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: 2017-10-04

Patch-Name: gssapi.patch

1 files changed, 111 insertions, 1 deletions

diff --git a/sshd.c b/sshd.c
index 51a1aaf6e..45e50fac3 100644
--- a/sshd.c
+++ b/sshd.c
@@ -122,6 +122,10 @@
 #include "version.h"
 #include "ssherr.h"
 
+#ifdef USE_SECURITY_SESSION_API
+#include <Security/AuthSession.h>
+#endif
+
 /* Re-exec fds */
 #define REEXEC_DEVCRYPTO_RESERVED_FD    (STDERR_FILENO + 1)
 #define REEXEC_STARTUP_PIPE_FD          (STDERR_FILENO + 2)
@@ -529,7 +533,7 @@ privsep_preauth_child(void)
 
 #ifdef GSSAPI
         /* Cache supported mechanism OIDs for later use */
-        if (options.gss_authentication)
+        if (options.gss_authentication || options.gss_keyex)
                 ssh_gssapi_prepare_supported_oids();
 #endif
 
@@ -1708,10 +1712,13 @@ main(int ac, char **av)
                     key ? "private" : "agent", i, sshkey_ssh_name(pubkey), fp);
                 free(fp);
         }
+#ifndef GSSAPI
+        /* The GSSAPI key exchange can run without a host key */
         if (!sensitive_data.have_ssh2_key) {
                 logit("sshd: no hostkeys available -- exiting.");
                 exit(1);
         }
+#endif
 
         /*
          * Load certificates. They are stored in an array at identical
@@ -1987,6 +1994,60 @@ main(int ac, char **av)
             remote_ip, remote_port, laddr,  ssh_local_port(ssh));
         free(laddr);
 
+#ifdef USE_SECURITY_SESSION_API
+        /*
+         * Create a new security session for use by the new user login if
+         * the current session is the root session or we are not launched
+         * by inetd (eg: debugging mode or server mode).  We do not
+         * necessarily need to create a session if we are launched from
+         * inetd because Panther xinetd will create a session for us.
+         *
+         * The only case where this logic will fail is if there is an
+         * inetd running in a non-root session which is not creating
+         * new sessions for us.  Then all the users will end up in the
+         * same session (bad).
+         *
+         * When the client exits, the session will be destroyed for us
+         * automatically.
+         *
+         * We must create the session before any credentials are stored
+         * (including AFS pags, which happens a few lines below).
+         */
+        {
+                OSStatus err = 0;
+                SecuritySessionId sid = 0;
+                SessionAttributeBits sattrs = 0;
+
+                err = SessionGetInfo(callerSecuritySession, &sid, &sattrs);
+                if (err)
+                        error("SessionGetInfo() failed with error %.8X",
+                            (unsigned) err);
+                else
+                        debug("Current Session ID is %.8X / Session Attributes are %.8X",
+                            (unsigned) sid, (unsigned) sattrs);
+
+                if (inetd_flag && !(sattrs & sessionIsRoot))
+                        debug("Running in inetd mode in a non-root session... "
+                            "assuming inetd created the session for us.");
+                else {
+                        debug("Creating new security session...");
+                        err = SessionCreate(0, sessionHasTTY | sessionIsRemote);
+                        if (err)
+                                error("SessionCreate() failed with error %.8X",
+                                    (unsigned) err);
+
+                        err = SessionGetInfo(callerSecuritySession, &sid, 
+                            &sattrs);
+                        if (err)
+                                error("SessionGetInfo() failed with error %.8X",
+                                    (unsigned) err);
+                        else
+                                debug("New Session ID is %.8X / Session Attributes are %.8X",
+                                    (unsigned) sid, (unsigned) sattrs);
+                }
+        }
+#endif
+
         /*
          * We don't want to listen forever unless the other side
          * successfully authenticates itself.  So we set up an alarm which is
@@ -2170,6 +2231,48 @@ do_ssh2_kex(void)
         myproposal[PROPOSAL_SERVER_HOST_KEY_ALGS] = compat_pkalg_proposal(
             list_hostkey_types());
 
+#ifdef GSSAPI
+        {
+        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(active_state, myproposal)) != 0)
                 fatal("kex_setup: %s", ssh_err(r));
@@ -2187,6 +2290,13 @@ do_ssh2_kex(void)
 # endif
 #endif
         kex->kex[KEX_C25519_SHA256] = kexc25519_server;
+#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_GEX_SHA1] = kexgss_server;
+        }
+#endif
         kex->server = 1;
         kex->client_version_string=client_version_string;
         kex->server_version_string=server_version_string;