Re: perf: use-after-free in perf_release

[Peter Zijlstra <peterz@infradead.org>]

On Tue, Mar 14, 2017 at 04:02:41PM +0100, Peter Zijlstra wrote:
> 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().

Do we want a WARN_ON_ONCE(atomic_read(&tsk->usage)); in free_task()?
Because in the above scenario we're freeing it with references on.