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Date:   Wed, 29 Sep 2021 20:00:42 +0100
From:   Mark Brown <broonie@kernel.org>
To:     Kees Cook <keescook@chromium.org>
Cc:     Josh Poimboeuf <jpoimboe@redhat.com>,
        Mark Rutland <mark.rutland@arm.com>,
        Vito Caputo <vcaputo@pengaru.com>,
        Jann Horn <jannh@google.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        "H. Peter Anvin" <hpa@zytor.com>, Jens Axboe <axboe@kernel.dk>,
        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 =?iso-8859-1?Q?Wei=DF?= 
        <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>,
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        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
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On Wed, Sep 29, 2021 at 11:54:55AM -0700, Kees Cook wrote:
> On Mon, Sep 27, 2021 at 01:50:56PM -0700, Josh Poimboeuf wrote:

> > Even with that patch, it doesn't lock the task's runqueue before reading
> > the stack, so there's still the possibility of the task running on
> > another CPU and the unwinder going off the rails a bit, which might be
> > used by an attacker in creative ways similar to the /proc/<pid>/stack
> > vulnerability Jann mentioned earlier.

> Since I think we're considering get_wchan() to be slow-path, can we just
> lock the runqueue and use arch_stack_walk_reliable()?

Unfortunately arch_stack_walk_reliable() is only available for powerpc,
s390 and x86 currently - work is in progress to implement it for arm64
as well but it's not there yet.

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