slub: fix a possible memleak in __slab_alloc()

Zhihua Che reported a possible memleak in slub allocator on
CONFIG_PREEMPT=y builds.

It is possible current thread migrates right before disabling irqs in
__slab_alloc(). We must check again c->freelist, and perform a normal
allocation instead of scratching c->freelist.

Many thanks to Zhihua Che for spotting this bug, introduced in 2.6.39

V2: Its also possible an IRQ freed one (or several) object(s) and
populated c->freelist, so its not a CONFIG_PREEMPT only problem.

Cc: <stable@vger.kernel.org>        [2.6.39+]
Reported-by: Zhihua Che <zhihua.che@gmail.com>
Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Acked-by: Christoph Lameter <cl@linux.com>
Signed-off-by: Pekka Enberg <penberg@kernel.org>

1 files changed, 5 insertions, 0 deletions

diff --git a/mm/slub.c b/mm/slub.c
index 108ed03fb42..5e410a95aba 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -2169,6 +2169,11 @@ redo:
                 goto new_slab;
         }
 
+        /* must check again c->freelist in case of cpu migration or IRQ */
+        object = c->freelist;
+        if (object)
+                goto load_freelist;
+
         stat(s, ALLOC_SLOWPATH);
 
         do {