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SubjectRe: [PATCH v2] mm: memcontrol: fix use after free in mem_cgroup_iter()
On Fri 26-07-19 14:49:33, Michal Hocko wrote:
> On Fri 26-07-19 10:12:47, Miles Chen wrote:
> > This patch is sent to report an use after free in mem_cgroup_iter()
> > after merging commit: be2657752e9e "mm: memcg: fix use after free in
> > mem_cgroup_iter()".
> >
> > I work with android kernel tree (4.9 & 4.14), and the commit:
> > be2657752e9e "mm: memcg: fix use after free in mem_cgroup_iter()" has
> > been merged to the trees. However, I can still observe use after free
> > issues addressed in the commit be2657752e9e.
> > (on low-end devices, a few times this month)
> >
> > backtrace:
> > css_tryget <- crash here
> > mem_cgroup_iter
> > shrink_node
> > shrink_zones
> > do_try_to_free_pages
> > try_to_free_pages
> > __perform_reclaim
> > __alloc_pages_direct_reclaim
> > __alloc_pages_slowpath
> > __alloc_pages_nodemask
> >
> > To debug, I poisoned mem_cgroup before freeing it:
> >
> > static void __mem_cgroup_free(struct mem_cgroup *memcg)
> > for_each_node(node)
> > free_mem_cgroup_per_node_info(memcg, node);
> > free_percpu(memcg->stat);
> > + /* poison memcg before freeing it */
> > + memset(memcg, 0x78, sizeof(struct mem_cgroup));
> > kfree(memcg);
> > }
> >
> > The coredump shows the position=0xdbbc2a00 is freed.
> >
> > (gdb) p/x ((struct mem_cgroup_per_node *)0xe5009e00)->iter[8]
> > $13 = {position = 0xdbbc2a00, generation = 0x2efd}
> >
> > 0xdbbc2a00: 0xdbbc2e00 0x00000000 0xdbbc2800 0x00000100
> > 0xdbbc2a10: 0x00000200 0x78787878 0x00026218 0x00000000
> > 0xdbbc2a20: 0xdcad6000 0x00000001 0x78787800 0x00000000
> > 0xdbbc2a30: 0x78780000 0x00000000 0x0068fb84 0x78787878
> > 0xdbbc2a40: 0x78787878 0x78787878 0x78787878 0xe3fa5cc0
> > 0xdbbc2a50: 0x78787878 0x78787878 0x00000000 0x00000000
> > 0xdbbc2a60: 0x00000000 0x00000000 0x00000000 0x00000000
> > 0xdbbc2a70: 0x00000000 0x00000000 0x00000000 0x00000000
> > 0xdbbc2a80: 0x00000000 0x00000000 0x00000000 0x00000000
> > 0xdbbc2a90: 0x00000001 0x00000000 0x00000000 0x00100000
> > 0xdbbc2aa0: 0x00000001 0xdbbc2ac8 0x00000000 0x00000000
> > 0xdbbc2ab0: 0x00000000 0x00000000 0x00000000 0x00000000
> > 0xdbbc2ac0: 0x00000000 0x00000000 0xe5b02618 0x00001000
> > 0xdbbc2ad0: 0x00000000 0x78787878 0x78787878 0x78787878
> > 0xdbbc2ae0: 0x78787878 0x78787878 0x78787878 0x78787878
> > 0xdbbc2af0: 0x78787878 0x78787878 0x78787878 0x78787878
> > 0xdbbc2b00: 0x78787878 0x78787878 0x78787878 0x78787878
> > 0xdbbc2b10: 0x78787878 0x78787878 0x78787878 0x78787878
> > 0xdbbc2b20: 0x78787878 0x78787878 0x78787878 0x78787878
> > 0xdbbc2b30: 0x78787878 0x78787878 0x78787878 0x78787878
> > 0xdbbc2b40: 0x78787878 0x78787878 0x78787878 0x78787878
> > 0xdbbc2b50: 0x78787878 0x78787878 0x78787878 0x78787878
> > 0xdbbc2b60: 0x78787878 0x78787878 0x78787878 0x78787878
> > 0xdbbc2b70: 0x78787878 0x78787878 0x78787878 0x78787878
> > 0xdbbc2b80: 0x78787878 0x78787878 0x00000000 0x78787878
> > 0xdbbc2b90: 0x78787878 0x78787878 0x78787878 0x78787878
> > 0xdbbc2ba0: 0x78787878 0x78787878 0x78787878 0x78787878
> >
> > In the reclaim path, try_to_free_pages() does not setup
> > sc.target_mem_cgroup and sc is passed to do_try_to_free_pages(), ...,
> > shrink_node().
> >
> > In mem_cgroup_iter(), root is set to root_mem_cgroup because
> > sc->target_mem_cgroup is NULL.
> > It is possible to assign a memcg to root_mem_cgroup.nodeinfo.iter in
> > mem_cgroup_iter().
> >
> > try_to_free_pages
> > struct scan_control sc = {...}, target_mem_cgroup is 0x0;
> > do_try_to_free_pages
> > shrink_zones
> > shrink_node
> > mem_cgroup *root = sc->target_mem_cgroup;
> > memcg = mem_cgroup_iter(root, NULL, &reclaim);
> > mem_cgroup_iter()
> > if (!root)
> > root = root_mem_cgroup;
> > ...
> >
> > css = css_next_descendant_pre(css, &root->css);
> > memcg = mem_cgroup_from_css(css);
> > cmpxchg(&iter->position, pos, memcg);
> >
> > My device uses memcg non-hierarchical mode.
> > When we release a memcg: invalidate_reclaim_iterators() reaches only
> > dead_memcg and its parents. If non-hierarchical mode is used,
> > invalidate_reclaim_iterators() never reaches root_mem_cgroup.
> >
> > static void invalidate_reclaim_iterators(struct mem_cgroup *dead_memcg)
> > {
> > struct mem_cgroup *memcg = dead_memcg;
> >
> > for (; memcg; memcg = parent_mem_cgroup(memcg)
> > ...
> > }
> >
> > So the use after free scenario looks like:
> >
> > CPU1 CPU2
> >
> > try_to_free_pages
> > do_try_to_free_pages
> > shrink_zones
> > shrink_node
> > mem_cgroup_iter()
> > if (!root)
> > root = root_mem_cgroup;
> > ...
> > css = css_next_descendant_pre(css, &root->css);
> > memcg = mem_cgroup_from_css(css);
> > cmpxchg(&iter->position, pos, memcg);
> >
> > invalidate_reclaim_iterators(memcg);
> > ...
> > __mem_cgroup_free()
> > kfree(memcg);
> >
> > try_to_free_pages
> > do_try_to_free_pages
> > shrink_zones
> > shrink_node
> > mem_cgroup_iter()
> > if (!root)
> > root = root_mem_cgroup;
> > ...
> > mz = mem_cgroup_nodeinfo(root, reclaim->pgdat->node_id);
> > iter = &mz->iter[reclaim->priority];
> > pos = READ_ONCE(iter->position);
> > css_tryget(&pos->css) <- use after free
>
> Thanks for the write up. This is really useful.
>
> > To avoid this, we should also invalidate root_mem_cgroup.nodeinfo.iter in
> > invalidate_reclaim_iterators().
>
> I am sorry, I didn't get to comment an earlier version but I am
> wondering whether it makes more sense to do and explicit invalidation.
>
> [...]
> > +static void invalidate_reclaim_iterators(struct mem_cgroup *dead_memcg)
> > +{
> > + struct mem_cgroup *memcg = dead_memcg;
> > + int invalidate_root = 0;
> > +
> > + for (; memcg; memcg = parent_mem_cgroup(memcg))
> > + __invalidate_reclaim_iterators(memcg, dead_memcg);
>
> /* here goes your comment */
> if (!dead_memcg->use_hierarchy)
> __invalidate_reclaim_iterators(root_mem_cgroup, dead_memcg);
> > +
> > +}
>
> Other than that the patch looks good to me.
>
> Acked-by: Michal Hocko <mhocko@suse.com>

Btw. I believe we want to push this to stable trees as well. I think it
goes all the way down to 5ac8fb31ad2e ("mm: memcontrol: convert reclaim
iterator to simple css refcounting"). Unless I am missing something a
Fixes: tag would be really helpful.
--
Michal Hocko
SUSE Labs

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