git: e87e8c20e647 - releng/12.4 - ssh: fix double-free caused by compat_kex_proposal()

The branch releng/12.4 has been updated by gordon:

URL: https://cgit.FreeBSD.org/src/commit/?id=e87e8c20e64753a25743d546e2a8b8619efd83c6

commit e87e8c20e64753a25743d546e2a8b8619efd83c6
Author:     Ed Maste <emaste@FreeBSD.org>
AuthorDate: 2023-02-06 16:26:08 +0000
Commit:     Gordon Tetlow <gordon@FreeBSD.org>
CommitDate: 2023-02-16 17:12:39 +0000

    ssh: fix double-free caused by compat_kex_proposal()
    
    Approved by:    so
    Security:       FreeBSD-SA-23:02.openssh
    Security:       CVE-2023-25136
    Obtained from:  OpenSSH-portable commit 12da78233364
    Sponsored by:   The FreeBSD Foundation
    
    (cherry picked from commit fe1371e8f3d7336748d291a7360b2aacce943fb1)
    (cherry picked from commit 296ec8eae0c834088a491643a937d881bfb4b5dd)
    (cherry picked from commit 2caac96a666ce99703b16e4d59a417bab8f91d3c)
---
 crypto/openssh/compat.c | 16 ++++++++--------
 1 file changed, 8 insertions(+), 8 deletions(-)

diff --git a/crypto/openssh/compat.c b/crypto/openssh/compat.c
index 46dfe3a9c2e7..478a9403eeaa 100644
--- a/crypto/openssh/compat.c
+++ b/crypto/openssh/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.
  *
@@ -190,26 +190,26 @@ 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;
 }