futex: Take mmap_sem for get_user_pages in fault_in_user_writeable

get_user_pages() must be called with mmap_sem held.
Signed-off-by: Andi Kleen <ak@linux.intel.com>
Cc: stable@kernel.org
Cc: Andrew Morton <akpm@linuxfoundation.org>
Cc: Nick Piggin <npiggin@suse.de>
Cc: Darren Hart <dvhltc@us.ibm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
LKML-Reference: <20091208121942.GA21298@basil.fritz.box>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

kernel/futex.c

@@ -304,8 +304,14 @@ void put_futex_key(int fshared, union futex_key *key)
  */
 static int fault_in_user_writeable(u32 __user *uaddr)
 {
-	int ret = get_user_pages(current, current->mm, (unsigned long)uaddr,
-				 1, 1, 0, NULL, NULL);
+	struct mm_struct *mm = current->mm;
+	int ret;
+
+	down_read(&mm->mmap_sem);
+	ret = get_user_pages(current, mm, (unsigned long)uaddr,
+			     1, 1, 0, NULL, NULL);
+	up_read(&mm->mmap_sem);
+
 	return ret < 0 ? ret : 0;
 }