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upstream: ssh-agent side of binding
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record session ID/hostkey/forwarding status for each active socket.

Attempt to parse data-to-be-signed at signature request time and extract
session ID from the blob if it is a pubkey userauth request.

ok markus@

OpenBSD-Commit-ID: a80fd41e292b18b67508362129e9fed549abd318
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@djmdjm
djmdjm committed Dec 19, 2021
1 parent e9497ec commit 4c1e3ce
Showing 1 changed file with 168 additions and 9 deletions.
177 changes: 168 additions & 9 deletions ssh-agent.c
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Original file line number Diff line number Diff line change
@@ -1,4 +1,4 @@
/* $OpenBSD: ssh-agent.c,v 1.279 2021/11/18 03:31:44 djm Exp $ */
/* $OpenBSD: ssh-agent.c,v 1.280 2021/12/19 22:09:23 djm Exp $ */
/*
* Author: Tatu Ylonen <ylo@cs.hut.fi>
* Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
Expand Down Expand Up @@ -98,22 +98,36 @@
#endif

/* Maximum accepted message length */
#define AGENT_MAX_LEN (256*1024)
#define AGENT_MAX_LEN (256*1024)
/* Maximum bytes to read from client socket */
#define AGENT_RBUF_LEN (4096)
#define AGENT_RBUF_LEN (4096)
/* Maximum number of recorded session IDs/hostkeys per connection */
#define AGENT_MAX_SESSION_IDS 16
/* Maximum size of session ID */
#define AGENT_MAX_SID_LEN 128

/* XXX store hostkey_sid in a refcounted tree */

typedef enum {
AUTH_UNUSED = 0,
AUTH_SOCKET = 1,
AUTH_CONNECTION = 2,
} sock_type;

struct hostkey_sid {
struct sshkey *key;
struct sshbuf *sid;
int forwarded;
};

typedef struct socket_entry {
int fd;
sock_type type;
struct sshbuf *input;
struct sshbuf *output;
struct sshbuf *request;
size_t nsession_ids;
struct hostkey_sid *session_ids;
} SocketEntry;

u_int sockets_alloc = 0;
Expand Down Expand Up @@ -174,10 +188,17 @@ static int restrict_websafe = 1;
static void
close_socket(SocketEntry *e)
{
size_t i;

close(e->fd);
sshbuf_free(e->input);
sshbuf_free(e->output);
sshbuf_free(e->request);
for (i = 0; i < e->nsession_ids; i++) {
sshkey_free(e->session_ids[i].key);
sshbuf_free(e->session_ids[i].sid);
}
free(e->session_ids);
memset(e, '\0', sizeof(*e));
e->fd = -1;
e->type = AUTH_UNUSED;
Expand Down Expand Up @@ -422,16 +443,28 @@ check_websafe_message_contents(struct sshkey *key, struct sshbuf *data)
return 0;
}

static int
buf_equal(const struct sshbuf *a, const struct sshbuf *b)
{
if (sshbuf_ptr(a) == NULL || sshbuf_ptr(b) == NULL)
return SSH_ERR_INVALID_ARGUMENT;
if (sshbuf_len(a) != sshbuf_len(b))
return SSH_ERR_INVALID_FORMAT;
if (timingsafe_bcmp(sshbuf_ptr(a), sshbuf_ptr(b), sshbuf_len(a)) != 0)
return SSH_ERR_INVALID_FORMAT;
return 0;
}

/* ssh2 only */
static void
process_sign_request2(SocketEntry *e)
{
u_char *signature = NULL;
size_t slen = 0;
size_t i, slen = 0;
u_int compat = 0, flags;
int r, ok = -1;
char *fp = NULL;
struct sshbuf *msg = NULL, *data = NULL;
char *fp = NULL, *user = NULL, *sig_dest = NULL;
struct sshbuf *msg = NULL, *data = NULL, *sid = NULL;
struct sshkey *key = NULL;
struct identity *id;
struct notifier_ctx *notifier = NULL;
Expand All @@ -451,7 +484,33 @@ process_sign_request2(SocketEntry *e)
verbose_f("%s key not found", sshkey_type(key));
goto send;
}
if (id->confirm && confirm_key(id, NULL) != 0) {
/*
* If session IDs were recorded for this socket, then use them to
* annotate the confirmation messages with the host keys.
*/
if (e->nsession_ids > 0 &&
parse_userauth_request(data, key, &user, &sid) == 0) {
/*
* session ID from userauth request should match the final
* ID in the list recorded in the socket, unless the ssh
* client at that point lacks the binding extension (or if
* an attacker is trying to steal use of the agent).
*/
i = e->nsession_ids - 1;
if (buf_equal(sid, e->session_ids[i].sid) == 0) {
if ((fp = sshkey_fingerprint(e->session_ids[i].key,
SSH_FP_HASH_DEFAULT, SSH_FP_DEFAULT)) == NULL)
fatal_f("fingerprint failed");
debug3_f("destination %s %s (slot %zu)",
sshkey_type(e->session_ids[i].key), fp, i);
xasprintf(&sig_dest, "public key request for "
"target user \"%s\" to %s %s", user,
sshkey_type(e->session_ids[i].key), fp);
free(fp);
fp = NULL;
}
}
if (id->confirm && confirm_key(id, sig_dest) != 0) {
verbose_f("user refused key");
goto send;
}
Expand All @@ -466,8 +525,10 @@ process_sign_request2(SocketEntry *e)
SSH_FP_DEFAULT)) == NULL)
fatal_f("fingerprint failed");
notifier = notify_start(0,
"Confirm user presence for key %s %s",
sshkey_type(id->key), fp);
"Confirm user presence for key %s %s%s%s",
sshkey_type(id->key), fp,
sig_dest == NULL ? "" : "\n",
sig_dest == NULL ? "" : sig_dest);
}
}
/* XXX support PIN required FIDO keys */
Expand All @@ -492,11 +553,14 @@ process_sign_request2(SocketEntry *e)
if ((r = sshbuf_put_stringb(e->output, msg)) != 0)
fatal_fr(r, "enqueue");

sshbuf_free(sid);
sshbuf_free(data);
sshbuf_free(msg);
sshkey_free(key);
free(fp);
free(signature);
free(sig_dest);
free(user);
}

/* shared */
Expand Down Expand Up @@ -958,6 +1022,98 @@ process_remove_smartcard_key(SocketEntry *e)
}
#endif /* ENABLE_PKCS11 */

static int
process_ext_session_bind(SocketEntry *e)
{
int r, sid_match, key_match;
struct sshkey *key = NULL;
struct sshbuf *sid = NULL, *sig = NULL;
char *fp = NULL;
u_char fwd;
size_t i;

debug2_f("entering");
if ((r = sshkey_froms(e->request, &key)) != 0 ||
(r = sshbuf_froms(e->request, &sid)) != 0 ||
(r = sshbuf_froms(e->request, &sig)) != 0 ||
(r = sshbuf_get_u8(e->request, &fwd)) != 0) {
error_fr(r, "parse");
goto out;
}
if ((fp = sshkey_fingerprint(key, SSH_FP_HASH_DEFAULT,
SSH_FP_DEFAULT)) == NULL)
fatal_f("fingerprint failed");
/* check signature with hostkey on session ID */
if ((r = sshkey_verify(key, sshbuf_ptr(sig), sshbuf_len(sig),
sshbuf_ptr(sid), sshbuf_len(sid), NULL, 0, NULL)) != 0) {
error_fr(r, "sshkey_verify for %s %s", sshkey_type(key), fp);
goto out;
}
/* check whether sid/key already recorded */
for (i = 0; i < e->nsession_ids; i++) {
sid_match = buf_equal(sid, e->session_ids[i].sid) == 0;
key_match = sshkey_equal(key, e->session_ids[i].key);
if (sid_match && key_match) {
debug_f("session ID already recorded for %s %s",
sshkey_type(key), fp);
r = 0;
goto out;
} else if (sid_match) {
error_f("session ID recorded against different key "
"for %s %s", sshkey_type(key), fp);
r = -1;
goto out;
}
/*
* new sid with previously-seen key can happen, e.g. multiple
* connections to the same host.
*/
}
/* record new key/sid */
if (e->nsession_ids >= AGENT_MAX_SESSION_IDS) {
error_f("too many session IDs recorded");
goto out;
}
e->session_ids = xrecallocarray(e->session_ids, e->nsession_ids,
e->nsession_ids + 1, sizeof(*e->session_ids));
i = e->nsession_ids++;
debug_f("recorded %s %s (slot %zu of %d)", sshkey_type(key), fp, i,
AGENT_MAX_SESSION_IDS);
e->session_ids[i].key = key;
e->session_ids[i].forwarded = fwd != 0;
key = NULL; /* transferred */
/* can't transfer sid; it's refcounted and scoped to request's life */
if ((e->session_ids[i].sid = sshbuf_new()) == NULL)
fatal_f("sshbuf_new");
if ((r = sshbuf_putb(e->session_ids[i].sid, sid)) != 0)
fatal_fr(r, "sshbuf_putb session ID");
/* success */
r = 0;
out:
sshkey_free(key);
sshbuf_free(sid);
sshbuf_free(sig);
return r == 0 ? 1 : 0;
}

static void
process_extension(SocketEntry *e)
{
int r, success = 0;
char *name;

debug2_f("entering");
if ((r = sshbuf_get_cstring(e->request, &name, NULL)) != 0) {
error_fr(r, "parse");
goto send;
}
if (strcmp(name, "session-bind@openssh.com") == 0)
success = process_ext_session_bind(e);
else
debug_f("unsupported extension \"%s\"", name);
send:
send_status(e, success);
}
/*
* dispatch incoming message.
* returns 1 on success, 0 for incomplete messages or -1 on error.
Expand Down Expand Up @@ -1050,6 +1206,9 @@ process_message(u_int socknum)
process_remove_smartcard_key(e);
break;
#endif /* ENABLE_PKCS11 */
case SSH_AGENTC_EXTENSION:
process_extension(e);
break;
default:
/* Unknown message. Respond with failure. */
error("Unknown message %d", type);
Expand Down

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