fix double-free caused by compat_kex_proposal(); bz3522

by dtucker@, ok me

diff --git a/usr.bin/ssh/compat.c b/usr.bin/ssh/compat.c
index babd941..38c7aaa 100644 (file)
--- a/usr.bin/ssh/compat.c
+++ b/usr.bin/ssh/compat.c
@@ -1,4 +1,4 @@
-/* $OpenBSD: compat.c,v 1.120 2022/07/01 03:35:45 dtucker Exp $ */
+/* $OpenBSD: compat.c,v 1.121 2023/02/02 12:10:05 djm Exp $ */
 /*
  * Copyright (c) 1999, 2000, 2001, 2002 Markus Friedl.  All rights reserved.
  *
@@ -188,26 +188,26 @@ compat_pkalg_proposal(struct ssh *ssh, char *pkalg_prop)
 char *
 compat_kex_proposal(struct ssh *ssh, char *p)
 {
-       char *cp = NULL;
+       char *cp = NULL, *cp2 = NULL;
 
        if ((ssh->compat & (SSH_BUG_CURVE25519PAD|SSH_OLD_DHGEX)) == 0)
                return xstrdup(p);
        debug2_f("original KEX proposal: %s", p);
        if ((ssh->compat & SSH_BUG_CURVE25519PAD) != 0)
-               if ((p = match_filter_denylist(p,
+               if ((cp = match_filter_denylist(p,
                    "curve25519-sha256@libssh.org")) == NULL)
                        fatal("match_filter_denylist failed");
        if ((ssh->compat & SSH_OLD_DHGEX) != 0) {
-               cp = p;
-               if ((p = match_filter_denylist(p,
+               if ((cp2 = match_filter_denylist(cp ? cp : p,
                    "diffie-hellman-group-exchange-sha256,"
                    "diffie-hellman-group-exchange-sha1")) == NULL)
                        fatal("match_filter_denylist failed");
                free(cp);
+               cp = cp2;
        }
-       debug2_f("compat KEX proposal: %s", p);
-       if (*p == '\0')
+       if (cp == NULL || *cp == '\0')
                fatal("No supported key exchange algorithms found");
-       return p;
+       debug2_f("compat KEX proposal: %s", cp);
+       return cp;
 }