[PATCH] perf: Avoid double-free of AUX buffer

[Ben Hutchings]

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().

Fixes: 57ffc5ca679f ("perf: Fix AUX buffer refcounting")
Signed-off-by: Ben Hutchings <b...@decadent.org.uk>
---
Found by inspection, and the fix is compile-tested only.

I think both commit 57ffc5ca679f and this should go to stable, but
that's up to you.

Ben.

 kernel/events/ring_buffer.c | 10 ++++++----
 1 file changed, 6 insertions(+), 4 deletions(-)

diff --git a/kernel/events/ring_buffer.c b/kernel/events/ring_buffer.c
index b2be01b..c8aa3f7 100644
--- a/kernel/events/ring_buffer.c
+++ b/kernel/events/ring_buffer.c
@@ -559,11 +559,13 @@ static void __rb_free_aux(struct ring_buffer *rb)
                rb->aux_priv = NULL;
        }
 
-       for (pg = 0; pg < rb->aux_nr_pages; pg++)
-               rb_free_aux_page(rb, pg);
+       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;
+               kfree(rb->aux_pages);
+               rb->aux_nr_pages = 0;
+       }
 }
 
 void rb_free_aux(struct ring_buffer *rb)
-- 
Ben Hutchings
Any sufficiently advanced bug is indistinguishable from a feature.

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