[Acked] [CVE-2014-3122][Lucid][Precise/lts-backport-quantal] mm: try_to_unmap_cluster() should lock_page() before mlocking

[Andy Whitcroft]

On Thu, May 08, 2014 at 10:28:02AM +0100, Luis Henriques wrote:
> From: Vlastimil Babka <vbabka at suse.cz>
> 
> A BUG_ON(!PageLocked) was triggered in mlock_vma_page() by Sasha Levin
> fuzzing with trinity.  The call site try_to_unmap_cluster() does not lock
> the pages other than its check_page parameter (which is already locked).
> 
> The BUG_ON in mlock_vma_page() is not documented and its purpose is
> somewhat unclear, but apparently it serializes against page migration,
> which could otherwise fail to transfer the PG_mlocked flag.  This would
> not be fatal, as the page would be eventually encountered again, but
> NR_MLOCK accounting would become distorted nevertheless.  This patch adds
> a comment to the BUG_ON in mlock_vma_page() and munlock_vma_page() to that
> effect.
> 
> The call site try_to_unmap_cluster() is fixed so that for page !=
> check_page, trylock_page() is attempted (to avoid possible deadlocks as we
> already have check_page locked) and mlock_vma_page() is performed only
> upon success.  If the page lock cannot be obtained, the page is left
> without PG_mlocked, which is again not a problem in the whole unevictable
> memory design.
> 
> Signed-off-by: Vlastimil Babka <vbabka at suse.cz>
> Signed-off-by: Bob Liu <bob.liu at oracle.com>
> Reported-by: Sasha Levin <sasha.levin at oracle.com>
> Cc: Wanpeng Li <liwanp at linux.vnet.ibm.com>
> Cc: Michel Lespinasse <walken at google.com>
> Cc: KOSAKI Motohiro <kosaki.motohiro at jp.fujitsu.com>
> Acked-by: Rik van Riel <riel at redhat.com>
> Cc: David Rientjes <rientjes at google.com>
> Cc: Mel Gorman <mgorman at suse.de>
> Cc: Hugh Dickins <hughd at google.com>
> Cc: Joonsoo Kim <iamjoonsoo.kim at lge.com>
> Cc: <stable at vger.kernel.org>
> Signed-off-by: Andrew Morton <akpm at linux-foundation.org>
> Signed-off-by: Linus Torvalds <torvalds at linux-foundation.org>
> (back ported from commit 57e68e9cd65b4b8eb4045a1e0d0746458502554c)
> CVE-2014-3122
> BugLink: http://bugs.launchpad.net/bugs/1316268
> Signed-off-by: Luis Henriques <luis.henriques at canonical.com>
> ---
>  mm/mlock.c |  2 ++
>  mm/rmap.c  | 14 ++++++++++++--
>  2 files changed, 14 insertions(+), 2 deletions(-)
> 
> diff --git a/mm/mlock.c b/mm/mlock.c
> index 32832726..019586f 100644
> --- a/mm/mlock.c
> +++ b/mm/mlock.c
> @@ -78,6 +78,7 @@ void __clear_page_mlock(struct page *page)
>   */
>  void mlock_vma_page(struct page *page)
>  {
> +	/* Serialize with page migration */
>  	BUG_ON(!PageLocked(page));
>  
>  	if (!TestSetPageMlocked(page)) {
> @@ -105,6 +106,7 @@ void mlock_vma_page(struct page *page)
>   */
>  void munlock_vma_page(struct page *page)
>  {
> +	/* For try_to_munlock() and to serialize with page migration */
>  	BUG_ON(!PageLocked(page));
>  
>  	if (TestClearPageMlocked(page)) {
> diff --git a/mm/rmap.c b/mm/rmap.c
> index 2b03b31..e5b4407 100644
> --- a/mm/rmap.c
> +++ b/mm/rmap.c
> @@ -1441,9 +1441,19 @@ static int try_to_unmap_cluster(unsigned long cursor, unsigned int *mapcount,
>  		BUG_ON(!page || PageAnon(page));
>  
>  		if (locked_vma) {
> -			mlock_vma_page(page);   /* no-op if already mlocked */
> -			if (page == check_page)
> +			if (page == check_page) {
> +				/* we know we have check_page locked */
> +				mlock_vma_page(page);
>  				ret = SWAP_MLOCK;
> +			} else if (trylock_page(page)) {
> +				/*
> +				 * If we can lock the page, perform mlock.
> +				 * Otherwise leave the page alone, it will be
> +				 * eventually encountered again later.
> +				 */
> +				mlock_vma_page(page);
> +				unlock_page(page);
> +			}
>  			continue;	/* don't unmap */
>  		}
>  
> -- 
> 1.9.1
> 
> -- 
> kernel-team mailing list
> kernel-team at lists.ubuntu.com
> https://lists.ubuntu.com/mailman/listinfo/kernel-team

Looks good to me, looks to be the upstream change.  Indeed, I am unable
to see how it is not a cherry-pick it is so similar :).  So:

Acked-by: Andy Whitcroft <apw at canonical.com>

-apw