lib/list_sort: do not pass bad pointers to cmp callback

If the original list is a POT in length, the first callback from line 73
will pass a==b both pointing to the original list_head.  This is dangerous
because the 'list_sort()' user can use 'container_of()' and accesses the
"containing" object, which does not necessary exist for the list head.  So
the user can access RAM which does not belong to him.  If this is a write
access, we can end up with memory corruption.

Signed-off-by: Don Mullis <[email protected]>
Tested-by: Artem Bityutskiy <[email protected]>
Signed-off-by: Artem Bityutskiy <[email protected]>
Cc: <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

lib/list_sort.c

@@ -70,7 +70,7 @@ static void merge_and_restore_back_links(void *priv,
 		 * element comparison is needed, so the client's cmp()
 		 * routine can invoke cond_resched() periodically.
 		 */
-		(*cmp)(priv, tail, tail);
+		(*cmp)(priv, tail->next, tail->next);
 
 		tail->next->prev = tail;
 		tail = tail->next;