upstream: only consider the ext-info-c extension during the initial

KEX. It shouldn't be sent in subsequent ones, but if it is present we should
ignore it.

This prevents sshd from sending a SSH_MSG_EXT_INFO for REKEX for buggy
these clients. Reported by Jakub Jelen via bz2929; ok dtucker@

OpenBSD-Commit-ID: 91564118547f7807030ec537480303e2371902f9

2 files changed, 6 insertions, 3 deletions

diff --git a/kex.c b/kex.c
index 25f9f66f6..3823a9544 100644
--- a/kex.c
+++ b/kex.c
@@ -1,4 +1,4 @@
-/* $OpenBSD: kex.c,v 1.141 2018/07/09 13:37:10 sf Exp $ */
+/* $OpenBSD: kex.c,v 1.142 2018/12/07 03:39:40 djm Exp $ */
 /*
  * Copyright (c) 2000, 2001 Markus Friedl.  All rights reserved.
  *
@@ -487,6 +487,7 @@ kex_input_newkeys(int type, u_int32_t seq, struct ssh *ssh)
         if ((r = ssh_set_newkeys(ssh, MODE_IN)) != 0)
                 return r;
         kex->done = 1;
+        kex->flags &= ~KEX_INITIAL;
         sshbuf_reset(kex->peer);
         /* sshbuf_reset(kex->my); */
         kex->flags &= ~KEX_INIT_SENT;
@@ -594,6 +595,7 @@ kex_new(struct ssh *ssh, char *proposal[PROPOSAL_MAX], struct kex **kexp)
         if ((r = kex_prop2buf(kex->my, proposal)) != 0)
                 goto out;
         kex->done = 0;
+        kex->flags = KEX_INITIAL;
         kex_reset_dispatch(ssh);
         ssh_dispatch_set(ssh, SSH2_MSG_KEXINIT, &kex_input_kexinit);
         r = 0;
@@ -839,7 +841,7 @@ kex_choose_conf(struct ssh *ssh)
         }
 
         /* Check whether client supports ext_info_c */
-        if (kex->server) {
+        if (kex->server && (kex->flags & KEX_INITIAL)) {
                 char *ext;
 
                 ext = match_list("ext-info-c", peer[PROPOSAL_KEX_ALGS], NULL);
diff --git a/kex.h b/kex.h
index 593de1208..0f67f58db 100644
--- a/kex.h
+++ b/kex.h
@@ -1,4 +1,4 @@
-/* $OpenBSD: kex.h,v 1.91 2018/07/11 18:53:29 markus Exp $ */
+/* $OpenBSD: kex.h,v 1.92 2018/12/07 03:39:40 djm Exp $ */
 
 /*
  * Copyright (c) 2000, 2001 Markus Friedl.  All rights reserved.
@@ -104,6 +104,7 @@ enum kex_exchange {
 };
 
 #define KEX_INIT_SENT   0x0001
+#define KEX_INITIAL     0x0002
 
 struct sshenc {
         char    *name;