+ mm-memcg-slab-fix-racy-access-to-page-mem_cgroup-in-mem_cgroup_from_obj.patch added to -mm tree

[akpm@linux-foundation.org]

The patch titled
     Subject: mm: memcg/slab: fix racy access to page->mem_cgroup in mem_cgroup_from_obj()
has been added to the -mm tree.  Its filename is
     mm-memcg-slab-fix-racy-access-to-page-mem_cgroup-in-mem_cgroup_from_obj.patch

This patch should soon appear at
    https://ozlabs.org/~akpm/mmots/broken-out/mm-memcg-slab-fix-racy-access-to-page-mem_cgroup-in-mem_cgroup_from_obj.patch
and later at
    https://ozlabs.org/~akpm/mmotm/broken-out/mm-memcg-slab-fix-racy-access-to-page-mem_cgroup-in-mem_cgroup_from_obj.patch

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------------------------------------------------------
From: Roman Gushchin <guro@fb.com>
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/20200910022435.2773735-1-guro@fb.com
Fixes: 10befea91b61 ("mm: memcg/slab: use a single set of kmem_caches for all allocations")
Signed-off-by: Roman Gushchin <guro@fb.com>
Reviewed-by: Shakeel Butt <shakeelb@google.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

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

--- 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.
_

Patches currently in -mm which might be from guro@fb.com are

mm-memcg-slab-fix-racy-access-to-page-mem_cgroup-in-mem_cgroup_from_obj.patch
mm-vmstat-fix-proc-sys-vm-stat_refresh-generating-false-warnings.patch
mm-vmstat-fix-proc-sys-vm-stat_refresh-generating-false-warnings-fix.patch
mm-rework-remote-memcg-charging-api-to-support-nesting.patch
mm-kmem-move-memcg_kmem_bypass-calls-to-get_mem-obj_cgroup_from_current.patch
mm-kmem-remove-redundant-checks-from-get_obj_cgroup_from_current.patch
mm-kmem-prepare-remote-memcg-charging-infra-for-interrupt-contexts.patch
mm-kmem-enable-kernel-memcg-accounting-from-interrupt-contexts.patch