support for requiring user verified FIDO keys in sshd

This adds a "verify-required" authorized_keys flag and a corresponding
sshd_config option that tells sshd to require that FIDO keys verify the
user identity before completing the signing/authentication attempt.
Whether or not user verification was performed is already baked into the
signature made on the FIDO token, so this is just plumbing that flag
through and adding ways to require it.

feedback and ok markus@

1 files changed, 16 insertions, 2 deletions

diff --git a/usr.bin/ssh/auth2-pubkey.c b/usr.bin/ssh/auth2-pubkey.c
index c9f6c73ba50..8210f404564 100644
--- a/usr.bin/ssh/auth2-pubkey.c
+++ b/usr.bin/ssh/auth2-pubkey.c
@@ -1,4 +1,4 @@
-/* $OpenBSD: auth2-pubkey.c,v 1.99 2020/02/06 22:30:54 naddy Exp $ */
+/* $OpenBSD: auth2-pubkey.c,v 1.100 2020/08/27 01:07:09 djm Exp $ */
 /*
  * Copyright (c) 2000 Markus Friedl.  All rights reserved.
  *
@@ -94,7 +94,7 @@ userauth_pubkey(struct ssh *ssh)
 	u_char *pkblob = NULL, *sig = NULL, have_sig;
 	size_t blen, slen;
 	int r, pktype;
-	int req_presence = 0, authenticated = 0;
+	int req_presence = 0, req_verify = 0, authenticated = 0;
 	struct sshauthopt *authopts = NULL;
 	struct sshkey_sig_details *sig_details = NULL;
 
@@ -236,6 +236,20 @@ userauth_pubkey(struct ssh *ssh)
 				authenticated = 0;
 				goto done;
 			}
+			req_verify = (options.pubkey_auth_options &
+			    PUBKEYAUTH_VERIFY_REQUIRED) ||
+			    authopts->require_verify;
+			if (req_verify && (sig_details->sk_flags &
+			    SSH_SK_USER_VERIFICATION_REQD) == 0) {
+				error("public key %s signature for %s%s from "
+				    "%.128s port %d rejected: user "
+				    "verification requirement not met ", key_s,
+				    authctxt->valid ? "" : "invalid user ",
+				    authctxt->user, ssh_remote_ipaddr(ssh),
+				    ssh_remote_port(ssh));
+				authenticated = 0;
+				goto done;
+			}
 		}
 		auth2_record_key(authctxt, authenticated, key);
 	} else {