keys: Namespace keyring names

Keyring names are held in a single global list that any process can pick
from by means of keyctl_join_session_keyring (provided the keyring grants
Search permission).  This isn't very container friendly, however.

Make the following changes:

 (1) Make default session, process and thread keyring names begin with a
     '.' instead of '_'.

 (2) Keyrings whose names begin with a '.' aren't added to the list.  Such
     keyrings are system specials.

 (3) Replace the global list with per-user_namespace lists.  A keyring adds
     its name to the list for the user_namespace that it is currently in.

 (4) When a user_namespace is deleted, it just removes itself from the
     keyring name list.

The global keyring_name_lock is retained for accessing the name lists.
This allows (4) to work.

This can be tested by:

	# keyctl newring foo @s
	995906392
	# unshare -U
	$ keyctl show
	...
	 995906392 --alswrv  65534 65534   \_ keyring: foo
	...
	$ keyctl session foo
	Joined session keyring: 935622349

As can be seen, a new session keyring was created.

The capability bit KEYCTL_CAPS1_NS_KEYRING_NAME is set if the kernel is
employing this feature.

Signed-off-by: David Howells <dhowells@redhat.com>
cc: Eric W. Biederman <ebiederm@xmission.com>

include/uapi/linux/keyctl.h

@@ -128,5 +128,6 @@ struct keyctl_pkey_params {
 #define KEYCTL_CAPS0_INVALIDATE		0x20 /* KEYCTL_INVALIDATE supported */
 #define KEYCTL_CAPS0_RESTRICT_KEYRING	0x40 /* KEYCTL_RESTRICT_KEYRING supported */
 #define KEYCTL_CAPS0_MOVE		0x80 /* KEYCTL_MOVE supported */
+#define KEYCTL_CAPS1_NS_KEYRING_NAME	0x01 /* Keyring names are per-user_namespace */
 
 #endif /*  _LINUX_KEYCTL_H */