From: Roman Gushchin guro@fb.com
[ Upstream commit 19b629c9795bfe67bf77be8fb611b84424b56d91 ]
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 10befea91b61 ("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: 10befea91b61 ("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 Signed-off-by: Sasha Levin sashal@kernel.org --- mm/memcontrol.c | 11 +++++++++++ 1 file changed, 11 insertions(+)
diff --git a/mm/memcontrol.c b/mm/memcontrol.c index 6877c765b8d03..b9688a4b1d550 100644 --- 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