GSSAPI key exchange support

This patch has been rejected upstream: "None of the OpenSSH developers are
in favour of adding this, and this situation has not changed for several
years.  This is not a slight on Simon's patch, which is of fine quality, but
just that a) we don't trust GSSAPI implementations that much and b) we don't
like adding new KEX since they are pre-auth attack surface.  This one is
particularly scary, since it requires hooks out to typically root-owned
system resources."

However, quite a lot of people rely on this in Debian, and it's better to
have it merged into the main openssh package rather than having separate
-krb5 packages (as we used to have).  It seems to have a generally good
security history.

Bug: https://bugzilla.mindrot.org/show_bug.cgi?id=1242
Last-Updated: 2018-10-20

Patch-Name: gssapi.patch

1 files changed, 131 insertions, 2 deletions

diff --git a/sshconnect2.c b/sshconnect2.c
index 1675f3935..8c872a4fb 100644
--- a/sshconnect2.c
+++ b/sshconnect2.c
@@ -162,6 +162,11 @@ ssh_kex2(char *host, struct sockaddr *hostaddr, u_short port)
         struct kex *kex;
         int r;
 
+#ifdef GSSAPI
+        char *orig = NULL, *gss = NULL;
+        char *gss_host = NULL;
+#endif
+
         xxx_host = host;
         xxx_hostaddr = hostaddr;
 
@@ -194,6 +199,35 @@ ssh_kex2(char *host, struct sockaddr *hostaddr, u_short port)
                     order_hostkeyalgs(host, hostaddr, port));
         }
 
+#ifdef GSSAPI
+        if (options.gss_keyex) {
+                /* Add the GSSAPI mechanisms currently supported on this
+                 * client to the key exchange algorithm proposal */
+                orig = myproposal[PROPOSAL_KEX_ALGS];
+
+                if (options.gss_server_identity)
+                        gss_host = xstrdup(options.gss_server_identity);
+                else if (options.gss_trust_dns)
+                        gss_host = remote_hostname(active_state);
+                else
+                        gss_host = xstrdup(host);
+
+                gss = ssh_gssapi_client_mechanisms(gss_host,
+                    options.gss_client_identity);
+                if (gss) {
+                        debug("Offering GSSAPI proposal: %s", gss);
+                        xasprintf(&myproposal[PROPOSAL_KEX_ALGS],
+                            "%s,%s", gss, orig);
+
+                        /* If we've got GSSAPI algorithms, then we also
+                         * support the 'null' hostkey, as a last resort */
+                        orig = myproposal[PROPOSAL_SERVER_HOST_KEY_ALGS];
+                        xasprintf(&myproposal[PROPOSAL_SERVER_HOST_KEY_ALGS],
+                            "%s,null", orig);
+                }
+        }
+#endif
+
         if (options.rekey_limit || options.rekey_interval)
                 packet_set_rekey_limits(options.rekey_limit,
                     options.rekey_interval);
@@ -215,15 +249,41 @@ ssh_kex2(char *host, struct sockaddr *hostaddr, u_short port)
 # endif
 #endif
         kex->kex[KEX_C25519_SHA256] = kexc25519_client;
+#ifdef GSSAPI
+        if (options.gss_keyex) {
+                kex->kex[KEX_GSS_GRP1_SHA1] = kexgss_client;
+                kex->kex[KEX_GSS_GRP14_SHA1] = kexgss_client;
+                kex->kex[KEX_GSS_GEX_SHA1] = kexgss_client;
+        }
+#endif
         kex->client_version_string=client_version_string;
         kex->server_version_string=server_version_string;
         kex->verify_host_key=&verify_host_key_callback;
 
+#ifdef GSSAPI
+        if (options.gss_keyex) {
+                kex->gss_deleg_creds = options.gss_deleg_creds;
+                kex->gss_trust_dns = options.gss_trust_dns;
+                kex->gss_client = options.gss_client_identity;
+                kex->gss_host = gss_host;
+        }
+#endif
+
         ssh_dispatch_run_fatal(active_state, DISPATCH_BLOCK, &kex->done);
 
         /* remove ext-info from the KEX proposals for rekeying */
         myproposal[PROPOSAL_KEX_ALGS] =
             compat_kex_proposal(options.kex_algorithms);
+#ifdef GSSAPI
+        /* repair myproposal after it was crumpled by the */
+        /* ext-info removal above */
+        if (gss) {
+                orig = myproposal[PROPOSAL_KEX_ALGS];
+                xasprintf(&myproposal[PROPOSAL_KEX_ALGS],
+                    "%s,%s", gss, orig);
+                free(gss);
+        }
+#endif
         if ((r = kex_prop2buf(kex->my, myproposal)) != 0)
                 fatal("kex_prop2buf: %s", ssh_err(r));
 
@@ -314,6 +374,7 @@ int	input_gssapi_token(int type, u_int32_t, struct ssh *);
 int     input_gssapi_hash(int type, u_int32_t, struct ssh *);
 int     input_gssapi_error(int, u_int32_t, struct ssh *);
 int     input_gssapi_errtok(int, u_int32_t, struct ssh *);
+int     userauth_gsskeyex(Authctxt *authctxt);
 #endif
 
 void    userauth(Authctxt *, char *);
@@ -330,6 +391,11 @@ static char *authmethods_get(void);
 
 Authmethod authmethods[] = {
 #ifdef GSSAPI
+        {"gssapi-keyex",
+                userauth_gsskeyex,
+                NULL,
+                &options.gss_authentication,
+                NULL},
         {"gssapi-with-mic",
                 userauth_gssapi,
                 NULL,
@@ -686,25 +752,40 @@ userauth_gssapi(Authctxt *authctxt)
         static u_int mech = 0;
         OM_uint32 min;
         int r, ok = 0;
+        char *gss_host;
+
+        if (options.gss_server_identity)
+                gss_host = xstrdup(options.gss_server_identity);
+        else if (options.gss_trust_dns)
+                gss_host = remote_hostname(active_state);
+        else
+                gss_host = xstrdup(authctxt->host);
 
         /* Try one GSSAPI method at a time, rather than sending them all at
          * once. */
 
         if (gss_supported == NULL)
-                gss_indicate_mechs(&min, &gss_supported);
+                if (GSS_ERROR(gss_indicate_mechs(&min, &gss_supported))) {
+                        gss_supported = NULL;
+                        free(gss_host);
+                        return 0;
+                }
 
         /* Check to see if the mechanism is usable before we offer it */
         while (mech < gss_supported->count && !ok) {
                 /* My DER encoding requires length<128 */
                 if (gss_supported->elements[mech].length < 128 &&
                     ssh_gssapi_check_mechanism(&gssctxt,
-                    &gss_supported->elements[mech], authctxt->host)) {
+                    &gss_supported->elements[mech], gss_host,
+                    options.gss_client_identity)) {
                         ok = 1; /* Mechanism works */
                 } else {
                         mech++;
                 }
         }
 
+        free(gss_host);
+
         if (!ok)
                 return 0;
 
@@ -935,6 +1016,54 @@ input_gssapi_error(int type, u_int32_t plen, struct ssh *ssh)
         free(lang);
         return r;
 }
+
+int
+userauth_gsskeyex(Authctxt *authctxt)
+{
+        struct ssh *ssh = active_state; /* XXX */
+        struct sshbuf *b;
+        gss_buffer_desc gssbuf;
+        gss_buffer_desc mic = GSS_C_EMPTY_BUFFER;
+        OM_uint32 ms;
+        int r;
+
+        static int attempt = 0;
+        if (attempt++ >= 1)
+                return (0);
+
+        if (gss_kex_context == NULL) {
+                debug("No valid Key exchange context"); 
+                return (0);
+        }
+
+        if ((b = sshbuf_new()) == NULL)
+                fatal("%s: sshbuf_new failed", __func__);
+        ssh_gssapi_buildmic(b, authctxt->server_user, authctxt->service,
+            "gssapi-keyex");
+
+        if ((gssbuf.value = sshbuf_mutable_ptr(b)) == NULL)
+                fatal("%s: sshbuf_mutable_ptr failed", __func__);
+        gssbuf.length = sshbuf_len(b);
+
+        if (GSS_ERROR(ssh_gssapi_sign(gss_kex_context, &gssbuf, &mic))) {
+                sshbuf_free(b);
+                return (0);
+        }
+
+        if ((r = sshpkt_start(ssh, SSH2_MSG_USERAUTH_REQUEST)) != 0 ||
+            (r = sshpkt_put_cstring(ssh, authctxt->server_user)) != 0 ||
+            (r = sshpkt_put_cstring(ssh, authctxt->service)) != 0 ||
+            (r = sshpkt_put_cstring(ssh, authctxt->method->name)) != 0 ||
+            (r = sshpkt_put_string(ssh, mic.value, mic.length)) != 0 ||
+            (r = sshpkt_send(ssh)) != 0)
+                fatal("%s: %s", __func__, ssh_err(r));
+
+        sshbuf_free(b);
+        gss_release_buffer(&ms, &mic);
+
+        return (1);
+}
+
 #endif /* GSSAPI */
 
 int