diff options
author | Roman Gushchin <guro@fb.com> | 2020-10-14 02:52:42 +0300 |
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committer | Linus Torvalds <torvalds@linux-foundation.org> | 2020-10-14 04:38:30 +0300 |
commit | 19b629c9795bfe67bf77be8fb611b84424b56d91 (patch) | |
tree | 922a3d1d35d84659bc2e20bb62e10b983f376630 /mm/memcontrol.c | |
parent | 61e604e636ab9614de49df9149ef92cae17e9701 (diff) | |
download | linux-19b629c9795bfe67bf77be8fb611b84424b56d91.tar.xz |
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.
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>
Diffstat (limited to 'mm/memcontrol.c')
-rw-r--r-- | mm/memcontrol.c | 11 |
1 files changed, 11 insertions, 0 deletions
diff --git a/mm/memcontrol.c b/mm/memcontrol.c index e71e8440de6b..ba9f5404b8cf 100644 --- a/mm/memcontrol.c +++ b/mm/memcontrol.c @@ -2888,6 +2888,17 @@ 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 * the page->obj_cgroups. |