[patch] mm: memcontrol: rewrite uncharge API fix - double migration

From: Johannes Weiner
Date: Wed Jul 09 2014 - 21:02:11 EST

Hugh reports:

VM_BUG_ON_PAGE(!(pc->flags & PCG_MEM))
page had count 1 mapcount 0 mapping anon index 0x196
flags locked uptodate reclaim swapbacked, pcflags 1, memcg not root
mem_cgroup_migrate < move_to_new_page < migrate_pages < compact_zone <
compact_zone_order < try_to_compact_pages < __alloc_pages_direct_compact <
__alloc_pages_nodemask < alloc_pages_vma < do_huge_pmd_anonymous_page <
handle_mm_fault < __do_page_fault

mem_cgroup_migrate() assumes that a page is only migrated once and
then freed immediately after.

However, putting the page back on the LRU list and dropping the
isolation refcount is not done atomically. This allows a PFN-based
migrator like compaction to isolate the page, see the expected
anonymous page refcount of 1, and migrate the page once more.

Furthermore, once the charges are transferred to the new page, the old
page no longer has a pin on the memcg, which might get released before
the page itself now. pc->mem_cgroup is invalid at this point, but
PCG_USED suggests otherwise, provoking use-after-free.

Properly uncharge the page after it's been migrated, including the
clearing of PCG_USED, so that a subsequent charge migration attempt
will be able to detect it and bail out.

Signed-off-by: Johannes Weiner <hannes@xxxxxxxxxxx>
Reported-by: Hugh Dickins <hughd@xxxxxxxxxx>
mm/memcontrol.c | 8 +++++++-
1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 1e3b27f8dc2f..1439537fe7c9 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -6655,7 +6655,6 @@ void mem_cgroup_migrate(struct page *oldpage, struct page *newpage,

VM_BUG_ON_PAGE(!(pc->flags & PCG_MEM), oldpage);
VM_BUG_ON_PAGE(do_swap_account && !(pc->flags & PCG_MEMSW), oldpage);
- pc->flags &= ~(PCG_MEM | PCG_MEMSW);

if (PageTransHuge(oldpage)) {
nr_pages <<= compound_order(oldpage);
@@ -6663,6 +6662,13 @@ void mem_cgroup_migrate(struct page *oldpage, struct page *newpage,
VM_BUG_ON_PAGE(!PageTransHuge(newpage), newpage);

+ pc->flags = 0;
+ local_irq_disable();
+ mem_cgroup_charge_statistics(pc->mem_cgroup, oldpage, -nr_pages);
+ memcg_check_events(pc->mem_cgroup, oldpage);
+ local_irq_enable();
commit_charge(newpage, pc->mem_cgroup, nr_pages, lrucare);


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