linux-kernel - Re: [PATCH 08/12] x86/dumpstack: Pin the target stack in save_stack_trace

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Message-ID: <20160915191938.mm2k7ccv6d2o622t@treble>
Date:   Thu, 15 Sep 2016 14:19:38 -0500
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Andy Lutomirski <luto@...capital.net>
Cc:     Andy Lutomirski <luto@...nel.org>, X86 ML <x86@...nel.org>,
        Borislav Petkov <bp@...en8.de>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Brian Gerst <brgerst@...il.com>, Jann Horn <jann@...jh.net>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>, live-patching@...r.kernel.org
Subject: Re: [PATCH 08/12] x86/dumpstack: Pin the target stack in
 save_stack_trace_tsk()

On Thu, Sep 15, 2016 at 11:41:25AM -0700, Andy Lutomirski wrote:
> I also wouldn't mind trying to do something to prevent ever dumping
> the stack of an actively running task.  It's definitely safe to dump:
> 
>  - current
> 
>  - any task that's stopped via ptrace, etc
> 
>  - any task on the current CPU if running atomically enough that the
> task can't migrate (which probably covers the nasty NMI cases, I hope)
> 
> What's *not* safe AFAIK is /proc/PID/stack.  I don't know if we can
> somehow fix that short of actually sending an interrupt or NMI to
> freeze the task if it's running.  I'm also not sure it's worth
> worrying about it.

Yeah, I proposed a fix for /proc/PID/stack a while back:

  https://lkml.kernel.org/r/cover.1424109806.git.jpoimboe@redhat.com

My idea was to use task_rq_lock() to lock the runqueue and then check
tsk->on_cpu.  I think Peter wasn't too keen on it.

We'll need something similar for the livepatch consistency model.

-- 
Josh