Messages in this thread Patch in this message | | | From | Muchun Song <> | Subject | [PATCH v2] mm: memcg/slab: Fix use after free in obj_cgroup_charge | Date | Wed, 28 Oct 2020 11:50:11 +0800 |
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The rcu_read_lock/unlock only can guarantee that the memcg will not be freed, but it cannot guarantee the success of css_get to memcg.
If the whole process of a cgroup offlining is completed between reading a objcg->memcg pointer and bumping the css reference on another CPU, and there are exactly 0 external references to this memory cgroup (how we get to the obj_cgroup_charge() then?), css_get() can change the ref counter from 0 back to 1.
Fixes: bf4f059954dc ("mm: memcg/slab: obj_cgroup API") Signed-off-by: Muchun Song <songmuchun@bytedance.com> Acked-by: Roman Gushchin <guro@fb.com> --- changelog in v2: 1. Add unlikely and update the commit log suggested by Roman.
mm/memcontrol.c | 4 +++- 1 file changed, 3 insertions(+), 1 deletion(-)
diff --git a/mm/memcontrol.c b/mm/memcontrol.c index 8c8b4c3ed5a0..d9cdf899c6fc 100644 --- a/mm/memcontrol.c +++ b/mm/memcontrol.c @@ -3221,8 +3221,10 @@ int obj_cgroup_charge(struct obj_cgroup *objcg, gfp_t gfp, size_t size) * independently later. */ rcu_read_lock(); +retry: memcg = obj_cgroup_memcg(objcg); - css_get(&memcg->css); + if (unlikely(!css_tryget(&memcg->css))) + goto retry; rcu_read_unlock(); nr_pages = size >> PAGE_SHIFT; -- 2.20.1
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