KEYS: Allow special keyrings to be cleared

The kernel contains some special internal keyrings, for instance the DNS
resolver keyring :

2a93faf1 I----- 1 perm 1f030000 0 0 keyring .dns_resolver: empty

It would occasionally be useful to allow the contents of such keyrings to be
flushed by root (cache invalidation).

Allow a flag to be set on a keyring to mark that someone possessing the
sysadmin capability can clear the keyring, even without normal write access to
the keyring.

Set this flag on the special keyrings created by the DNS resolver, the NFS
identity mapper and the CIFS identity mapper.

Signed-off-by: David Howells <[e-mail]>
Acked-by: Jeff Layton <[e-mail]>
Acked-by: Steve Dickson <[e-mail]>
Signed-off-by: James Morris <[e-mail]>

Documentation/networking/dns_resolver.txt

···
      If _expiry is non-NULL, the expiry time (TTL) of the result will be
      returned also.
 
+The kernel maintains an internal keyring in which it caches looked up keys.
+This can be cleared by any process that has the CAP_SYS_ADMIN capability by
+the use of KEYCTL_KEYRING_CLEAR on the keyring ID.
+
 
 ===============================
 READING DNS KEYS FROM USERSPACE

Documentation/security/keys.txt

···
      process must have write permission on the keyring, and it must be a
      keyring (or else error ENOTDIR will result).
 
+     This function can also be used to clear special kernel keyrings if they
+     are appropriately marked if the user has CAP_SYS_ADMIN capability.  The
+     DNS resolver cache keyring is an example of this.
+
 
  (*) Link a key into a keyring:
 

fs/cifs/cifsacl.c

···
 
 	/* instruct request_key() to use this special keyring as a cache for
 	 * the results it looks up */
+	set_bit(KEY_FLAG_ROOT_CAN_CLEAR, &keyring->flags);
 	cred->thread_keyring = keyring;
 	cred->jit_keyring = KEY_REQKEY_DEFL_THREAD_KEYRING;
 	root_cred = cred;

fs/nfs/idmap.c

···
 	if (ret < 0)
 		goto failed_put_key;
 
+	set_bit(KEY_FLAG_ROOT_CAN_CLEAR, &keyring->flags);
 	cred->thread_keyring = keyring;
 	cred->jit_keyring = KEY_REQKEY_DEFL_THREAD_KEYRING;
 	id_resolver_cache = cred;

include/linux/key.h

···
 #define KEY_FLAG_IN_QUOTA	3	/* set if key consumes quota */
 #define KEY_FLAG_USER_CONSTRUCT	4	/* set if key is being constructed in userspace */
 #define KEY_FLAG_NEGATIVE	5	/* set if key is negative */
+#define KEY_FLAG_ROOT_CAN_CLEAR	6	/* set if key can be cleared by root without permission */
 
 	/* the description string
 	 * - this is used to match a key against search criteria

net/dns_resolver/dns_key.c

···
 
 	/* instruct request_key() to use this special keyring as a cache for
 	 * the results it looks up */
+	set_bit(KEY_FLAG_ROOT_CAN_CLEAR, &keyring->flags);
 	cred->thread_keyring = keyring;
 	cred->jit_keyring = KEY_REQKEY_DEFL_THREAD_KEYRING;
 	dns_resolver_cache = cred;

security/keys/keyctl.c

···
 	keyring_ref = lookup_user_key(ringid, KEY_LOOKUP_CREATE, KEY_WRITE);
 	if (IS_ERR(keyring_ref)) {
 		ret = PTR_ERR(keyring_ref);
+
+		/* Root is permitted to invalidate certain special keyrings */
+		if (capable(CAP_SYS_ADMIN)) {
+			keyring_ref = lookup_user_key(ringid, 0, 0);
+			if (IS_ERR(keyring_ref))
+				goto error;
+			if (test_bit(KEY_FLAG_ROOT_CAN_CLEAR,
+				     &key_ref_to_ptr(keyring_ref)->flags))
+				goto clear;
+			goto error_put;
+		}
+
 		goto error;
 	}
 
+clear:
 	ret = keyring_clear(key_ref_to_ptr(keyring_ref));
-
+error_put:
 	key_ref_put(keyring_ref);
 error:
 	return ret;