upstream: sshd side of hostbound public key auth

This is identical to the standard "publickey" method, but it also includes
the initial server hostkey in the message signed by the client.

feedback / ok markus@

OpenBSD-Commit-ID: 7ea01bb7238a560c1bfb426fda0c10a8aac07862

1 files changed, 21 insertions, 5 deletions

diff --git a/monitor.c b/monitor.c
index 74c803e1..a6429852 100644
--- a/monitor.c
+++ b/monitor.c
@@ -1,4 +1,4 @@
-/* $OpenBSD: monitor.c,v 1.228 2021/08/11 05:20:17 djm Exp $ */
+/* $OpenBSD: monitor.c,v 1.229 2021/12/19 22:12:30 djm Exp $ */
 /*
  * Copyright 2002 Niels Provos <provos@citi.umich.edu>
  * Copyright 2002 Markus Friedl <markus@openbsd.org>
@@ -1246,11 +1246,12 @@ static int
 monitor_valid_userblob(struct ssh *ssh, const u_char *data, u_int datalen)
 {
 	struct sshbuf *b;
+	struct sshkey *hostkey = NULL;
 	const u_char *p;
 	char *userstyle, *cp;
 	size_t len;
 	u_char type;
-	int r, fail = 0;
+	int hostbound = 0, r, fail = 0;
 
 	if ((b = sshbuf_from(data, datalen)) == NULL)
 		fatal_f("sshbuf_from");
@@ -1291,19 +1292,34 @@ monitor_valid_userblob(struct ssh *ssh, const u_char *data, u_int datalen)
 	if ((r = sshbuf_skip_string(b)) != 0 ||	/* service */
 	    (r = sshbuf_get_cstring(b, &cp, NULL)) != 0)
 		fatal_fr(r, "parse method");
-	if (strcmp("publickey", cp) != 0)
-		fail++;
+	if (strcmp("publickey", cp) != 0) {
+		if (strcmp("publickey-hostbound-v00@openssh.com", cp) == 0)
+			hostbound = 1;
+		else
+			fail++;
+	}
 	free(cp);
 	if ((r = sshbuf_get_u8(b, &type)) != 0)
 		fatal_fr(r, "parse pktype");
 	if (type == 0)
 		fail++;
 	if ((r = sshbuf_skip_string(b)) != 0 ||	/* pkalg */
-	    (r = sshbuf_skip_string(b)) != 0)	/* pkblob */
+	    (r = sshbuf_skip_string(b)) != 0 ||	/* pkblob */
+	    (hostbound && (r = sshkey_froms(b, &hostkey)) != 0))
 		fatal_fr(r, "parse pk");
 	if (sshbuf_len(b) != 0)
 		fail++;
 	sshbuf_free(b);
+	if (hostkey != NULL) {
+		/*
+		 * Ensure this is actually one of our hostkeys; unfortunately
+		 * can't check ssh->kex->initial_hostkey directly at this point
+		 * as packet state has not yet been exported to monitor.
+		 */
+		if (get_hostkey_index(hostkey, 1, ssh) == -1)
+			fatal_f("hostbound hostkey does not match");
+		sshkey_free(hostkey);
+	}
 	return (fail == 0);
 }