Re: [PATCH v2] mm: memcontrol: fix use after free in mem_cgroup_iter()

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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@xxxxxxxx>

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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