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Date: Tue, 14 Mar 2017 16:02:41 +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 03:30:11PM +0100, Oleg Nesterov wrote:

> But. perf_event_init_task() adds child_event to parent_event->child_list.
> 
> If perf_event_release_kernel(parent_event) is called before copy_process()
> does perf_event_free_task() which (in particular) removes it from child_list,
> perf_event_release_kernel() can find this child_event and do get_ctx(ctx)
> (under the list_for_each_entry(child, &event->child_list, child_list) loop).

Right; the child_list is the only thing that is exposed. And yes, it
looks like that can interleave just right.

> Then it does put_ctx(ctx), but ctx->task can be already freed by
> copy_process()->free_task() in this case.


	Task1				Task2

	fork()
	  perf_event_init_task()
	  /* ... */
	  goto bad_fork_$foo;
	  /* ... */
	  perf_event_free_task()
	    mutex_lock(ctx->lock)
	    perf_free_event(B)

					perf_event_release_kernel(A)
					  mutex_lock(A->child_mutex)
					  list_for_each_entry(child, ...) {
					    /* child == B */
					    ctx = B->ctx;
					    get_ctx(ctx);
					    mutex_unlock(A->child_mutex);

	      mutex_lock(A->child_mutex)
	      list_del_init(B->child_list)
	      mutex_unlock(A->child_mutex)

	      /* ... */

	    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?


Let me see if it makes sense to retain perf_event_free_task() at all;
maybe we should always do perf_event_exit_task().