Always return allocated strings from the kex filtering so that we

can free them later.  Fix one leak in compat_kex_proposal.  Based
on github PR#324 from ZoltanFridrich with some simplications by me.
ok djm@

diff --git a/usr.bin/ssh/compat.c b/usr.bin/ssh/compat.c
index 554edb9..babd941 100644 (file)
--- a/usr.bin/ssh/compat.c
+++ b/usr.bin/ssh/compat.c
@@ -1,4 +1,4 @@
-/* $OpenBSD: compat.c,v 1.119 2021/09/10 05:46:09 djm Exp $ */
+/* $OpenBSD: compat.c,v 1.120 2022/07/01 03:35:45 dtucker Exp $ */
 /*
  * Copyright (c) 1999, 2000, 2001, 2002 Markus Friedl.  All rights reserved.
  *
@@ -154,11 +154,12 @@ compat_banner(struct ssh *ssh, const char *version)
        debug_f("no match: %s", version);
 }
 
+/* Always returns pointer to allocated memory, caller must free. */
 char *
 compat_cipher_proposal(struct ssh *ssh, char *cipher_prop)
 {
        if (!(ssh->compat & SSH_BUG_BIGENDIANAES))
-               return cipher_prop;
+               return xstrdup(cipher_prop);
        debug2_f("original cipher proposal: %s", cipher_prop);
        if ((cipher_prop = match_filter_denylist(cipher_prop, "aes*")) == NULL)
                fatal("match_filter_denylist failed");
@@ -168,11 +169,12 @@ compat_cipher_proposal(struct ssh *ssh, char *cipher_prop)
        return cipher_prop;
 }
 
+/* Always returns pointer to allocated memory, caller must free. */
 char *
 compat_pkalg_proposal(struct ssh *ssh, char *pkalg_prop)
 {
        if (!(ssh->compat & SSH_BUG_RSASIGMD5))
-               return pkalg_prop;
+               return xstrdup(pkalg_prop);
        debug2_f("original public key proposal: %s", pkalg_prop);
        if ((pkalg_prop = match_filter_denylist(pkalg_prop, "ssh-rsa")) == NULL)
                fatal("match_filter_denylist failed");
@@ -182,21 +184,26 @@ compat_pkalg_proposal(struct ssh *ssh, char *pkalg_prop)
        return pkalg_prop;
 }
 
+/* Always returns pointer to allocated memory, caller must free. */
 char *
 compat_kex_proposal(struct ssh *ssh, char *p)
 {
+       char *cp = NULL;
+
        if ((ssh->compat & (SSH_BUG_CURVE25519PAD|SSH_OLD_DHGEX)) == 0)
-               return p;
+               return xstrdup(p);
        debug2_f("original KEX proposal: %s", p);
        if ((ssh->compat & SSH_BUG_CURVE25519PAD) != 0)
                if ((p = 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,
                    "diffie-hellman-group-exchange-sha256,"
                    "diffie-hellman-group-exchange-sha1")) == NULL)
                        fatal("match_filter_denylist failed");
+               free(cp);
        }
        debug2_f("compat KEX proposal: %s", p);
        if (*p == '\0')