On Thu, Jul 19, 2018 at 3:43 AM Michal Hocko <mhocko@xxxxxxxxxx> wrote: > > [CC Andrew] > > On Thu 19-07-18 18:06:47, Jing Xia wrote: > > It was reported that a kernel crash happened in mem_cgroup_iter(), > > which can be triggered if the legacy cgroup-v1 non-hierarchical > > mode is used. > > > > Unable to handle kernel paging request at virtual address 6b6b6b6b6b6b8f > > ...... > > Call trace: > > mem_cgroup_iter+0x2e0/0x6d4 > > shrink_zone+0x8c/0x324 > > balance_pgdat+0x450/0x640 > > kswapd+0x130/0x4b8 > > kthread+0xe8/0xfc > > ret_from_fork+0x10/0x20 > > > > mem_cgroup_iter(): > > ...... > > if (css_tryget(css)) <-- crash here > > break; > > ...... > > > > The crashing reason is that mem_cgroup_iter() uses the memcg object > > whose pointer is stored in iter->position, which has been freed before > > and filled with POISON_FREE(0x6b). > > > > And the root cause of the use-after-free issue is that > > invalidate_reclaim_iterators() fails to reset the value of > > iter->position to NULL when the css of the memcg is released in non- > > hierarchical mode. > > Well, spotted! > > I suspect > Fixes: 6df38689e0e9 ("mm: memcontrol: fix possible memcg leak due to interrupted reclaim") > > but maybe it goes further into past. I also suggest > Cc: stable > > even though the non-hierarchical mode is strongly discouraged. Why not set root_mem_cgroup's use_hierarchy to true by default on init? If someone wants non-hierarchical mode, they can explicitly set it to false. > A lack of > reports for 3 years is encouraging that not many people really use this > mode. > > > Signed-off-by: Jing Xia <jing.xia.mail@xxxxxxxxx> > > Acked-by: Michal Hocko <mhocko@xxxxxxxx> > > Thanks! > > --- > > mm/memcontrol.c | 2 +- > > 1 file changed, 1 insertion(+), 1 deletion(-) > > > > diff --git a/mm/memcontrol.c b/mm/memcontrol.c > > index e6f0d5e..8c0280b 100644 > > --- a/mm/memcontrol.c > > +++ b/mm/memcontrol.c > > @@ -850,7 +850,7 @@ static void invalidate_reclaim_iterators(struct mem_cgroup *dead_memcg) > > int nid; > > int i; > > > > - while ((memcg = parent_mem_cgroup(memcg))) { > > + for (; memcg; memcg = parent_mem_cgroup(memcg)) { > > for_each_node(nid) { > > mz = mem_cgroup_nodeinfo(memcg, nid); > > for (i = 0; i <= DEF_PRIORITY; i++) { > > -- > > 1.9.1 > > -- > Michal Hocko > SUSE Labs > -- > To unsubscribe from this list: send the line "unsubscribe cgroups" in > the body of a message to majordomo@xxxxxxxxxxxxxxx > More majordomo info at http://vger.kernel.org/majordomo-info.html -- To unsubscribe from this list: send the line "unsubscribe cgroups" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html