From: Muchun Song <songmuchun@xxxxxxxxxxxxx> Subject: mm: memcg/slab: fix use after free in obj_cgroup_charge 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. Link: https://lkml.kernel.org/r/20201028035013.99711-2-songmuchun@xxxxxxxxxxxxx Fixes: bf4f059954dc ("mm: memcg/slab: obj_cgroup API") Signed-off-by: Muchun Song <songmuchun@xxxxxxxxxxxxx> Acked-by: Roman Gushchin <guro@xxxxxx> Reviewed-by: Shakeel Butt <shakeelb@xxxxxxxxxx> Cc: Johannes Weiner <hannes@xxxxxxxxxxx> Cc: Michal Hocko <mhocko@xxxxxxxxxx> Cc: Vladimir Davydov <vdavydov.dev@xxxxxxxxx> Cc: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx> Cc: Yafang Shao <laoar.shao@xxxxxxxxx> Cc: Chris Down <chris@xxxxxxxxxxxxxx> Cc: Christian Brauner <christian.brauner@xxxxxxxxxx> Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> --- mm/memcontrol.c | 4 +++- 1 file changed, 3 insertions(+), 1 deletion(-) --- a/mm/memcontrol.c~mm-memcg-slab-fix-use-after-free-in-obj_cgroup_charge +++ a/mm/memcontrol.c @@ -3235,8 +3235,10 @@ int obj_cgroup_charge(struct obj_cgroup * 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; _
