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From: Mel Gorman on 12 Feb 2010 07:10
rmap_walk_anon() does not use page_lock_anon_vma() for looking up and
locking an anon_vma. One important difference between page_lock_anon_vma()
and rmap_walk_anon() is that the page_lock_anon_vma() takes the RCU lock
before the lookup so that the anon_vma does not disappear. There does not
appear to be locking in place that prevents the anon_vma disappearing before
the spinlock is taken.

This patch puts a rcu_read_lock() around the anon_vma lookup similar to
what page_lock_anon_vma() does to prevent an accidental use-after-free.

Signed-off-by: Mel Gorman <mel(a)csn.ul.ie>
---
mm/rmap.c | 7 ++++++-
1 files changed, 6 insertions(+), 1 deletions(-)

diff --git a/mm/rmap.c b/mm/rmap.c
index b468d5f..fb695d3 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -1233,9 +1233,10 @@ static int rmap_walk_anon(struct page *page, int (*rmap_one)(struct page *,
* This needs to be reviewed later: avoiding page_lock_anon_vma()
* is risky, and currently limits the usefulness of rmap_walk().
*/
+ rcu_read_lock();
anon_vma = page_anon_vma(page);
if (!anon_vma)
- return ret;
+ goto out_rcu_unlock;
spin_lock(&anon_vma->lock);

/*
@@ -1256,6 +1257,10 @@ static int rmap_walk_anon(struct page *page, int (*rmap_one)(struct page *,

out_anon_unlock:
spin_unlock(&anon_vma->lock);
+
+out_rcu_unlock:
+ rcu_read_unlock();
+
return ret;
}

--
1.6.5

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