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
