PKCS#7: Make trust determination dependent on contents of trust keyring

Make the determination of the trustworthiness of a key dependent on whether
a key that can verify it is present in the supplied ring of trusted keys
rather than whether or not the verifying key has KEY_FLAG_TRUSTED set.

verify_pkcs7_signature() will return -ENOKEY if the PKCS#7 message trust
chain cannot be verified.

Signed-off-by: David Howells <dhowells@redhat.com>

kernel/module_signing.c

@@ -81,6 +81,6 @@ int mod_verify_sig(const void *mod, unsigned long *_modlen)
 	}
 
 	return verify_pkcs7_signature(mod, modlen, mod + modlen, sig_len,
-				      NULL, -ENOKEY, VERIFYING_MODULE_SIGNATURE,
+				      NULL, VERIFYING_MODULE_SIGNATURE,
 				      NULL, NULL);
 }