linux-hardening - Re: [PATCH] proc: Disable /proc/$pid/wchan

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Date:   Mon, 27 Sep 2021 10:03:51 +0100
From:   Mark Rutland <mark.rutland@....com>
To:     Kees Cook <keescook@...omium.org>
Cc:     Vito Caputo <vcaputo@...garu.com>, Jann Horn <jannh@...gle.com>,
        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>,
        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 07:26:22AM -0700, Kees Cook wrote:
> On Fri, Sep 24, 2021 at 02:54:24PM +0100, Mark Rutland wrote:
> > On Thu, Sep 23, 2021 at 06:16:16PM -0700, Kees Cook wrote:
> > > On Thu, Sep 23, 2021 at 05:22:30PM -0700, Vito Caputo wrote:
> > > > Instead of unwinding stacks maybe the kernel should be sticking an
> > > > entrypoint address in the current task struct for get_wchan() to
> > > > access, whenever userspace enters the kernel?
> > > 
> > > wchan is supposed to show where the kernel is at the instant the
> > > get_wchan() happens. (i.e. recording it at syscall entry would just
> > > always show syscall entry.)
> > 
> > It's supposed to show where a blocked task is blocked; the "wait
> > channel".
> > 
> > I'd wanted to remove get_wchan since it requires cross-task stack
> > walking, which is generally painful.
> 
> Right -- this is the "fragile" part I'm worried about.
> 
> > We could instead have the scheduler entrypoints snapshot their caller
> > into a field in task_struct. If there are sufficiently few callers, that
> > could be an inline wrapper that passes a __func__ string. Otherwise, we
> > still need to symbolize.
> 
> Hmm. Does PREEMPT break this?

Within the core scheduler functions interrupts should be disabled, and
as long as we only update task_struct there we shouldn't have a race.

> Can we actually use __builtin_return_address(0) in __schedule?

We'd need to do this in a few entry points above __schedule, since the
currently get_wchan walks until !in_sched_functions(). It should be
possible, though we might need to make sure those the nexus points
aren't inlined.

Thanks,
Mark.