[tip:core/urgent] futexes: Fix infinite loop in get_futex_key() on huge page

[tip-bot for Sonny Rao <sonnyrao@us.ibm.com>]

Commit-ID:  ce2ae53b750abfaa012ce408e93da131a5b5649b
Gitweb:     http://git.kernel.org/tip/ce2ae53b750abfaa012ce408e93da131a5b5649b
Author:     Sonny Rao <sonnyrao@us.ibm.com>
AuthorDate: Fri, 10 Jul 2009 18:13:13 -0500
Committer:  Ingo Molnar <mingo@elte.hu>
CommitDate: Sat, 11 Jul 2009 12:40:44 +0200

futexes: Fix infinite loop in get_futex_key() on huge page

get_futex_key() can infinitely loop if it is called on a
virtual address that is within a huge page but not aligned to
the beginning of that page.  The call to get_user_pages_fast
will return the struct page for a sub-page within the huge page
and the check for page->mapping will always fail.

The fix is to call compound_head on the page before checking
that it's mapped.

Signed-off-by: Sonny Rao <sonnyrao@us.ibm.com>
Acked-by: Thomas Gleixner <tglx@linutronix.de>
Cc: stable@kernel.org
Cc: anton@samba.org
Cc: rajamony@us.ibm.com
Cc: speight@us.ibm.com
Cc: mstephen@us.ibm.com
Cc: grimm@us.ibm.com
Cc: mikey@ozlabs.au.ibm.com
LKML-Reference: <20090710231313.GA23572@us.ibm.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>


---
 kernel/futex.c |    1 +
 1 files changed, 1 insertions(+), 0 deletions(-)

diff --git a/kernel/futex.c b/kernel/futex.c
index 794c862..0672ff8 100644
--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -247,6 +247,7 @@ again:
 	if (err < 0)
 		return err;
 
+	page = compound_head(page);
 	lock_page(page);
 	if (!page->mapping) {
 		unlock_page(page);