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SubjectRe: [PATCH 02/11] mm,migration: Do not try to migrate unmapped anonymous pages
On Mon, Mar 15, 2010 at 09:48:49PM +0900, Minchan Kim wrote:
> On Mon, 2010-03-15 at 11:28 +0000, Mel Gorman wrote:
> > The use after free looks like
> >
> > 1. page_mapcount(page) was zero so anon_vma was no longer reliable
> > 2. rcu lock taken but the anon_vma at this point can already be garbage because the
> > process exited
> > 3. call try_to_unmap, looks up tha anon_vma and locks it. This causes problems
> >
> > I thought the race would be closed but there is still a very tiny window there all
> > right. The following alternative should close it. What do you think?
> >
> > if (PageAnon(page)) {
> > rcu_read_lock();
> >
> > /*
> > * 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_mapcount(page)) {
> > rcu_read_unlock();
> > goto uncharge;
> > }
> >
> > rcu_locked = 1;
> > anon_vma = page_anon_vma(page);
> > atomic_inc(&anon_vma->external_refcount);
> > }
> >
> > The rcu_unlock label is not used here because the reference counts were not taken in
> > the case where page_mapcount == 0.
> >
>
> Please, repost above code with your use-after-free scenario comment.
>

This will be the replacement patch so.

==== CUT HERE ====
mm,migration: Do not try to migrate unmapped anonymous pages

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@csn.ul.ie>
Acked-by: Rik van Riel <riel@redhat.com>
---
mm/migrate.c | 13 +++++++++++++
1 files changed, 13 insertions(+), 0 deletions(-)

diff --git a/mm/migrate.c b/mm/migrate.c
index 98eaaf2..6eb1efe 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -603,6 +603,19 @@ static int unmap_and_move(new_page_t get_new_page, unsigned long private,
*/
if (PageAnon(page)) {
rcu_read_lock();
+
+ /*
+ * 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_mapcount(page)) {
+ rcu_read_unlock();
+ goto uncharge;
+ }
+
rcu_locked = 1;
anon_vma = page_anon_vma(page);
atomic_inc(&anon_vma->migrate_refcount);

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