Re: [PATCH 02/11] mm,migration: Do not try to migrate unmapped anonymous pages

[Mel Gorman <mel@csn.ul.ie>]

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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