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, 58 insertions, 4 deletions

diff --git a/sshd.c b/sshd.c
index 6f8f11a3b..02fca5c28 100644
--- a/sshd.c
+++ b/sshd.c
@@ -816,8 +816,8 @@ notify_hostkeys(struct ssh *ssh)
         }
         debug3("%s: sent %u hostkeys", __func__, nkeys);
         if (nkeys == 0)
-                fatal("%s: no hostkeys", __func__);
-        if ((r = sshpkt_send(ssh)) != 0)
+                debug3("%s: no hostkeys", __func__);
+        else if ((r = sshpkt_send(ssh)) != 0)
                 sshpkt_fatal(ssh, r, "%s: send", __func__);
         sshbuf_free(buf);
 }
@@ -1851,7 +1851,8 @@ main(int ac, char **av)
                 free(fp);
         }
         accumulate_host_timing_secret(cfg, NULL);
-        if (!sensitive_data.have_ssh2_key) {
+        /* The GSSAPI key exchange can run without a host key */
+        if (!sensitive_data.have_ssh2_key && !options.gss_keyex) {
                 logit("sshd: no hostkeys available -- exiting.");
                 exit(1);
         }
@@ -2342,6 +2343,48 @@ do_ssh2_kex(struct ssh *ssh)
         myproposal[PROPOSAL_SERVER_HOST_KEY_ALGS] = compat_pkalg_proposal(
             list_hostkey_types());
 
+#if defined(GSSAPI) && defined(WITH_OPENSSL)
+        {
+        char *orig;
+        char *gss = NULL;
+        char *newstr = NULL;
+        orig = myproposal[PROPOSAL_KEX_ALGS];
+
+        /*
+         * If we don't have a host key, then there's no point advertising
+         * the other key exchange algorithms
+         */
+
+        if (strlen(myproposal[PROPOSAL_SERVER_HOST_KEY_ALGS]) == 0)
+                orig = NULL;
+
+        if (options.gss_keyex)
+                gss = ssh_gssapi_server_mechanisms();
+        else
+                gss = NULL;
+
+        if (gss && orig)
+                xasprintf(&newstr, "%s,%s", gss, orig);
+        else if (gss)
+                newstr = gss;
+        else if (orig)
+                newstr = orig;
+
+        /*
+         * If we've got GSSAPI mechanisms, then we've got the 'null' host
+         * key alg, but we can't tell people about it unless its the only
+         * host key algorithm we support
+         */
+        if (gss && (strlen(myproposal[PROPOSAL_SERVER_HOST_KEY_ALGS])) == 0)
+                myproposal[PROPOSAL_SERVER_HOST_KEY_ALGS] = "null";
+
+        if (newstr)
+                myproposal[PROPOSAL_KEX_ALGS] = newstr;
+        else
+                fatal("No supported key exchange algorithms");
+        }
+#endif
+
         /* start key exchange */
         if ((r = kex_setup(ssh, myproposal)) != 0)
                 fatal("kex_setup: %s", ssh_err(r));
@@ -2357,7 +2400,18 @@ do_ssh2_kex(struct ssh *ssh)
 # ifdef OPENSSL_HAS_ECC
         kex->kex[KEX_ECDH_SHA2] = kex_gen_server;
 # endif
-#endif
+# ifdef GSSAPI
+        if (options.gss_keyex) {
+                kex->kex[KEX_GSS_GRP1_SHA1] = kexgss_server;
+                kex->kex[KEX_GSS_GRP14_SHA1] = kexgss_server;
+                kex->kex[KEX_GSS_GRP14_SHA256] = kexgss_server;
+                kex->kex[KEX_GSS_GRP16_SHA512] = kexgss_server;
+                kex->kex[KEX_GSS_GEX_SHA1] = kexgssgex_server;
+                kex->kex[KEX_GSS_NISTP256_SHA256] = kexgss_server;
+                kex->kex[KEX_GSS_C25519_SHA256] = kexgss_server;
+        }
+# endif
+#endif /* WITH_OPENSSL */
         kex->kex[KEX_C25519_SHA256] = kex_gen_server;
         kex->kex[KEX_KEM_SNTRUP4591761X25519_SHA512] = kex_gen_server;
         kex->load_host_public_key=&get_hostkey_public_by_type;