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.

Origin: other, https://github.com/openssh-gsskex/openssh-gsskex/commits/debian/master
Bug: https://bugzilla.mindrot.org/show_bug.cgi?id=1242
Last-Updated: 2019-10-09

Patch-Name: gssapi.patch

1 files changed, 53 insertions, 3 deletions

diff --git a/auth2-gss.c b/auth2-gss.c
index 9351e0428..d6446c0cf 100644
--- a/auth2-gss.c
+++ b/auth2-gss.c
@@ -1,7 +1,7 @@
 /* $OpenBSD: auth2-gss.c,v 1.29 2018/07/31 03:10:27 djm Exp $ */
 
 /*
- * Copyright (c) 2001-2003 Simon Wilkinson. All rights reserved.
+ * Copyright (c) 2001-2007 Simon Wilkinson. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
@@ -55,6 +55,48 @@ static int input_gssapi_exchange_complete(int type, u_int32_t plen, struct ssh *
 static int input_gssapi_errtok(int, u_int32_t, struct ssh *);
 
 /*
+ * The 'gssapi_keyex' userauth mechanism.
+ */
+static int
+userauth_gsskeyex(struct ssh *ssh)
+{
+        Authctxt *authctxt = ssh->authctxt;
+        int r, authenticated = 0;
+        struct sshbuf *b = NULL;
+        gss_buffer_desc mic, gssbuf;
+        u_char *p;
+        size_t len;
+
+        if ((r = sshpkt_get_string(ssh, &p, &len)) != 0 ||
+            (r = sshpkt_get_end(ssh)) != 0)
+                fatal("%s: %s", __func__, ssh_err(r));
+
+        if ((b = sshbuf_new()) == NULL)
+                fatal("%s: sshbuf_new failed", __func__);
+
+        mic.value = p;
+        mic.length = len;
+
+        ssh_gssapi_buildmic(b, authctxt->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);
+
+        /* gss_kex_context is NULL with privsep, so we can't check it here */
+        if (!GSS_ERROR(PRIVSEP(ssh_gssapi_checkmic(gss_kex_context,
+            &gssbuf, &mic))))
+                authenticated = PRIVSEP(ssh_gssapi_userok(authctxt->user,
+                    authctxt->pw, 1));
+
+        sshbuf_free(b);
+        free(mic.value);
+
+        return (authenticated);
+}
+
+/*
  * We only support those mechanisms that we know about (ie ones that we know
  * how to check local user kuserok and the like)
  */
@@ -260,7 +302,8 @@ input_gssapi_exchange_complete(int type, u_int32_t plen, struct ssh *ssh)
         if ((r = sshpkt_get_end(ssh)) != 0)
                 fatal("%s: %s", __func__, ssh_err(r));
 
-        authenticated = PRIVSEP(ssh_gssapi_userok(authctxt->user));
+        authenticated = PRIVSEP(ssh_gssapi_userok(authctxt->user,
+            authctxt->pw, 1));
 
         if ((!use_privsep || mm_is_monitor()) &&
             (displayname = ssh_gssapi_displayname()) != NULL)
@@ -306,7 +349,8 @@ input_gssapi_mic(int type, u_int32_t plen, struct ssh *ssh)
         gssbuf.length = sshbuf_len(b);
 
         if (!GSS_ERROR(PRIVSEP(ssh_gssapi_checkmic(gssctxt, &gssbuf, &mic))))
-                authenticated = PRIVSEP(ssh_gssapi_userok(authctxt->user));
+                authenticated = PRIVSEP(ssh_gssapi_userok(authctxt->user,
+                    authctxt->pw, 0));
         else
                 logit("GSSAPI MIC check failed");
 
@@ -326,6 +370,12 @@ input_gssapi_mic(int type, u_int32_t plen, struct ssh *ssh)
         return 0;
 }
 
+Authmethod method_gsskeyex = {
+        "gssapi-keyex",
+        userauth_gsskeyex,
+        &options.gss_authentication
+};
+
 Authmethod method_gssapi = {
         "gssapi-with-mic",
         userauth_gssapi,