KEYS: Alter use of key instantiation link-to-keyring argument

Alter the use of the key instantiation and negation functions' link-to-keyring
arguments.  Currently this specifies a keyring in the target process to link
the key into, creating the keyring if it doesn't exist.  This, however, can be
a problem for copy-on-write credentials as it means that the instantiating
process can alter the credentials of the requesting process.

This patch alters the behaviour such that:

 (1) If keyctl_instantiate_key() or keyctl_negate_key() are given a specific
     keyring by ID (ringid >= 0), then that keyring will be used.

 (2) If keyctl_instantiate_key() or keyctl_negate_key() are given one of the
     special constants that refer to the requesting process's keyrings
     (KEY_SPEC_*_KEYRING, all <= 0), then:

     (a) If sys_request_key() was given a keyring to use (destringid) then the
     	 key will be attached to that keyring.

     (b) If sys_request_key() was given a NULL keyring, then the key being
     	 instantiated will be attached to the default keyring as set by
     	 keyctl_set_reqkey_keyring().

 (3) No extra link will be made.

Decision point (1) follows current behaviour, and allows those instantiators
who've searched for a specifically named keyring in the requestor's keyring so
as to partition the keys by type to still have their named keyrings.

Decision point (2) allows the requestor to make sure that the key or keys that
get produced by request_key() go where they want, whilst allowing the
instantiator to request that the key is retained.  This is mainly useful for
situations where the instantiator makes a secondary request, the key for which
should be retained by the initial requestor:

	+-----------+        +--------------+        +--------------+
	|           |        |              |        |              |
	| Requestor |------->| Instantiator |------->| Instantiator |
	|           |        |              |        |              |
	+-----------+        +--------------+        +--------------+
	           request_key()           request_key()

This might be useful, for example, in Kerberos, where the requestor requests a
ticket, and then the ticket instantiator requests the TGT, which someone else
then has to go and fetch.  The TGT, however, should be retained in the
keyrings of the requestor, not the first instantiator.  To make this explict
an extra special keyring constant is also added.

Signed-off-by: David Howells <dhowells@redhat.com>
Reviewed-by: James Morris <jmorris@namei.org>
Signed-off-by: James Morris <jmorris@namei.org>

1 files changed, 8 insertions, 8 deletions

diff --git a/include/linux/key.h b/include/linux/key.h
index 1b70e35a71e3..df709e1af3cd 100644
--- a/include/linux/key.h
+++ b/include/linux/key.h
@@ -287,11 +287,11 @@ extern void key_fsuid_changed(struct task_struct *tsk);
 extern void key_fsgid_changed(struct task_struct *tsk);
 extern void key_init(void);
 
-#define __install_session_keyring(tsk, keyring)			\
-({								\
-	struct key *old_session = tsk->signal->session_keyring;	\
-	tsk->signal->session_keyring = keyring;			\
-	old_session;						\
+#define __install_session_keyring(keyring)				\
+({									\
+	struct key *old_session = current->signal->session_keyring;	\
+	current->signal->session_keyring = keyring;			\
+	old_session;							\
 })
 
 #else /* CONFIG_KEYS */
@@ -302,11 +302,11 @@ extern void key_init(void);
 #define key_revoke(k)			do { } while(0)
 #define key_put(k)			do { } while(0)
 #define key_ref_put(k)			do { } while(0)
-#define make_key_ref(k, p)			({ NULL; })
-#define key_ref_to_ptr(k)		({ NULL; })
+#define make_key_ref(k, p)		NULL
+#define key_ref_to_ptr(k)		NULL
 #define is_key_possessed(k)		0
 #define switch_uid_keyring(u)		do { } while(0)
-#define __install_session_keyring(t, k)	({ NULL; })
+#define __install_session_keyring(k)	({ NULL; })
 #define copy_keys(f,t)			0
 #define copy_thread_group_keys(t)	0
 #define exit_keys(t)			do { } while(0)