[PATCH 4.4 13/15] mm/mempool: avoid KASAN marking mempool poison checks as use-after-free

[Greg Kroah-Hartman]

4.4-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Matthew Dawson <matthew@xxxxxxxxxxxxx>

commit 7640131032db9118a78af715ac77ba2debeeb17c upstream.

When removing an element from the mempool, mark it as unpoisoned in KASAN
before verifying its contents for SLUB/SLAB debugging.  Otherwise KASAN
will flag the reads checking the element use-after-free writes as
use-after-free reads.

Signed-off-by: Matthew Dawson <matthew@xxxxxxxxxxxxx>
Acked-by: Andrey Ryabinin <aryabinin@xxxxxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Andrii Bordunov <aborduno@xxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

---
 mm/mempool.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

--- a/mm/mempool.c
+++ b/mm/mempool.c
@@ -135,8 +135,8 @@ static void *remove_element(mempool_t *p
 	void *element = pool->elements[--pool->curr_nr];
 
 	BUG_ON(pool->curr_nr < 0);
-	check_element(pool, element);
 	kasan_unpoison_element(pool, element);
+	check_element(pool, element);
 	return element;
 }