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From: Mel Gorman on 30 Mar 2010 05:20
rmap_walk_anon() was triggering errors in memory compaction that look like
use-after-free errors. The problem is that between the page being isolated
from the LRU and rcu_read_lock() being taken, the mapcount of the page
dropped to 0 and the anon_vma gets freed. This can happen during memory
compaction if pages being migrated belong to a process that exits before
migration completes. Hence, the use-after-free race looks like

1. Page isolated for migration
2. Process exits
3. page_mapcount(page) drops to zero so anon_vma was no longer reliable
4. unmap_and_move() takes the rcu_lock but the anon_vma is already garbage
4. call try_to_unmap, looks up tha anon_vma and "locks" it but the lock
is garbage.

This patch checks the mapcount after the rcu lock is taken. If the
mapcount is zero, the anon_vma is assumed to be freed and no further
action is taken.

Signed-off-by: Mel Gorman <mel(a)csn.ul.ie>
Acked-by: Rik van Riel <riel(a)redhat.com>
Reviewed-by: Minchan Kim <minchan.kim(a)gmail.com>
Reviewed-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu(a)jp.fujitsu.com>
---
mm/migrate.c | 11 +++++++++++
1 files changed, 11 insertions(+), 0 deletions(-)

diff --git a/mm/migrate.c b/mm/migrate.c
index 06e6316..5c5c1bd 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -599,6 +599,17 @@ static int unmap_and_move(new_page_t get_new_page, unsigned long private,
if (PageAnon(page)) {
rcu_read_lock();
rcu_locked = 1;
+
+ /*
+ * If the page has no mappings any more, just bail. An
+ * unmapped anon page is likely to be freed soon but worse,
+ * it's possible its anon_vma disappeared between when
+ * the page was isolated and when we reached here while
+ * the RCU lock was not held
+ */
+ if (!page_mapped(page))
+ goto rcu_unlock;
+
anon_vma = page_anon_vma(page);
atomic_inc(&anon_vma->migrate_refcount);
}
--
1.6.5

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