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authorRoman Gushchin <guro@fb.com>2020-10-13 16:52:42 -0700
committerLinus Torvalds <torvalds@linux-foundation.org>2020-10-13 18:38:30 -0700
commit19b629c9795bfe67bf77be8fb611b84424b56d91 (patch)
tree922a3d1d35d84659bc2e20bb62e10b983f376630 /mm/memcontrol.c
parent61e604e636ab9614de49df9149ef92cae17e9701 (diff)
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.c11
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.