keys: Add a keyctl to move a key between keyrings

Add a keyctl to atomically move a link to a key from one keyring to
another.  The key must exist in "from" keyring and a flag can be given to
cause the operation to fail if there's a matching key already in the "to"
keyring.

This can be done with:

	keyctl(KEYCTL_MOVE,
	       key_serial_t key,
	       key_serial_t from_keyring,
	       key_serial_t to_keyring,
	       unsigned int flags);

The key being moved must grant Link permission and both keyrings must grant
Write permission.

flags should be 0 or KEYCTL_MOVE_EXCL, with the latter preventing
displacement of a matching key from the "to" keyring.

Signed-off-by: David Howells <dhowells@redhat.com>

security/keys/compat.c

@@ -159,6 +159,9 @@ COMPAT_SYSCALL_DEFINE5(keyctl, u32, option,
 		return keyctl_pkey_verify(compat_ptr(arg2), compat_ptr(arg3),
 					  compat_ptr(arg4), compat_ptr(arg5));
 
+	case KEYCTL_MOVE:
+		return keyctl_keyring_move(arg2, arg3, arg4, arg5);
+
 	default:
 		return -EOPNOTSUPP;
 	}