Re: [Bugme-new] [Bug 33502] New: Caught 64-bit read from uninitialized memory in __alloc_skb

Christoph Lameter
2011-05-10T17:43:32Z

Draft for a patch

Subject: slub: Make CONFIG_PAGE_ALLOC work with new fastpath

Fastpath can do a speculative access to a page that CONFIG_PAGE_ALLOC may have
marked as invalid to retrieve the pointer to the next free object.

Probe that address before dereferencing the pointer to the page.
All of that needs to occur with interrupts disabled since an interrupt
could cause the page status to change (as pointed out by Eric).

Signed-off-by: Christoph Lameter <cl@linux.com>
---
mm/slub.c | 23 ++++++++++++++++++++++-
1 file changed, 22 insertions(+), 1 deletion(-)

Index: linux-2.6/mm/slub.c
===================================================================
--- linux-2.6.orig/mm/slub.c 2011-05-10 12:35:30.000000000 -0500
+++ linux-2.6/mm/slub.c 2011-05-10 12:38:53.000000000 -0500
@@ -261,6 +261,27 @@ static inline void *get_freepointer(stru
return *(void **)(object + s->offset);
}

+static inline void *get_freepointer_safe(struct kmem_cache *s, void *object)
+{
+ void *p;
+
+#ifdef CONFIG_PAGE_ALLOC
+ unsigned long flags;
+
+ local_irq_save(flags);
+
+ if (probe_kernel_address(object))
+ p = NULL; /* Invalid */
+ else
+ p = get_freepointer(s, object);
+
+ local_irq_restore(flags);
+#else
+ p = get_freepointer(s, object);
+#endif
+ return p;
+}
+
static inline void set_freepointer(struct kmem_cache *s, void *object, void *fp)
{
*(void **)(object + s->offset) = fp;
@@ -1933,7 +1954,7 @@ redo:
if (unlikely(!irqsafe_cpu_cmpxchg_double(
s->cpu_slab->freelist, s->cpu_slab->tid,
object, tid,
- get_freepointer(s, object), next_tid(tid)))) {
+ get_freepointer_safe(s, object), next_tid(tid)))) {

note_cmpxchg_failure("slab_alloc", s, tid);
goto redo;