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Date: Tue, 14 Mar 2017 16:26:25 +0100
From: Peter Zijlstra <peterz@infradead.org>
To: Oleg Nesterov <oleg@redhat.com>
Cc: Dmitry Vyukov <dvyukov@google.com>, Ingo Molnar <mingo@redhat.com>,
        Arnaldo Carvalho de Melo <acme@kernel.org>,
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        LKML <linux-kernel@vger.kernel.org>,
        Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
        syzkaller <syzkaller@googlegroups.com>
Subject: Re: perf: use-after-free in perf_release
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On Tue, Mar 14, 2017 at 04:19:10PM +0100, Oleg Nesterov wrote:
> On 03/14, Peter Zijlstra wrote:
> >
> > 	    mutex_unlock(ctx->lock);
> > 	    put_ctx() /* >0 */
> > 	  free_task();
> > 					    mutex_lock(ctx->lock);
> > 					    mutex_lock(A->child_mutex);
> > 					    /* ... */
> > 					    mutex_unlock(A->child_mutex);
> > 					    mutex_unlock(ctx->lock)
> > 					    put_ctx() /* 0 */
> > 					      ctx->task && !TOMBSTONE
> > 					        put_task_struct() /* UAF */
> >
> >
> > Something like that, right?
> 
> Yes, exactly.
> 
> > Let me see if it makes sense to retain perf_event_free_task() at all;
> > maybe we should always do perf_event_exit_task().
> 
> Yes, perhaps... but this needs changes too. Say, WARN_ON_ONCE(child != current)
> in perf_event_exit_task_context(). And even perf_event_task(new => F) does not
> look right in this case. In fact it would be simply buggy to do this, this task
> was not fully constructed yet, so even perf_event_pid(task) is not safe.

Yeah; there's a fair amount of stuff like that. I'm afraid crafting
exceptions for all that will just end up with more of a mess than we
safe by merging the two :/

A well.. I'll go do the 'trivial' patch then.