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>

include/uapi/linux/keyctl.h

@@ -67,6 +67,7 @@
 #define KEYCTL_PKEY_SIGN		27	/* Create a public key signature */
 #define KEYCTL_PKEY_VERIFY		28	/* Verify a public key signature */
 #define KEYCTL_RESTRICT_KEYRING		29	/* Restrict keys allowed to link to a keyring */
+#define KEYCTL_MOVE			30	/* Move keys between keyrings */
 
 /* keyctl structures */
 struct keyctl_dh_params {
@@ -112,4 +113,6 @@ struct keyctl_pkey_params {
 	__u32		__spare[7];
 };
 
+#define KEYCTL_MOVE_EXCL	0x00000001 /* Do not displace from the to-keyring */
+
 #endif /*  _LINUX_KEYCTL_H */