diff --git a/mm/mempolicy.c b/mm/mempolicy.c
index b1f70d6eec3..9c51f9f58ca 100644
--- a/mm/mempolicy.c
+++ b/mm/mempolicy.c
@@ -1411,7 +1411,9 @@ asmlinkage long compat_sys_get_mempolicy(int __user *policy,
 	err = sys_get_mempolicy(policy, nm, nr_bits+1, addr, flags);
 
 	if (!err && nmask) {
-		err = copy_from_user(bm, nm, alloc_size);
+		unsigned long copy_size;
+		copy_size = min_t(unsigned long, sizeof(bm), alloc_size);
+		err = copy_from_user(bm, nm, copy_size);
 		/* ensure entire bitmap is zeroed */
 		err |= clear_user(nmask, ALIGN(maxnode-1, 8) / 8);
 		err |= compat_put_bitmap(nmask, bm, nr_bits);