On Mon, 2019-07-29 at 12:06 -0400, Johannes Weiner wrote:
> On Fri, Jul 26, 2019 at 10:12:47AM +0800, 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
> >
> > To avoid this, we should also invalidate root_mem_cgroup.nodeinfo.iter in
> > invalidate_reclaim_iterators().
> >
> > Change since v1:
> > Add a comment to explain why we need to handle root_mem_cgroup separately.
> > Rename invalid_root to invalidate_root.
> >
> > Cc: Johannes Weiner <hannes@xxxxxxxxxxx>
> > Signed-off-by: Miles Chen <miles.chen@xxxxxxxxxxxx>
> > ---
> > mm/memcontrol.c | 38 ++++++++++++++++++++++++++++----------
> > 1 file changed, 28 insertions(+), 10 deletions(-)
> >
> > diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> > index cdbb7a84cb6e..09f2191f113b 100644
> > --- a/mm/memcontrol.c
> > +++ b/mm/memcontrol.c
> > @@ -1130,26 +1130,44 @@ void mem_cgroup_iter_break(struct mem_cgroup *root,
> > css_put(&prev->css);
> > }
> >
> > -static void invalidate_reclaim_iterators(struct mem_cgroup *dead_memcg)
> > +static void __invalidate_reclaim_iterators(struct mem_cgroup *from,
> > + struct mem_cgroup *dead_memcg)
> > {
> > - struct mem_cgroup *memcg = dead_memcg;
> > struct mem_cgroup_reclaim_iter *iter;
> > struct mem_cgroup_per_node *mz;
> > int nid;
> > int i;
> >
> > - for (; memcg; memcg = parent_mem_cgroup(memcg)) {
> > - for_each_node(nid) {
> > - mz = mem_cgroup_nodeinfo(memcg, nid);
> > - for (i = 0; i <= DEF_PRIORITY; i++) {
> > - iter = &mz->iter[i];
> > - cmpxchg(&iter->position,
> > - dead_memcg, NULL);
> > - }
> > + for_each_node(nid) {
> > + mz = mem_cgroup_nodeinfo(from, nid);
> > + for (i = 0; i <= DEF_PRIORITY; i++) {
> > + iter = &mz->iter[i];
> > + cmpxchg(&iter->position,
> > + dead_memcg, NULL);
> > }
> > }
> > }
> >
> > +/*
> > + * When cgruop1 non-hierarchy mode is used, parent_mem_cgroup() does
> > + * not walk all the way up to the cgroup root (root_mem_cgroup). So
> > + * we have to handle dead_memcg from cgroup root separately.
> > + */
> > +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);
> > + if (memcg == root_mem_cgroup)
> > + invalidate_root = 1;
> > + }
> > +
> > + if (!invalidate_root)
> > + __invalidate_reclaim_iterators(root_mem_cgroup, dead_memcg);
>
> "invalidate_root" suggests we still have to invalidate the root, but
> the variable works the opposite way. How about dropping it altogether
> and moving the comment directly to where the decision is made:
>
> struct mem_cgroup *memcg = dead_memcg;
>
> do {
> __invalidate_reclaim_iterators(memcg, dead_memcg);
> last = memcg;
> } while ((memcg = parent_mem_cgroup(memcg)));
>
> /*
> * When cgruop1 non-hierarchy mode is used,
> * parent_mem_cgroup() does not walk all the way up to the
> * cgroup root (root_mem_cgroup). So we have to handle
> * dead_memcg from cgroup root separately.
> */
> if (last != root_mem_cgroup)
> __invalidate_reclaim_iterators(root_mem_cgroup, dead_memcg);
Thanks for the suggestion, the code is easier to read this way.
I'll submit patch v4 with this and the fixed tags.
Miles
