[PATCH] futex: Take mmap_sem for get_user_pages in fault_in_user_writeable

[Andi Kleen <andi@firstfloor.org>]

futex: Take mmap_sem for get_user_pages in fault_in_user_writeable

gup() requires mmap_sem, so take it before running it. The
recent fault_in_user_writeable() didn't do that.

I didn't find a good futex test case, so untested.

Likely a stable candidate.

Cc: npiggin@suse.de
Cc: tglx@linutronix.de
Cc: dvhltc@us.ibm.com
Signed-off-by: Andi Kleen <ak@linux.intel.com>

---
 kernel/futex.c |    7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

Index: linux-2.6.32-ak/kernel/futex.c
===================================================================
--- linux-2.6.32-ak.orig/kernel/futex.c
+++ linux-2.6.32-ak/kernel/futex.c
@@ -304,8 +304,13 @@ void put_futex_key(int fshared, union fu
  */
 static int fault_in_user_writeable(u32 __user *uaddr)
 {
-	int ret = get_user_pages(current, current->mm, (unsigned long)uaddr,
+	int ret;
+	struct mm_struct *mm = current->mm;
+
+	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;
 }