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

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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>
This commit is contained in:
Roman Gushchin 2020-10-13 16:52:42 -07:00 committed by Linus Torvalds
parent 61e604e636
commit 19b629c979
1 changed files with 11 additions and 0 deletions
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11
mm/memcontrol.c
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@ -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