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SubjectRe: [PATCH v2] mm: memcontrol: fix use after free in mem_cgroup_iter()
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On Sat, 2019-07-27 at 07:47 +0800, Miles Chen wrote:
> On Fri, 2019-07-26 at 14:55 +0200, Michal Hocko wrote:
> > 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.
> > >
>
> I think we should keep the original v2 version, the reason is the
> !use_hierarchy does not imply we can reach root_mem_cgroup:

Sorry I want to correct myself:

(dead_memcg->use_hierarchy == true) does not guarantee that we can
reach root_mem_cgroup by parent_mem_cgroup(dead_memcg)

> cd /sys/fs/cgroup/memory/0
> mkdir 1
> cd /sys/fs/cgroup/memory/0/1
> echo 1 > memory.use_hierarchy // only 1 and its children has
> use_hierarchy set
> mkdir 2
>
> rmdir 2 // parent_mem_cgroup(2) goes up to 1
>
> > > [...]
> > > > +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.
>
> No problem. I'll add the fix tag to patch v3.
> Fixes: 5ac8fb31ad2e ("mm: memcontrol: convert reclaim iterator to simple
> css refcounting")
>
>
> Miles
>
>


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