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SubjectRe: [PATCH v2] mm: memcontrol: fix use after free in mem_cgroup_iter()
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>
--
Michal Hocko
SUSE Labs

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