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.

Author: Simon Wilkinson <simon@sxw.org.uk>
Author: Colin Watson <cjwatson@debian.org>
Author: Jakub Jelen <jjelen@redhat.com>
Origin: other, https://github.com/openssh-gsskex/openssh-gsskex/commits/debian/master
Bug: https://bugzilla.mindrot.org/show_bug.cgi?id=1242
Last-Updated: 2020-06-07

Patch-Name: gssapi.patch

1 files changed, 149 insertions, 5 deletions

diff --git a/sshconnect2.c b/sshconnect2.c
index 1a6545edf..79a22e600 100644
--- a/sshconnect2.c
+++ b/sshconnect2.c
@@ -80,8 +80,6 @@
 #endif
 
 /* import */
-extern char *client_version_string;
-extern char *server_version_string;
 extern Options options;
 
 /*
@@ -163,6 +161,11 @@ ssh_kex2(struct ssh *ssh, char *host, struct sockaddr *hostaddr, u_short port)
         char *s, *all_key;
         int r, use_known_hosts_order = 0;
 
+#if defined(GSSAPI) && defined(WITH_OPENSSL)
+        char *orig = NULL, *gss = NULL;
+        char *gss_host = NULL;
+#endif
+
         xxx_host = host;
         xxx_hostaddr = hostaddr;
 
@@ -206,6 +209,41 @@ ssh_kex2(struct ssh *ssh, char *host, struct sockaddr *hostaddr, u_short port)
                     compat_pkalg_proposal(options.hostkeyalgorithms);
         }
 
+#if defined(GSSAPI) && defined(WITH_OPENSSL)
+        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(ssh);
+                        /* Fall back to specified host if we are using proxy command
+                         * and can not use DNS on that socket */
+                        if (strcmp(gss_host, "UNKNOWN") == 0) {
+                                gss_host = xstrdup(host);
+                        }
+                } else {
+                        gss_host = xstrdup(host);
+                }
+
+                gss = ssh_gssapi_client_mechanisms(gss_host,
+                    options.gss_client_identity, options.gss_kex_algorithms);
+                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)
                 ssh_packet_set_rekey_limits(ssh, options.rekey_limit,
                     options.rekey_interval);
@@ -224,16 +262,46 @@ ssh_kex2(struct ssh *ssh, char *host, struct sockaddr *hostaddr, u_short port)
 # ifdef OPENSSL_HAS_ECC
         ssh->kex->kex[KEX_ECDH_SHA2] = kex_gen_client;
 # endif
-#endif
+# ifdef GSSAPI
+        if (options.gss_keyex) {
+                ssh->kex->kex[KEX_GSS_GRP1_SHA1] = kexgss_client;
+                ssh->kex->kex[KEX_GSS_GRP14_SHA1] = kexgss_client;
+                ssh->kex->kex[KEX_GSS_GRP14_SHA256] = kexgss_client;
+                ssh->kex->kex[KEX_GSS_GRP16_SHA512] = kexgss_client;
+                ssh->kex->kex[KEX_GSS_GEX_SHA1] = kexgssgex_client;
+                ssh->kex->kex[KEX_GSS_NISTP256_SHA256] = kexgss_client;
+                ssh->kex->kex[KEX_GSS_C25519_SHA256] = kexgss_client;
+        }
+# endif
+#endif /* WITH_OPENSSL */
         ssh->kex->kex[KEX_C25519_SHA256] = kex_gen_client;
         ssh->kex->kex[KEX_KEM_SNTRUP4591761X25519_SHA512] = kex_gen_client;
         ssh->kex->verify_host_key=&verify_host_key_callback;
 
+#if defined(GSSAPI) && defined(WITH_OPENSSL)
+        if (options.gss_keyex) {
+                ssh->kex->gss_deleg_creds = options.gss_deleg_creds;
+                ssh->kex->gss_trust_dns = options.gss_trust_dns;
+                ssh->kex->gss_client = options.gss_client_identity;
+                ssh->kex->gss_host = gss_host;
+        }
+#endif
+
         ssh_dispatch_run_fatal(ssh, DISPATCH_BLOCK, &ssh->kex->done);
 
         /* remove ext-info from the KEX proposals for rekeying */
         myproposal[PROPOSAL_KEX_ALGS] =
             compat_kex_proposal(options.kex_algorithms);
+#if defined(GSSAPI) && defined(WITH_OPENSSL)
+        /* 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(ssh->kex->my, myproposal)) != 0)
                 fatal("kex_prop2buf: %s", ssh_err(r));
 
@@ -330,6 +398,7 @@ static int input_gssapi_response(int type, u_int32_t, struct ssh *);
 static int input_gssapi_token(int type, u_int32_t, struct ssh *);
 static int input_gssapi_error(int, u_int32_t, struct ssh *);
 static int input_gssapi_errtok(int, u_int32_t, struct ssh *);
+static int userauth_gsskeyex(struct ssh *);
 #endif
 
 void    userauth(struct ssh *, char *);
@@ -346,6 +415,11 @@ static char *authmethods_get(void);
 
 Authmethod authmethods[] = {
 #ifdef GSSAPI
+        {"gssapi-keyex",
+                userauth_gsskeyex,
+                NULL,
+                &options.gss_keyex,
+                NULL},
         {"gssapi-with-mic",
                 userauth_gssapi,
                 userauth_gssapi_cleanup,
@@ -716,12 +790,31 @@ userauth_gssapi(struct ssh *ssh)
         OM_uint32 min;
         int r, ok = 0;
         gss_OID mech = NULL;
+        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(ssh);
+                /* Fall back to specified host if we are using proxy command
+                 * and can not use DNS on that socket */
+                if (strcmp(gss_host, "UNKNOWN") == 0) {
+                        gss_host = authctxt->host;
+                }
+        } else {
+                gss_host = xstrdup(authctxt->host);
+        }
 
         /* Try one GSSAPI method at a time, rather than sending them all at
          * once. */
 
         if (authctxt->gss_supported_mechs == NULL)
-                gss_indicate_mechs(&min, &authctxt->gss_supported_mechs);
+                if (GSS_ERROR(gss_indicate_mechs(&min,
+                    &authctxt->gss_supported_mechs))) {
+                        authctxt->gss_supported_mechs = NULL;
+                        free(gss_host);
+                        return 0;
+                }
 
         /* Check to see whether the mechanism is usable before we offer it */
         while (authctxt->mech_tried < authctxt->gss_supported_mechs->count &&
@@ -730,13 +823,15 @@ userauth_gssapi(struct ssh *ssh)
                     elements[authctxt->mech_tried];
                 /* My DER encoding requires length<128 */
                 if (mech->length < 128 && ssh_gssapi_check_mechanism(&gssctxt,
-                    mech, authctxt->host)) {
+                    mech, gss_host, options.gss_client_identity)) {
                         ok = 1; /* Mechanism works */
                 } else {
                         authctxt->mech_tried++;
                 }
         }
 
+        free(gss_host);
+
         if (!ok || mech == NULL)
                 return 0;
 
@@ -976,6 +1071,55 @@ input_gssapi_error(int type, u_int32_t plen, struct ssh *ssh)
         free(lang);
         return r;
 }
+
+int
+userauth_gsskeyex(struct ssh *ssh)
+{
+        struct sshbuf *b = NULL;
+        Authctxt *authctxt = ssh->authctxt;
+        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 */
 
 static int