org/lkml/20210922001537.4ktg3r2ky3b3r6yp@treble/ > > > > Cc: Josh Poimboeuf <jpoimboe@...hat.com> > > Cc: Vito Caputo <vcaputo@...garu.com> > > Signed-off-by: Kees Cook <keescook@...omium.org> > <snip> > > > Please don't deliberately break WCHANs wholesale. This is a very > useful tool for sysadmins to get a vague sense of where processes are > spending time in the kernel on production systems without affecting > performance or having to restart things under instrumentation. Wouldn't /proc/$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?)
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