mm: memcg/slab: fix racy access to page->mem_cgroup in mem_cgroup_from_obj()

mem_cgroup_from_obj() checks the lowest bit of the page->mem_cgroup
pointer to determine if the page has an attached obj_cgroup vector instead
of a regular memcg pointer.  If it's not set, it simple returns the
page->mem_cgroup value as a struct mem_cgroup pointer.

The commit faa69fb579fc ("mm: memcg/slab: use a single set of kmem_caches
for all allocations") changed the moment when this bit is set: if
previously it was set on the allocation of the slab page, now it can be
set well after, when the first accounted object is allocated on this page.

It opened a race: if page->mem_cgroup is set concurrently after the first
page_has_obj_cgroups(page) check, a pointer to the obj_cgroups array can
be returned as a memory cgroup pointer.

A simple check for page->mem_cgroup pointer for NULL before the
page_has_obj_cgroups() check fixes the race.  Indeed, if the pointer is
not NULL, it's either a simple mem_cgroup pointer or a pointer to
obj_cgroup vector.  The pointer can be asynchronously changed from NULL to
(obj_cgroup_vec | 0x1UL), but can't be changed from a valid memcg pointer
to objcg vector or back.

If the object passed to mem_cgroup_from_obj() is a slab object and
page->mem_cgroup is NULL, it means that the object is not accounted, so
the function must return NULL.

I've discovered the race looking at the code, so far I haven't seen it in
the wild.

Fixes: faa69fb579fc ("mm: memcg/slab: use a single set of kmem_caches for all allocations")
Signed-off-by: Roman Gushchin <guro@fb.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Reviewed-by: Shakeel Butt <shakeelb@google.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Vlastimil Babka <vbabka@suse.cz>
Link: https://lkml.kernel.org/r/20200910022435.2773735-1-guro@fb.com
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index e71e8440de6b38a4c6d70586b81880502ad95671..ba9f5404b8cf13014c6f2a3cba142242ae8b7a44 100644 (file)
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -2887,6 +2887,17 @@ struct mem_cgroup *mem_cgroup_from_obj(void *p)
 
        page = virt_to_head_page(p);
 
+       /*
+        * If page->mem_cgroup is set, it's either a simple mem_cgroup pointer
+        * or a pointer to obj_cgroup vector. In the latter case the lowest
+        * bit of the pointer is set.
+        * The page->mem_cgroup pointer can be asynchronously changed
+        * from NULL to (obj_cgroup_vec | 0x1UL), but can't be changed
+        * from a valid memcg pointer to objcg vector or back.
+        */
+       if (!page->mem_cgroup)
+               return NULL;
+
        /*
         * Slab objects are accounted individually, not per-page.
         * Memcg membership data for each individual object is saved in