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:
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 <mho...@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
