upstream: When using HostkeyAlgorithms to merely append or remove

algorithms from the default set (i.e. HostkeyAlgorithms=+/-...), retain the
default behaviour of preferring those algorithms that have existing keys in
known_hosts; ok markus

OpenBSD-Commit-ID: 040e7fcc38ea00146b5d224ce31ce7a1795ee6ed

1 files changed, 26 insertions, 15 deletions

diff --git a/sshconnect2.c b/sshconnect2.c
index 4c4a61ba0..af00fb30c 100644
--- a/sshconnect2.c
+++ b/sshconnect2.c
@@ -1,4 +1,4 @@
-/* $OpenBSD: sshconnect2.c,v 1.319 2020/02/06 22:30:54 naddy Exp $ */
+/* $OpenBSD: sshconnect2.c,v 1.320 2020/02/06 22:48:23 djm Exp $ */
 /*
  * Copyright (c) 2000 Markus Friedl.  All rights reserved.
  * Copyright (c) 2008 Damien Miller.  All rights reserved.
@@ -119,7 +119,7 @@ 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]);
 
-        oavail = avail = xstrdup(kex_default_pk_alg());
+        oavail = avail = xstrdup(options.hostkeyalgorithms);
         maxlen = strlen(avail) + 1;
         first = xmalloc(maxlen);
         last = xmalloc(maxlen);
@@ -161,11 +161,28 @@ ssh_kex2(struct ssh *ssh, char *host, struct sockaddr *hostaddr, u_short port)
 {
         char *myproposal[PROPOSAL_MAX] = { KEX_CLIENT };
         char *s, *all_key;
-        int r;
+        int r, use_known_hosts_order = 0;
 
         xxx_host = host;
         xxx_hostaddr = hostaddr;
 
+        /*
+         * If the user has not specified HostkeyAlgorithms, or has only
+         * appended or removed algorithms from that list then prefer algorithms
+         * that are in the list that are supported by known_hosts keys.
+         */
+        if (options.hostkeyalgorithms == NULL ||
+            options.hostkeyalgorithms[0] == '-' ||
+            options.hostkeyalgorithms[0] == '+')
+                use_known_hosts_order = 1;
+
+        /* Expand or fill in HostkeyAlgorithms */
+        all_key = sshkey_alg_list(0, 0, 1, ',');
+        if (kex_assemble_names(&options.hostkeyalgorithms,
+            kex_default_pk_alg(), all_key) != 0)
+                fatal("%s: kex_assemble_namelist", __func__);
+        free(all_key);
+
         if ((s = kex_names_cat(options.kex_algorithms, "ext-info-c")) == NULL)
                 fatal("%s: kex_names_cat", __func__);
         myproposal[PROPOSAL_KEX_ALGS] = compat_kex_proposal(s);
@@ -178,21 +195,15 @@ ssh_kex2(struct ssh *ssh, char *host, struct sockaddr *hostaddr, u_short port)
             (char *)compression_alg_list(options.compression);
         myproposal[PROPOSAL_MAC_ALGS_CTOS] =
             myproposal[PROPOSAL_MAC_ALGS_STOC] = options.macs;
-        if (options.hostkeyalgorithms != NULL) {
-                all_key = sshkey_alg_list(0, 0, 1, ',');
-                if (kex_assemble_names(&options.hostkeyalgorithms,
-                    kex_default_pk_alg(), all_key) != 0)
-                        fatal("%s: kex_assemble_namelist", __func__);
-                free(all_key);
-                myproposal[PROPOSAL_SERVER_HOST_KEY_ALGS] =
-                    compat_pkalg_proposal(options.hostkeyalgorithms);
-        } else {
-                /* Enforce default */
-                options.hostkeyalgorithms = xstrdup(kex_default_pk_alg());
-                /* Prefer algorithms that we already have keys for */
+        if (use_known_hosts_order) {
+                /* Query known_hosts and prefer algorithms that appear there */
                 myproposal[PROPOSAL_SERVER_HOST_KEY_ALGS] =
                     compat_pkalg_proposal(
                     order_hostkeyalgs(host, hostaddr, port));
+        } else {
+                /* Use specified HostkeyAlgorithms exactly */
+                myproposal[PROPOSAL_SERVER_HOST_KEY_ALGS] =
+                    compat_pkalg_proposal(options.hostkeyalgorithms);
         }
 
         if (options.rekey_limit || options.rekey_interval)