[PATCH rfc 1/5] mm: memcg/slab: fix racy access to page->mem_cgroup in mem_cgroup_from_obj()

[Roman Gushchin <guro@fb.com>]

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>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Shakeel Butt <shakeelb@google.com>
---
 mm/memcontrol.c | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 75cd1a1e66c8..093526fec4bf 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -2923,6 +2923,17 @@ 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
-- 
2.26.2