From: Roman Gushchin <guro@fb.com> To: Muchun Song <songmuchun@bytedance.com> Cc: <hannes@cmpxchg.org>, <mhocko@kernel.org>, <vdavydov.dev@gmail.com>, <akpm@linux-foundation.org>, <shakeelb@google.com>, <iamjoonsoo.kim@lge.com>, <laoar.shao@gmail.com>, <chris@chrisdown.name>, <christian.brauner@ubuntu.com>, <peterz@infradead.org>, <mingo@kernel.org>, <keescook@chromium.org>, <tglx@linutronix.de>, <esyr@redhat.com>, <surenb@google.com>, <areber@redhat.com>, <elver@google.com>, <linux-kernel@vger.kernel.org>, <cgroups@vger.kernel.org>, <linux-mm@kvack.org> Subject: Re: [PATCH 2/5] mm: memcg/slab: Fix use after free in obj_cgroup_charge Date: Tue, 27 Oct 2020 11:31:32 -0700 [thread overview] Message-ID: <20201027183132.GC725724@carbon.dhcp.thefacebook.com> (raw) In-Reply-To: <20201027080256.76497-3-songmuchun@bytedance.com> On Tue, Oct 27, 2020 at 04:02:53PM +0800, Muchun Song wrote: > 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. This can be happened when we reparent the memcg. So using > css_tryget instead of css_get. Hm, I really doubt that it can be reproduced in the real life, but I think you're formally correct. 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. Is this a good description of the problem? If so, I think it's worth a more detailed description and mentioning that it's not a real-life bug. It also probably doesn't deserve a stable@ backport. > > Fixes: bf4f059954dc ("mm: memcg/slab: obj_cgroup API") > Signed-off-by: Muchun Song <songmuchun@bytedance.com> > --- > mm/memcontrol.c | 4 +++- > 1 file changed, 3 insertions(+), 1 deletion(-) > > diff --git a/mm/memcontrol.c b/mm/memcontrol.c > index fcbd79c5023e..c0c27bee23ad 100644 > --- a/mm/memcontrol.c > +++ b/mm/memcontrol.c > @@ -3223,8 +3223,10 @@ 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 (!css_tryget(&memcg->css)) if (unlikely(!css_tryget(&memcg->css))) ^^^^^^^^ maybe? > + goto retry; Otherwise the fix looks good to me. Please, feel free to add Acked-by: Roman Gushchin <guro@fb.com> after extending the commit description. Thanks!
WARNING: multiple messages have this Message-ID (diff)
From: Roman Gushchin <guro@fb.com> To: Muchun Song <songmuchun@bytedance.com> Cc: hannes@cmpxchg.org, mhocko@kernel.org, vdavydov.dev@gmail.com, akpm@linux-foundation.org, shakeelb@google.com, iamjoonsoo.kim@lge.com, laoar.shao@gmail.com, chris@chrisdown.name, christian.brauner@ubuntu.com, peterz@infradead.org, mingo@kernel.org, keescook@chromium.org, tglx@linutronix.de, esyr@redhat.com, surenb@google.com, areber@redhat.com, elver@google.com, linux-kernel@vger.kernel.org, cgroups@vger.kernel.org, linux-mm@kvack.org Subject: Re: [PATCH 2/5] mm: memcg/slab: Fix use after free in obj_cgroup_charge Date: Tue, 27 Oct 2020 11:31:32 -0700 [thread overview] Message-ID: <20201027183132.GC725724@carbon.dhcp.thefacebook.com> (raw) In-Reply-To: <20201027080256.76497-3-songmuchun@bytedance.com> On Tue, Oct 27, 2020 at 04:02:53PM +0800, Muchun Song wrote: > 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. This can be happened when we reparent the memcg. So using > css_tryget instead of css_get. Hm, I really doubt that it can be reproduced in the real life, but I think you're formally correct. 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. Is this a good description of the problem? If so, I think it's worth a more detailed description and mentioning that it's not a real-life bug. It also probably doesn't deserve a stable@ backport. > > Fixes: bf4f059954dc ("mm: memcg/slab: obj_cgroup API") > Signed-off-by: Muchun Song <songmuchun@bytedance.com> > --- > mm/memcontrol.c | 4 +++- > 1 file changed, 3 insertions(+), 1 deletion(-) > > diff --git a/mm/memcontrol.c b/mm/memcontrol.c > index fcbd79c5023e..c0c27bee23ad 100644 > --- a/mm/memcontrol.c > +++ b/mm/memcontrol.c > @@ -3223,8 +3223,10 @@ 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 (!css_tryget(&memcg->css)) if (unlikely(!css_tryget(&memcg->css))) ^^^^^^^^ maybe? > + goto retry; Otherwise the fix looks good to me. Please, feel free to add Acked-by: Roman Gushchin <guro@fb.com> after extending the commit description. Thanks!
next prev parent reply other threads:[~2020-10-27 18:32 UTC|newest] Thread overview: 41+ messages / expand[flat|nested] mbox.gz Atom feed top 2020-10-27 8:02 [PATCH 0/5] Fix some bugs in memcg/slab Muchun Song 2020-10-27 8:02 ` Muchun Song 2020-10-27 8:02 ` [PATCH 1/5] mm: memcg/slab: Fix return child memcg objcg for root memcg Muchun Song 2020-10-27 17:42 ` Roman Gushchin 2020-10-27 17:42 ` Roman Gushchin 2020-10-27 19:05 ` Roman Gushchin 2020-10-27 19:05 ` Roman Gushchin 2020-10-27 8:02 ` [PATCH 2/5] mm: memcg/slab: Fix use after free in obj_cgroup_charge Muchun Song 2020-10-27 18:31 ` Roman Gushchin [this message] 2020-10-27 18:31 ` Roman Gushchin 2020-10-27 8:02 ` [PATCH 3/5] mm: memcg/slab: Rename *_lruvec_slab_state to *_lruvec_kmem_state Muchun Song 2020-10-27 8:02 ` Muchun Song 2020-10-27 18:37 ` Roman Gushchin 2020-10-27 18:37 ` Roman Gushchin 2020-10-27 8:02 ` [PATCH 4/5] mm: memcg/slab: Fix root memcg vmstats Muchun Song 2020-10-27 8:02 ` Muchun Song 2020-10-27 18:48 ` Roman Gushchin 2020-10-27 18:48 ` Roman Gushchin 2020-10-28 2:56 ` [External] " Muchun Song 2020-10-28 2:56 ` Muchun Song 2020-10-28 2:56 ` Muchun Song 2020-10-28 3:47 ` Muchun Song 2020-10-28 3:47 ` Muchun Song 2020-10-29 0:14 ` Roman Gushchin 2020-10-29 0:14 ` Roman Gushchin 2020-10-29 6:15 ` Muchun Song 2020-10-29 6:15 ` Muchun Song 2020-11-10 1:32 ` Roman Gushchin 2020-11-10 1:32 ` Roman Gushchin 2020-11-10 2:57 ` Muchun Song 2020-11-10 2:57 ` Muchun Song 2020-11-10 2:57 ` Muchun Song 2020-10-27 8:02 ` [PATCH 5/5] mm: memcontrol: Simplify the mem_cgroup_page_lruvec Muchun Song 2020-10-27 8:02 ` Muchun Song 2020-10-27 13:36 ` Michal Hocko 2020-10-27 13:36 ` Michal Hocko 2020-10-27 14:15 ` [External] " Muchun Song 2020-10-27 14:15 ` Muchun Song 2020-10-27 14:15 ` Muchun Song 2020-10-27 14:31 ` Michal Hocko 2020-10-27 14:31 ` Michal Hocko
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