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openssh/openssh-8.7p1-minimize-sha1-use.patch
Dmitry Belyavskiy a0db6b2b7f Avoid spirous message on connecting to the machine with ssh-rsa keys 
Related: rhbz#2115246
2022-08-16 14:32:50 +02:00

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diff --color -ru a/clientloop.c b/clientloop.c
--- a/clientloop.c 2022-06-29 16:35:06.677597259 +0200
+++ b/clientloop.c 2022-06-29 16:40:29.737926205 +0200
@@ -116,6 +116,9 @@
#include "ssh-gss.h"
#endif
+/* Permitted RSA signature algorithms for UpdateHostkeys proofs */
+#define HOSTKEY_PROOF_RSA_ALGS "rsa-sha2-512,rsa-sha2-256"
+
/* import options */
extern Options options;
@@ -2110,8 +2113,10 @@
struct hostkeys_update_ctx *ctx = (struct hostkeys_update_ctx *)_ctx;
size_t i, ndone;
struct sshbuf *signdata;
- int r, kexsigtype, use_kexsigtype;
+ int r, plaintype;
const u_char *sig;
+ const char *rsa_kexalg = NULL;
+ char *alg = NULL;
size_t siglen;
if (ctx->nnew == 0)
@@ -2122,9 +2127,9 @@
hostkeys_update_ctx_free(ctx);
return;
}
- 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)
+ rsa_kexalg = ssh->kex->hostkey_alg;
if ((signdata = sshbuf_new()) == NULL)
fatal_f("sshbuf_new failed");
/*
@@ -2135,6 +2140,7 @@
for (ndone = i = 0; i < ctx->nkeys; i++) {
if (ctx->keys_match[i])
continue;
+ plaintype = sshkey_type_plain(ctx->keys[i]->type);
/* Prepare data to be signed: session ID, unique string, key */
sshbuf_reset(signdata);
if ( (r = sshbuf_put_cstring(signdata,
@@ -2148,19 +2154,33 @@
error_fr(r, "parse sig");
goto out;
}
+ if ((r = sshkey_get_sigtype(sig, siglen, &alg)) != 0) {
+ error_fr(r, "server gave unintelligible signature "
+ "for %s key %zu", sshkey_type(ctx->keys[i]), i);
+ goto out;
+ }
/*
- * For RSA keys, prefer to use the signature type negotiated
- * during KEX to the default (SHA1).
+ * Special case for RSA keys: if a RSA hostkey was negotiated,
+ * then use its signature type for verification of RSA hostkey
+ * proofs. Otherwise, accept only RSA-SHA256/512 signatures.
*/
- use_kexsigtype = kexsigtype == KEY_RSA &&
- sshkey_type_plain(ctx->keys[i]->type) == KEY_RSA;
- debug3_f("verify %s key %zu using %s sigalg",
- sshkey_type(ctx->keys[i]), i,
- use_kexsigtype ? ssh->kex->hostkey_alg : "default");
+ if (plaintype == KEY_RSA && rsa_kexalg == NULL &&
+ match_pattern_list(alg, HOSTKEY_PROOF_RSA_ALGS, 0) != 1) {
+ debug_f("server used untrusted RSA signature algorithm "
+ "%s for key %zu, disregarding", alg, i);
+ free(alg);
+ /* zap the key from the list */
+ sshkey_free(ctx->keys[i]);
+ ctx->keys[i] = NULL;
+ ndone++;
+ continue;
+ }
+ debug3_f("verify %s key %zu using sigalg %s",
+ sshkey_type(ctx->keys[i]), i, alg);
+ free(alg);
if ((r = sshkey_verify(ctx->keys[i], sig, siglen,
sshbuf_ptr(signdata), sshbuf_len(signdata),
- use_kexsigtype ? ssh->kex->hostkey_alg : NULL, 0,
- NULL)) != 0) {
+ plaintype == KEY_RSA ? rsa_kexalg : NULL, 0, NULL)) != 0) {
error_fr(r, "server gave bad signature for %s key %zu",
sshkey_type(ctx->keys[i]), i);
goto out;
diff --git a/hostfile.c b/hostfile.c
index a035b381..bd49e3ac 100644
--- a/hostfile.c
+++ b/hostfile.c
@@ -642,7 +642,7 @@ hostfile_replace_entries(const char *filename, const char *host, const char *ip,
/* Re-add the requested keys */
want = HKF_MATCH_HOST | (ip == NULL ? 0 : HKF_MATCH_IP);
for (i = 0; i < nkeys; i++) {
- if ((want & ctx.match_keys[i]) == want)
+ if (keys[i] == NULL || (want & ctx.match_keys[i]) == want)
continue;
if ((fp = sshkey_fingerprint(keys[i], hash_alg,
SSH_FP_DEFAULT)) == NULL) {
diff --color -ru a/kex.c b/kex.c
--- a/kex.c 2022-06-29 16:35:06.775599179 +0200
+++ b/kex.c 2022-06-29 16:42:00.839710940 +0200
@@ -959,6 +959,18 @@
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)
{
@@ -994,6 +1006,16 @@
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) {
diff --color -ru a/kex.h b/kex.h
--- a/kex.h 2022-06-29 16:35:06.766599003 +0200
+++ b/kex.h 2022-06-29 16:42:24.199168567 +0200
@@ -116,6 +116,8 @@
#define KEX_INIT_SENT 0x0001
#define KEX_INITIAL 0x0002
+#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;
diff --color -ru a/serverloop.c b/serverloop.c
--- a/serverloop.c 2021-08-20 06:03:49.000000000 +0200
+++ b/serverloop.c 2022-06-29 16:45:05.902336428 +0200
@@ -684,16 +684,18 @@
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;
@@ -726,16 +728,24 @@
* 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;
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