mempolicy: mask off internal flags for userspace API

Flags considered internal to the mempolicy kernel code are stored as part
of the "flags" member of struct mempolicy.

Before exposing a policy type to userspace via get_mempolicy(), these
internal flags must be masked.  Flags exposed to userspace, however,
should still be returned to the user.
Signed-off-by: David Rientjes <rientjes@google.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/mempolicy.c

@@ -729,7 +729,11 @@ static long do_get_mempolicy(int *policy, nodemask_t *nmask,
 	} else {
 		*policy = pol == &default_policy ? MPOL_DEFAULT :
 						pol->mode;
-		*policy |= pol->flags;
+		/*
+		 * Internal mempolicy flags must be masked off before exposing
+		 * the policy to userspace.
+		 */
+		*policy |= (pol->flags & MPOL_MODE_FLAGS);
 	}
 
 	if (vma) {