KEYS: Don't permit request_key() to construct a new keyring

If request_key() is used to find a keyring, only do the search part - don't
do the construction part if the keyring was not found by the search.  We
don't really want keyrings in the negative instantiated state since the
rejected/negative instantiation error value in the payload is unioned with
keyring metadata.

Now the kernel gives an error:

	request_key("keyring", "#selinux,bdekeyring", "keyring", KEY_SPEC_USER_SESSION_KEYRING) = -1 EPERM (Operation not permitted)

Signed-off-by: David Howells <dhowells@redhat.com>
CVE-2015-7872
Signed-off-by: Kevin F. Haggerty <haggertk@lineageos.org>

Change-Id: I3603fec8fab929d7636d7223901f16dc8d8026cc

security/keys/request_key.c

@@ -481,6 +481,9 @@ static struct key *construct_key_and_link(struct key_type *type,
 
 	kenter("");
 
+	if (type == &key_type_keyring)
+		return ERR_PTR(-EPERM);
+
 	ret = construct_get_dest_keyring(&dest_keyring);
 	if (ret)
 		goto error;