In slab_alloc_node(), after the cpu_slab is assigned, if the task is
preempted and moves to another CPU, there's nothing keeping the page and
object in sync. The -rt kernel crashed because page was NULL and object
was not, and the node_match() dereferences page. Even though the crash
happened on -rt, there's nothing that's keeping this from happening on
mainline.
The easiest fix is to disable interrupts for the entire time from
acquiring the current CPU cpu_slab and assigning the object and page.
After that, it's fine to allow preemption.
Signed-off-by: Steven Rostedt <rostedt@xxxxxxxxxxx>
diff --git a/mm/slub.c b/mm/slub.c
index 2fdd96f..c00ce80 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -2317,6 +2317,8 @@ static __always_inline void *slab_alloc(struct kmem_cache *s,
struct kmem_cache_cpu *c;
struct page *page;
unsigned long tid;
+ unsigned long flags;
+ int new_object;
if (slab_pre_alloc_hook(s, gfpflags))
return NULL;
@@ -2328,7 +2330,10 @@ redo:
* enabled. We may switch back and forth between cpus while
* reading from one cpu area. That does not matter as long
* as we end up on the original cpu again when doing the cmpxchg.
+ *
+ * But we need to sync the setting of page and object.
*/
+ local_irq_save(flags);
c = __this_cpu_ptr(s->cpu_slab);
/*
@@ -2338,11 +2343,14 @@ redo:
* linked list in between.
*/
tid = c->tid;
- barrier();
object = c->freelist;
page = c->page;
- if (unlikely(!object || !node_match(page, node)))
+
+ new_object = !object || !node_match(page, node);
+ local_irq_restore(flags);
+
+ if (new_object)
object = __slab_alloc(s, gfpflags, node, addr, c);
else {
--
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