perf: Fix double-free of the AUX buffer

If rb->aux_refcount is decremented to zero before rb->refcount,
__rb_free_aux() may be called twice resulting in a double free of
rb->aux_pages.  Fix this by adding a check to __rb_free_aux().
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Arnaldo Carvalho de Melo <acme@kernel.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: stable@vger.kernel.org
Fixes: 57ffc5ca ("perf: Fix AUX buffer refcounting")
Link: http://lkml.kernel.org/r/1437953468.12842.17.camel@decadent.org.ukSigned-off-by: Ingo Molnar <mingo@kernel.org>

kernel/events/ring_buffer.c

@@ -559,11 +559,13 @@ static void __rb_free_aux(struct ring_buffer *rb)
 		rb->aux_priv = NULL;
 	}
 
+	if (rb->aux_nr_pages) {
 		for (pg = 0; pg < rb->aux_nr_pages; pg++)
 			rb_free_aux_page(rb, pg);
 
 		kfree(rb->aux_pages);
 		rb->aux_nr_pages = 0;
+	}
 }
 
 void rb_free_aux(struct ring_buffer *rb)