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upstream: Fix signature algorithm selection logic for 

UpdateHostkeys on the server side. The previous code tried to prefer RSA/SHA2
for hostkey proofs of RSA keys, but missed some cases. This will use RSA/SHA2
signatures for RSA keys if the client proposed these algorithms in initial
KEX. bz3375

Mostly by Dmitry Belyavskiy with some tweaks by me.

ok markus@

OpenBSD-Commit-ID: c17ba0c3236340d2c6a248158ebed042ac6a8029
This commit is contained in:
djm@openbsd.org 2022-01-06 21:55:23 +00:00 committed by Damien Miller
parent 17877bc81d
commit 0fa3368322
3 changed files with 44 additions and 11 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

24
kex.c
View File

@ -1,4 +1,4 @@
/* $OpenBSD: kex.c,v 1.170 2021/12/19 22:13:12 djm Exp $ */
/* $OpenBSD: kex.c,v 1.171 2022/01/06 21:55:23 djm Exp $ */
/*
* Copyright (c) 2000, 2001 Markus Friedl. All rights reserved.
*
@ -900,6 +900,18 @@ proposals_match(char *my[PROPOSAL_MAX], char *peer[PROPOSAL_MAX])
return (1);
}
/* returns non-zero if proposal contains any algorithm from algs */
static int
has_any_alg(const char *proposal, const char *algs)
{
char *cp;
if ((cp = match_list(proposal, algs, NULL)) == NULL)
return 0;
free(cp);
return 1;
}
static int
kex_choose_conf(struct ssh *ssh)
{
@ -935,6 +947,16 @@ kex_choose_conf(struct ssh *ssh)
free(ext);
}
/* Check whether client supports rsa-sha2 algorithms */
if (kex->server && (kex->flags & KEX_INITIAL)) {
if (has_any_alg(peer[PROPOSAL_SERVER_HOST_KEY_ALGS],
"rsa-sha2-256,rsa-sha2-256-cert-v01@openssh.com"))
kex->flags |= KEX_RSA_SHA2_256_SUPPORTED;
if (has_any_alg(peer[PROPOSAL_SERVER_HOST_KEY_ALGS],
"rsa-sha2-512,rsa-sha2-512-cert-v01@openssh.com"))
kex->flags |= KEX_RSA_SHA2_512_SUPPORTED;
}
/* Algorithm Negotiation */
if ((r = choose_kex(kex, cprop[PROPOSAL_KEX_ALGS],
sprop[PROPOSAL_KEX_ALGS])) != 0) {

4
kex.h
View File

@ -1,4 +1,4 @@
/* $OpenBSD: kex.h,v 1.116 2021/12/19 22:12:54 djm Exp $ */
/* $OpenBSD: kex.h,v 1.117 2022/01/06 21:55:23 djm Exp $ */
/*
* Copyright (c) 2000, 2001 Markus Friedl. All rights reserved.
@ -109,6 +109,8 @@ enum kex_exchange {
#define KEX_INIT_SENT 0x0001
#define KEX_INITIAL 0x0002
#define KEX_HAS_PUBKEY_HOSTBOUND 0x0004
#define KEX_RSA_SHA2_256_SUPPORTED 0x0008 /* only set in server for now */
#define KEX_RSA_SHA2_512_SUPPORTED 0x0010 /* only set in server for now */
struct sshenc {
char *name;

27
serverloop.c
View File

@ -1,4 +1,4 @@
/* $OpenBSD: serverloop.c,v 1.229 2022/01/06 21:48:38 djm Exp $ */
/* $OpenBSD: serverloop.c,v 1.230 2022/01/06 21:55:23 djm Exp $ */
/*
* Author: Tatu Ylonen <ylo@cs.hut.fi>
* Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
@ -685,16 +685,17 @@ server_input_hostkeys_prove(struct ssh *ssh, struct sshbuf **respp)
struct sshbuf *resp = NULL;
struct sshbuf *sigbuf = NULL;
struct sshkey *key = NULL, *key_pub = NULL, *key_prv = NULL;
int r, ndx, kexsigtype, use_kexsigtype, success = 0;
int r, ndx, success = 0;
const u_char *blob;
const char *sigalg, *kex_rsa_sigalg = NULL;
u_char *sig = 0;
size_t blen, slen;
if ((resp = sshbuf_new()) == NULL || (sigbuf = sshbuf_new()) == NULL)
fatal_f("sshbuf_new");
kexsigtype = sshkey_type_plain(
sshkey_type_from_name(ssh->kex->hostkey_alg));
if (sshkey_type_plain(sshkey_type_from_name(
ssh->kex->hostkey_alg)) == KEY_RSA)
kex_rsa_sigalg = ssh->kex->hostkey_alg;
while (ssh_packet_remaining(ssh) > 0) {
sshkey_free(key);
key = NULL;
@ -727,16 +728,24 @@ server_input_hostkeys_prove(struct ssh *ssh, struct sshbuf **respp)
* For RSA keys, prefer to use the signature type negotiated
* during KEX to the default (SHA1).
*/
use_kexsigtype = kexsigtype == KEY_RSA &&
sshkey_type_plain(key->type) == KEY_RSA;
sigalg = NULL;
if (sshkey_type_plain(key->type) == KEY_RSA) {
if (kex_rsa_sigalg != NULL)
sigalg = kex_rsa_sigalg;
else if (ssh->kex->flags & KEX_RSA_SHA2_512_SUPPORTED)
sigalg = "rsa-sha2-512";
else if (ssh->kex->flags & KEX_RSA_SHA2_256_SUPPORTED)
sigalg = "rsa-sha2-256";
}
debug3_f("sign %s key (index %d) using sigalg %s",
sshkey_type(key), ndx, sigalg == NULL ? "default" : sigalg);
if ((r = sshbuf_put_cstring(sigbuf,
"hostkeys-prove-00@openssh.com")) != 0 ||
(r = sshbuf_put_stringb(sigbuf,
ssh->kex->session_id)) != 0 ||
(r = sshkey_puts(key, sigbuf)) != 0 ||
(r = ssh->kex->sign(ssh, key_prv, key_pub, &sig, &slen,
sshbuf_ptr(sigbuf), sshbuf_len(sigbuf),
use_kexsigtype ? ssh->kex->hostkey_alg : NULL)) != 0 ||
sshbuf_ptr(sigbuf), sshbuf_len(sigbuf), sigalg)) != 0 ||
(r = sshbuf_put_string(resp, sig, slen)) != 0) {
error_fr(r, "assemble signature");
goto out;