Reject multilayer certificates in check_cert.

Rejecting them in the CA config box reminded me that the main checking
code also ought to do the same thing.

crypto/openssh-certs.c

@@ -726,11 +726,40 @@ static bool opensshcert_check_cert(
 
     ptrlen signature = ptrlen_from_strbuf(ck->signature);
 
-    ca_key = opensshcert_ca_pub_key(ck, signature, NULL);
+    /*
+     * The OpenSSH certificate spec is one-layer only: it explicitly
+     * forbids using a certified key in turn as the CA.
+     *
+     * If it did not, then we'd also have to recursively verify
+     * everything up the CA chain until we reached the ultimate root,
+     * and then make sure _that_ was something we trusted. (Not to
+     * mention that there'd probably be an additional SSH_CERT_TYPE_CA
+     * or some such, and certificate options saying what kinds of
+     * certificate a CA was trusted to sign for, and ...)
+     */
+    ca_key = opensshcert_ca_pub_key(ck, make_ptrlen(NULL, 0), NULL);
     if (!ca_key) {
         put_fmt(error, "Certificate's signing key is invalid");
         goto out;
     }
+    if (ssh_key_alg(ca_key)->is_certificate) {
+        put_fmt(error, "Certificate is signed with a certified key "
+                "(forbidden by OpenSSH certificate specification)");
+        goto out;
+    }
+
+    /*
+     * Now re-instantiate the key in a way that matches the signature
+     * (i.e. so that if the key is an RSA one we get the right subtype
+     * of RSA).
+     */
+    ssh_key_free(ca_key);
+    ca_key = opensshcert_ca_pub_key(ck, signature, NULL);
+    if (!ca_key) {
+        put_fmt(error, "Certificate's signing key does not match "
+                "signature type");
+        goto out;
+    }
 
     /* Check which signature algorithm is actually in use, because
      * that might be a reason to reject the certificate (e.g. ssh-rsa

test/cryptsuite.py

@@ -2715,6 +2715,43 @@ Private-MAC: 5b1f6f4cc43eb0060d2c3e181bc0129343adba2b
                 False, b'username', 1000, '')
             self.assertEqual(result, True)
 
+            # Make a certificate on the CA key, and re-sign the main
+            # key using that, to ensure that two-level certs are rejected
+            ca_self_certificate = sign_cert_via_testcrypt(
+                make_signature_preimage(
+                    key_to_certify = ca_key.public_blob(),
+                    ca_key = ca_key,
+                    certtype = CertType.user,
+                    keyid = b'id',
+                    serial = 111,
+                    principals = [b"doesn't matter"],
+                    valid_after = 1000,
+                    valid_before = 2000), ca_key, signflags=ca_signflags)
+            import base64
+            print(base64.b64encode(ca_self_certificate))
+            self_signed_ca_key = ssh_key_new_pub(
+                alg + '-cert', ca_self_certificate)
+            print(self_signed_ca_key)
+            cert_pub = sign_cert_via_testcrypt(
+                make_signature_preimage(
+                    key_to_certify = base_key.public_blob(),
+                    ca_key = self_signed_ca_key,
+                    certtype = CertType.user,
+                    keyid = b'id',
+                    serial = 111,
+                    principals = [b'username'],
+                    valid_after = 1000,
+                    valid_before = 2000), ca_key, signflags=ca_signflags)
+            print(base64.b64encode(cert_pub))
+            certified_key = ssh_key_new_priv(alg + '-cert', cert_pub,
+                                             base_key.private_blob())
+            result, err = certified_key.check_cert(
+                False, b'username', 1500, '')
+            self.assertEqual(result, False)
+            self.assertEqual(
+                err, b'Certificate is signed with a certified key '
+                b'(forbidden by OpenSSH certificate specification)')
+
             # Now try a host certificate. We don't need to do _all_ the
             # checks over again, but at least make sure that setting
             # CertType.host leads to the certificate validating with