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Date: Fri, 16 Sep 2016 09:47:30 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Andy Lutomirski <luto@amacapital.net>,
        Andy Lutomirski <luto@kernel.org>, X86 ML <x86@kernel.org>,
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        Brian Gerst <brgerst@gmail.com>, Jann Horn <jann@thejh.net>,
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Subject: Re: [PATCH 08/12] x86/dumpstack: Pin the target stack in
 save_stack_trace_tsk()
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On Thu, Sep 15, 2016 at 02:19:38PM -0500, Josh Poimboeuf wrote:
> 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.

That basically allows a DoS on the scheduler, since a user can run tasks
on every cpu (through sys_sched_setaffinity()). Then doing while (1) cat
/proc/$PID/stack would saturate the rq->lock on every CPU.

The more tasks the merrier.