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

[KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>]

On Mon, 15 Mar 2010 14:21:24 +0000
Mel Gorman <mel@csn.ul.ie> wrote:

> 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>

Reviewd-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.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);
>