KEYS: prevent keys from being removed from specified keyrings

Userspace should not be allowed to remove keys from certain keyrings
(eg. blacklist), though the keys themselves can expire.

This patch defines a new key flag named KEY_FLAG_KEEP to prevent
userspace from being able to unlink, revoke, invalidate or timed
out a key on a keyring.  When this flag is set on the keyring, all
keys subsequently added are flagged.

In addition, when this flag is set, the keyring itself can not be
cleared.
Signed-off-by: Mimi Zohar <zohar@linux.vnet.ibm.com>
Cc: David Howells <dhowells@redhat.com>

include/linux/key.h

@@ -177,6 +177,7 @@ struct key {
 #define KEY_FLAG_TRUSTED_ONLY	9	/* set if keyring only accepts links to trusted keys */
 #define KEY_FLAG_BUILTIN	10	/* set if key is builtin */
 #define KEY_FLAG_ROOT_CAN_INVAL	11	/* set if key can be invalidated by root without permission */
+#define KEY_FLAG_KEEP		12	/* set if key should not be removed */
 
 	/* the key type and key description string
 	 * - the desc is used to match a key against search criteria

security/keys/key.c

@@ -429,8 +429,12 @@ static int __key_instantiate_and_link(struct key *key,
 				awaken = 1;
 
 			/* and link it into the destination keyring */
-			if (keyring)
+			if (keyring) {
+				if (test_bit(KEY_FLAG_KEEP, &keyring->flags))
+					set_bit(KEY_FLAG_KEEP, &key->flags);
+
 				__key_link(key, _edit);
+			}
 
 			/* disable the authorisation key */
 			if (authkey)

security/keys/keyctl.c

@@ -358,11 +358,14 @@ long keyctl_update_key(key_serial_t id,
  * and any links to the key will be automatically garbage collected after a
  * certain amount of time (/proc/sys/kernel/keys/gc_delay).
  *
+ * Keys with KEY_FLAG_KEEP set should not be revoked.
+ *
  * If successful, 0 is returned.
  */
 long keyctl_revoke_key(key_serial_t id)
 {
 	key_ref_t key_ref;
+	struct key *key;
 	long ret;
 
 	key_ref = lookup_user_key(id, 0, KEY_NEED_WRITE);
@@ -377,8 +380,13 @@ long keyctl_revoke_key(key_serial_t id)
 		}
 	}
 
-	key_revoke(key_ref_to_ptr(key_ref));
+	key = key_ref_to_ptr(key_ref);
+	if (test_bit(KEY_FLAG_KEEP, &key->flags))
+		return -EPERM;
+	else {
+		key_revoke(key);
 		ret = 0;
+	}
 
 	key_ref_put(key_ref);
 error:
@@ -392,11 +400,14 @@ long keyctl_revoke_key(key_serial_t id)
  * The key and any links to the key will be automatically garbage collected
  * immediately.
  *
+ * Keys with KEY_FLAG_KEEP set should not be invalidated.
+ *
  * If successful, 0 is returned.
  */
 long keyctl_invalidate_key(key_serial_t id)
 {
 	key_ref_t key_ref;
+	struct key *key;
 	long ret;
 
 	kenter("%d", id);
@@ -420,8 +431,13 @@ long keyctl_invalidate_key(key_serial_t id)
 	}
 
 invalidate:
-	key_invalidate(key_ref_to_ptr(key_ref));
+	key = key_ref_to_ptr(key_ref);
+	if (test_bit(KEY_FLAG_KEEP, &key->flags))
+		ret = -EPERM;
+	else {
+		key_invalidate(key);
 		ret = 0;
+	}
 error_put:
 	key_ref_put(key_ref);
 error:
@@ -433,12 +449,13 @@ long keyctl_invalidate_key(key_serial_t id)
  * Clear the specified keyring, creating an empty process keyring if one of the
  * special keyring IDs is used.
  *
- * The keyring must grant the caller Write permission for this to work.  If
- * successful, 0 will be returned.
+ * The keyring must grant the caller Write permission and not have
+ * KEY_FLAG_KEEP set for this to work.  If successful, 0 will be returned.
  */
 long keyctl_keyring_clear(key_serial_t ringid)
 {
 	key_ref_t keyring_ref;
+	struct key *keyring;
 	long ret;
 
 	keyring_ref = lookup_user_key(ringid, KEY_LOOKUP_CREATE, KEY_NEED_WRITE);
@@ -460,7 +477,11 @@ long keyctl_keyring_clear(key_serial_t ringid)
 	}
 
 clear:
-	ret = keyring_clear(key_ref_to_ptr(keyring_ref));
+	keyring = key_ref_to_ptr(keyring_ref);
+	if (test_bit(KEY_FLAG_KEEP, &keyring->flags))
+		ret = -EPERM;
+	else
+		ret = keyring_clear(keyring);
 error_put:
 	key_ref_put(keyring_ref);
 error:
@@ -511,11 +532,14 @@ long keyctl_keyring_link(key_serial_t id, key_serial_t ringid)
  * itself need not grant the caller anything.  If the last link to a key is
  * removed then that key will be scheduled for destruction.
  *
+ * Keys or keyrings with KEY_FLAG_KEEP set should not be unlinked.
+ *
  * If successful, 0 will be returned.
  */
 long keyctl_keyring_unlink(key_serial_t id, key_serial_t ringid)
 {
 	key_ref_t keyring_ref, key_ref;
+	struct key *keyring, *key;
 	long ret;
 
 	keyring_ref = lookup_user_key(ringid, 0, KEY_NEED_WRITE);
@@ -530,7 +554,13 @@ long keyctl_keyring_unlink(key_serial_t id, key_serial_t ringid)
 		goto error2;
 	}
 
-	ret = key_unlink(key_ref_to_ptr(keyring_ref), key_ref_to_ptr(key_ref));
+	keyring = key_ref_to_ptr(keyring_ref);
+	key = key_ref_to_ptr(key_ref);
+	if (test_bit(KEY_FLAG_KEEP, &keyring->flags) &&
+	    test_bit(KEY_FLAG_KEEP, &key->flags))
+		ret = -EPERM;
+	else
+		ret = key_unlink(keyring, key);
 
 	key_ref_put(key_ref);
 error2:
@@ -1289,6 +1319,8 @@ long keyctl_set_reqkey_keyring(int reqkey_defl)
  * the current time.  The key and any links to the key will be automatically
  * garbage collected after the timeout expires.
  *
+ * Keys with KEY_FLAG_KEEP set should not be timed out.
+ *
  * If successful, 0 is returned.
  */
 long keyctl_set_timeout(key_serial_t id, unsigned timeout)
@@ -1320,10 +1352,14 @@ long keyctl_set_timeout(key_serial_t id, unsigned timeout)
 
 okay:
 	key = key_ref_to_ptr(key_ref);
+	if (test_bit(KEY_FLAG_KEEP, &key->flags))
+		ret = -EPERM;
+	else {
 		key_set_timeout(key, timeout);
+		ret = 0;
+	}
 	key_put(key);
 
-	ret = 0;
 error:
 	return ret;
 }