[080/181] mm: memcg/slab: fix racy access to page->mem_cgroup in mem_cgroup_from_obj()

Commit Message

Andrew Morton Oct. 13, 2020, 11:52 p.m. UTC

From: Roman Gushchin <[e-mail]>
Subject: 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 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.

Link: https://lkml.kernel.org/r/[e-mail]
Fixes: 10befea91b61 ("mm: memcg/slab: use a single set of kmem_caches for all allocations")
Signed-off-by: Roman Gushchin <[e-mail]>
Reviewed-by: Shakeel Butt <[e-mail]>
Cc: Johannes Weiner <[e-mail]>
Cc: Vlastimil Babka <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
---

 mm/memcontrol.c |   11 +++++++++++
 1 file changed, 11 insertions(+)

Patch

--- a/mm/memcontrol.c~mm-memcg-slab-fix-racy-access-to-page-mem_cgroup-in-mem_cgroup_from_obj
+++ a/mm/memcontrol.c
@@ -2888,6 +2888,17 @@  struct mem_cgroup *mem_cgroup_from_obj(v
 	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
 	 * the page->obj_cgroups.