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Date: Thu, 23 Sep 2021 23:04:11 -0700 From: Kees Cook <keescook@...omium.org> To: Andrew Morton <akpm@...ux-foundation.org> 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>, "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/$pid/wchan On Thu, Sep 23, 2021 at 07:13:06PM -0700, Andrew Morton wrote: > On Thu, 23 Sep 2021 16:31:05 -0700 Kees Cook <keescook@...omium.org> wrote: > > > The /proc/$pid/wchan file has been broken by default on x86_64 for 4 > > years now[1]. > > [1] is hard to decrypt. I think it would be better if this changelog > were to describe the problem directly, completely and succinctly? > > > 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? > > Agree that returning a hard-wired "0\n" is the way to go. I must NAK my own patch. ;) It looks like this would be a breaking userspace-visible change[1]. We need to fix the two bugs though: 1) wchan broken under ORC (patch exists in the thread at [1]) 2) wchan leaking raw addresses (152c432b128c needs reverting from v5.12 and later) -Kees [1] https://lore.kernel.org/lkml/20210924054647.v6x6risoa4jhuu6s@shells.gnugeneration.com -- Kees Cook
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