KEYS: prevent creating a different user's keyrings

It was possible for an unprivileged user to create the user and user
session keyrings for another user.  For example:

    sudo -u '#3000' sh -c 'keyctl add keyring _uid.4000 "" @u
                           keyctl add keyring _uid_ses.4000 "" @u
                           sleep 15' &
    sleep 1
    sudo -u '#4000' keyctl describe @u
    sudo -u '#4000' keyctl describe @us

This is problematic because these "fake" keyrings won't have the right
permissions.  In particular, the user who created them first will own
them and will have full access to them via the possessor permissions,
which can be used to compromise the security of a user's keys:

    -4: alswrv-----v------------  3000     0 keyring: _uid.4000
    -5: alswrv-----v------------  3000     0 keyring: _uid_ses.4000

Fix it by marking user and user session keyrings with a flag
KEY_FLAG_UID_KEYRING.  Then, when searching for a user or user session
keyring by name, skip all keyrings that don't have the flag set.

Fixes: 69664cf1 ("keys: don't generate user and user session keyrings unless they're accessed")
Cc: <stable@vger.kernel.org>	[v2.6.26+]
Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: David Howells <dhowells@redhat.com>

include/linux/key.h

@@ -187,6 +187,7 @@ struct key {
 #define KEY_FLAG_BUILTIN	8	/* set if key is built in to the kernel */
 #define KEY_FLAG_ROOT_CAN_INVAL	9	/* set if key can be invalidated by root without permission */
 #define KEY_FLAG_KEEP		10	/* set if key should not be removed */
+#define KEY_FLAG_UID_KEYRING	11	/* set if key is a user or user session keyring */
 
 	/* the key type and key description string
 	 * - the desc is used to match a key against search criteria
@@ -243,6 +244,7 @@ extern struct key *key_alloc(struct key_type *type,
 #define KEY_ALLOC_NOT_IN_QUOTA		0x0002	/* not in quota */
 #define KEY_ALLOC_BUILT_IN		0x0004	/* Key is built into kernel */
 #define KEY_ALLOC_BYPASS_RESTRICTION	0x0008	/* Override the check on restricted keyrings */
+#define KEY_ALLOC_UID_KEYRING		0x0010	/* allocating a user or user session keyring */
 
 extern void key_revoke(struct key *key);
 extern void key_invalidate(struct key *key);

security/keys/internal.h

@@ -141,7 +141,7 @@ extern key_ref_t keyring_search_aux(key_ref_t keyring_ref,
 extern key_ref_t search_my_process_keyrings(struct keyring_search_context *ctx);
 extern key_ref_t search_process_keyrings(struct keyring_search_context *ctx);
 
-extern struct key *find_keyring_by_name(const char *name, bool skip_perm_check);
+extern struct key *find_keyring_by_name(const char *name, bool uid_keyring);
 
 extern int install_user_keyrings(void);
 extern int install_thread_keyring_to_cred(struct cred *);

security/keys/key.c

@@ -302,6 +302,8 @@ struct key *key_alloc(struct key_type *type, const char *desc,
 		key->flags |= 1 << KEY_FLAG_IN_QUOTA;
 	if (flags & KEY_ALLOC_BUILT_IN)
 		key->flags |= 1 << KEY_FLAG_BUILTIN;
+	if (flags & KEY_ALLOC_UID_KEYRING)
+		key->flags |= 1 << KEY_FLAG_UID_KEYRING;
 
 #ifdef KEY_DEBUGGING
 	key->magic = KEY_DEBUG_MAGIC;

security/keys/keyring.c

@@ -1097,15 +1097,15 @@ key_ref_t find_key_to_update(key_ref_t keyring_ref,
 /*
  * Find a keyring with the specified name.
  *
- * All named keyrings in the current user namespace are searched, provided they
- * grant Search permission directly to the caller (unless this check is
- * skipped).  Keyrings whose usage points have reached zero or who have been
- * revoked are skipped.
+ * Only keyrings that have nonzero refcount, are not revoked, and are owned by a
+ * user in the current user namespace are considered.  If @uid_keyring is %true,
+ * the keyring additionally must have been allocated as a user or user session
+ * keyring; otherwise, it must grant Search permission directly to the caller.
  *
  * Returns a pointer to the keyring with the keyring's refcount having being
  * incremented on success.  -ENOKEY is returned if a key could not be found.
  */
-struct key *find_keyring_by_name(const char *name, bool skip_perm_check)
+struct key *find_keyring_by_name(const char *name, bool uid_keyring)
 {
 	struct key *keyring;
 	int bucket;
@@ -1133,10 +1133,15 @@ struct key *find_keyring_by_name(const char *name, bool skip_perm_check)
 			if (strcmp(keyring->description, name) != 0)
 				continue;
 
-			if (!skip_perm_check &&
-			    key_permission(make_key_ref(keyring, 0),
+			if (uid_keyring) {
+				if (!test_bit(KEY_FLAG_UID_KEYRING,
+					      &keyring->flags))
+					continue;
+			} else {
+				if (key_permission(make_key_ref(keyring, 0),
 						   KEY_NEED_SEARCH) < 0)
 					continue;
+			}
 
 			/* we've got a match but we might end up racing with
 			 * key_cleanup() if the keyring is currently 'dead'

security/keys/process_keys.c

@@ -77,6 +77,7 @@ int install_user_keyrings(void)
 		if (IS_ERR(uid_keyring)) {
 			uid_keyring = keyring_alloc(buf, user->uid, INVALID_GID,
 						    cred, user_keyring_perm,
+						    KEY_ALLOC_UID_KEYRING |
 							KEY_ALLOC_IN_QUOTA,
 						    NULL, NULL);
 			if (IS_ERR(uid_keyring)) {
@@ -94,6 +95,7 @@ int install_user_keyrings(void)
 			session_keyring =
 				keyring_alloc(buf, user->uid, INVALID_GID,
 					      cred, user_keyring_perm,
+					      KEY_ALLOC_UID_KEYRING |
 						  KEY_ALLOC_IN_QUOTA,
 					      NULL, NULL);
 			if (IS_ERR(session_keyring)) {