linux-kernel - [PATCH 4.4 168/268] mm: fix races between address_space dereference and free in page

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Date:   Mon, 28 May 2018 12:02:22 +0200
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org, "Huang, Ying" <ying.huang@...el.com>,
        Jan Kara <jack@...e.cz>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Minchan Kim <minchan@...nel.org>,
        Johannes Weiner <hannes@...xchg.org>,
        Michal Hocko <mhocko@...e.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Sasha Levin <alexander.levin@...rosoft.com>
Subject: [PATCH 4.4 168/268] mm: fix races between address_space dereference and free in page_evicatable

4.4-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Huang Ying <ying.huang@...el.com>

[ Upstream commit e92bb4dd9673945179b1fc738c9817dd91bfb629 ]

When page_mapping() is called and the mapping is dereferenced in
page_evicatable() through shrink_active_list(), it is possible for the
inode to be truncated and the embedded address space to be freed at the
same time.  This may lead to the following race.

CPU1                                                CPU2

truncate(inode)                                     shrink_active_list()
  ...                                                 page_evictable(page)
  truncate_inode_page(mapping, page);
    delete_from_page_cache(page)
      spin_lock_irqsave(&mapping->tree_lock, flags);
        __delete_from_page_cache(page, NULL)
          page_cache_tree_delete(..)
            ...                                         mapping = page_mapping(page);
            page->mapping = NULL;
            ...
      spin_unlock_irqrestore(&mapping->tree_lock, flags);
      page_cache_free_page(mapping, page)
        put_page(page)
          if (put_page_testzero(page)) -> false
- inode now has no pages and can be freed including embedded address_space

                                                        mapping_unevictable(mapping)
							  test_bit(AS_UNEVICTABLE, &mapping->flags);
- we've dereferenced mapping which is potentially already free.

Similar race exists between swap cache freeing and page_evicatable()
too.

The address_space in inode and swap cache will be freed after a RCU
grace period.  So the races are fixed via enclosing the page_mapping()
and address_space usage in rcu_read_lock/unlock().  Some comments are
added in code to make it clear what is protected by the RCU read lock.

Link: http://lkml.kernel.org/r/20180212081227.1940-1-ying.huang@intel.com
Signed-off-by: "Huang, Ying" <ying.huang@...el.com>
Reviewed-by: Jan Kara <jack@...e.cz>
Reviewed-by: Andrew Morton <akpm@...ux-foundation.org>
Cc: Mel Gorman <mgorman@...hsingularity.net>
Cc: Minchan Kim <minchan@...nel.org>
Cc: "Huang, Ying" <ying.huang@...el.com>
Cc: Johannes Weiner <hannes@...xchg.org>
Cc: Michal Hocko <mhocko@...e.com>
Signed-off-by: Andrew Morton <akpm@...ux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@...ux-foundation.org>
Signed-off-by: Sasha Levin <alexander.levin@...rosoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
---
 mm/vmscan.c |    8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -3841,7 +3841,13 @@ int zone_reclaim(struct zone *zone, gfp_
  */
 int page_evictable(struct page *page)
 {
-	return !mapping_unevictable(page_mapping(page)) && !PageMlocked(page);
+	int ret;
+
+	/* Prevent address_space of inode and swap cache from being freed */
+	rcu_read_lock();
+	ret = !mapping_unevictable(page_mapping(page)) && !PageMlocked(page);
+	rcu_read_unlock();
+	return ret;
 }
 
 #ifdef CONFIG_SHMEM