upstream: tweak the client hostkey preference ordering algorithm to

prefer the default ordering if the user has a key that matches the
best-preference default algorithm.

feedback and ok markus@

OpenBSD-Commit-ID: a92dd7d7520ddd95c0a16786a7519e6d0167d35f

1 files changed, 38 insertions, 3 deletions

diff --git a/sshconnect2.c b/sshconnect2.c
index 347e348c6..f64aae66a 100644
--- a/sshconnect2.c
+++ b/sshconnect2.c
@@ -1,4 +1,4 @@
-/* $OpenBSD: sshconnect2.c,v 1.325 2020/08/27 01:06:18 djm Exp $ */
+/* $OpenBSD: sshconnect2.c,v 1.326 2020/09/18 05:23:03 djm Exp $ */
 /*
  * Copyright (c) 2000 Markus Friedl.  All rights reserved.
  * Copyright (c) 2008 Damien Miller.  All rights reserved.
@@ -102,12 +102,25 @@ verify_host_key_callback(struct sshkey *hostkey, struct ssh *ssh)
         return 0;
 }
 
+/* Returns the first item from a comma-separated algorithm list */
+static char *
+first_alg(const char *algs)
+{
+        char *ret, *cp;
+
+        ret = xstrdup(algs);
+        if ((cp = strchr(ret, ',')) != NULL)
+                *cp = '\0';
+        return ret;
+}
+
 static char *
 order_hostkeyalgs(char *host, struct sockaddr *hostaddr, u_short port)
 {
-        char *oavail, *avail, *first, *last, *alg, *hostname, *ret;
+        char *oavail = NULL, *avail = NULL, *first = NULL, *last = NULL;
+        char *alg = NULL, *hostname = NULL, *ret = NULL, *best = NULL;
         size_t maxlen;
-        struct hostkeys *hostkeys;
+        struct hostkeys *hostkeys = NULL;
         int ktype;
         u_int i;
 
@@ -119,6 +132,26 @@ order_hostkeyalgs(char *host, struct sockaddr *hostaddr, u_short port)
         for (i = 0; i < options.num_system_hostfiles; i++)
                 load_hostkeys(hostkeys, hostname, options.system_hostfiles[i]);
 
+        /*
+         * If a plain public key exists that matches the type of the best
+         * preference HostkeyAlgorithms, then use the whole list as is.
+         * Note that we ignore whether the best preference algorithm is a
+         * certificate type, as sshconnect.c will downgrade certs to
+         * plain keys if necessary.
+         */
+        best = first_alg(options.hostkeyalgorithms);
+        if (lookup_key_in_hostkeys_by_type(hostkeys,
+            sshkey_type_plain(sshkey_type_from_name(best)), NULL)) {
+                debug3("%s: have matching best-preference key type %s, "
+                    "using HostkeyAlgorithms verbatim", __func__, best);
+                ret = xstrdup(options.hostkeyalgorithms);
+                goto out;
+        }
+
+        /*
+         * Otherwise, prefer the host key algorithms that match known keys
+         * while keeping the ordering of HostkeyAlgorithms as much as possible.
+         */
         oavail = avail = xstrdup(options.hostkeyalgorithms);
         maxlen = strlen(avail) + 1;
         first = xmalloc(maxlen);
@@ -159,6 +192,8 @@ order_hostkeyalgs(char *host, struct sockaddr *hostaddr, u_short port)
         if (*first != '\0')
                 debug3("%s: prefer hostkeyalgs: %s", __func__, first);
 
+ out:
+        free(best);
         free(first);
         free(last);
         free(hostname);