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putty-source/ssh1connection-client.c
Simon Tatham b4e1110892 Relax criteria for accepting agent-forwarding channel-opens. 
Previously, the instant at which we send to the server a request to
enable agent forwarding (the "auth-agent-req@openssh.com" channel
request, or SSH1_CMSG_AGENT_REQUEST_FORWARDING) was also the instant
at which we set a flag indicating that we're prepared to accept
attempts from the server to open a channel to talk to the forwarded
agent. If the server attempts that when we haven't sent a forwarding
request, we treat it with suspicion, and reject it.

But it turns out that at least one SSH server does this, for what
seems to be a _somewhat_ sensible purpose, and OpenSSH accepts it. So,
on the basis that the @openssh.com domain suffix makes them the
arbiters of this part of the spec, I'm following their practice. I've
removed the 'agent_fwd_enabled' flag from both connection layer
implementations, together with the ConnectionLayer method that sets
it; now agent-forwarding CHANNEL_OPENs are gated only on the questions
of whether agent forwarding was permitted in the configuration and
whether an agent actually exists to talk to, and not also whether we
had previously sent a message to the server announcing it.

(The change to this condition is also applied in the SSH-1 agent
forwarding code, mostly for the sake of keeping things parallel where
possible. I think it doesn't actually make a difference in SSH-1,
because in SSH-1, it's not _possible_ for the server to try to open an
agent channel before the main channel is set up, due to the entirely
separate setup phase of the protocol.)

The use case is a proxy host which makes a secondary SSH connection to
a real destination host. A user has run into one of these recently,
announcing a version banner of "SSH-2.0-FudoSSH", which relies on
agent forwarding to authenticate the secondary connection. You connect
to the proxy host and authenticate with a username string of the form
"realusername#real.destination.host", and then, at the start of the
connection protocol, the server immediately opens a channel back to
your SSH agent which it uses to authenticate to the destination host.
And it delays answering any CHANNEL_OPEN requests from the client
until that's all done. For example (seen from the client's POV,
although the server's CHANNEL_OPEN may well have been _sent_ up front
rather than in response to the client's):

client: SSH2_MSG_CHANNEL_OPEN "session"
server: SSH2_MSG_CHANNEL_OPEN "auth-agent@openssh.com"
client: SSH2_MSG_CHANNEL_OPEN_CONFIRMATION to the auth-agent request
        <- data is exchanged on the agent channel; proxy host uses
           that signature to log in to the destination host ->
server: SSH2_MSG_CHANNEL_OPEN_CONFIRMATION to the session request

With PuTTY, this wasn't working, because at the point when the server
sends the auth-agent CHANNEL_OPEN, we had not yet had any opportunity
to send auth-agent-req (because that has to wait until we've had a
CHANNEL_OPEN_CONFIRMATION). So we were rejecting the server's
CHANNEL_OPEN, which broke this workflow:

client: SSH2_MSG_CHANNEL_OPEN "session"
server: SSH2_MSG_CHANNEL_OPEN "auth-agent@openssh.com"
client: SSH2_MSG_CHANNEL_OPEN_FAILURE to the auth-agent request
        (hey, I haven't told you you can do that yet!)
server: SSH2_MSG_CHANNEL_OPEN_FAILURE to the session request
        (in that case, no shell session for you!)
2020-12-23 22:26:44 +00:00

548 lines
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/*
* Client-specific parts of the SSH-1 connection layer.
*/
#include <assert.h>
#include "putty.h"
#include "ssh.h"
#include "sshbpp.h"
#include "sshppl.h"
#include "sshchan.h"
#include "sshcr.h"
#include "ssh1connection.h"
void ssh1_connection_direction_specific_setup(
struct ssh1_connection_state *s)
{
if (!s->mainchan) {
/*
* Start up the main session, by telling mainchan.c to do it
* all just as it would in SSH-2, and translating those
* concepts to SSH-1's non-channel-shaped idea of the main
* session.
*/
s->mainchan = mainchan_new(
&s->ppl, &s->cl, s->conf, s->term_width, s->term_height,
false /* is_simple */, NULL);
}
}
typedef void (*sf_handler_fn_t)(struct ssh1_connection_state *s,
bool success, void *ctx);
struct outstanding_succfail {
sf_handler_fn_t handler;
void *ctx;
struct outstanding_succfail *next;
/*
* The 'trivial' flag is set if this handler is in response to a
* request for which the SSH-1 protocol doesn't actually specify a
* response packet. The client of this system (mainchan.c) will
* expect to get an acknowledgment regardless, so we arrange to
* send that ack immediately after the rest of the queue empties.
*/
bool trivial;
};
static void ssh1_connection_process_trivial_succfails(void *vs);
static void ssh1_queue_succfail_handler(
struct ssh1_connection_state *s, sf_handler_fn_t handler, void *ctx,
bool trivial)
{
struct outstanding_succfail *osf = snew(struct outstanding_succfail);
osf->handler = handler;
osf->ctx = ctx;
osf->trivial = trivial;
osf->next = NULL;
if (s->succfail_tail)
s->succfail_tail->next = osf;
else
s->succfail_head = osf;
s->succfail_tail = osf;
/* In case this one was trivial and the queue was already empty,
* we should make sure we run the handler promptly, and the
* easiest way is to queue it anyway and then run a trivials pass
* by callback. */
queue_toplevel_callback(ssh1_connection_process_trivial_succfails, s);
}
static void ssh1_connection_process_succfail(
struct ssh1_connection_state *s, bool success)
{
struct outstanding_succfail *prevhead = s->succfail_head;
s->succfail_head = s->succfail_head->next;
if (!s->succfail_head)
s->succfail_tail = NULL;
prevhead->handler(s, success, prevhead->ctx);
sfree(prevhead);
}
static void ssh1_connection_process_trivial_succfails(void *vs)
{
struct ssh1_connection_state *s = (struct ssh1_connection_state *)vs;
while (s->succfail_head && s->succfail_head->trivial)
ssh1_connection_process_succfail(s, true);
}
bool ssh1_handle_direction_specific_packet(
struct ssh1_connection_state *s, PktIn *pktin)
{
PacketProtocolLayer *ppl = &s->ppl; /* for ppl_logevent */
PktOut *pktout;
struct ssh1_channel *c;
unsigned remid;
struct ssh_rportfwd pf, *pfp;
ptrlen host, data;
int port;
switch (pktin->type) {
case SSH1_SMSG_SUCCESS:
case SSH1_SMSG_FAILURE:
if (!s->succfail_head) {
ssh_remote_error(s->ppl.ssh,
"Received %s with no outstanding request",
ssh1_pkt_type(pktin->type));
return true;
}
ssh1_connection_process_succfail(
s, pktin->type == SSH1_SMSG_SUCCESS);
queue_toplevel_callback(
ssh1_connection_process_trivial_succfails, s);
return true;
case SSH1_SMSG_X11_OPEN:
remid = get_uint32(pktin);
/* Refuse if X11 forwarding is disabled. */
if (!s->X11_fwd_enabled) {
pktout = ssh_bpp_new_pktout(
s->ppl.bpp, SSH1_MSG_CHANNEL_OPEN_FAILURE);
put_uint32(pktout, remid);
pq_push(s->ppl.out_pq, pktout);
ppl_logevent("Rejected X11 connect request");
} else {
c = snew(struct ssh1_channel);
c->connlayer = s;
ssh1_channel_init(c);
c->remoteid = remid;
c->chan = x11_new_channel(s->x11authtree, &c->sc,
NULL, -1, false);
c->remoteid = remid;
c->halfopen = false;
pktout = ssh_bpp_new_pktout(
s->ppl.bpp, SSH1_MSG_CHANNEL_OPEN_CONFIRMATION);
put_uint32(pktout, c->remoteid);
put_uint32(pktout, c->localid);
pq_push(s->ppl.out_pq, pktout);
ppl_logevent("Opened X11 forward channel");
}
return true;
case SSH1_SMSG_AGENT_OPEN:
remid = get_uint32(pktin);
/* Refuse if agent forwarding is disabled. */
if (!ssh_agent_forwarding_permitted(&s->cl)) {
pktout = ssh_bpp_new_pktout(
s->ppl.bpp, SSH1_MSG_CHANNEL_OPEN_FAILURE);
put_uint32(pktout, remid);
pq_push(s->ppl.out_pq, pktout);
} else {
c = snew(struct ssh1_channel);
c->connlayer = s;
ssh1_channel_init(c);
c->remoteid = remid;
c->halfopen = false;
/*
* If possible, make a stream-oriented connection to the
* agent and set up an ordinary port-forwarding type
* channel over it.
*/
Plug *plug;
Channel *ch = portfwd_raw_new(&s->cl, &plug, true);
Socket *skt = agent_connect(plug);
if (!sk_socket_error(skt)) {
portfwd_raw_setup(ch, skt, &c->sc);
c->chan = ch;
} else {
portfwd_raw_free(ch);
/*
* Otherwise, fall back to the old-fashioned system of
* parsing the forwarded data stream ourselves for
* message boundaries, and passing each individual
* message to the one-off agent_query().
*/
c->chan = agentf_new(&c->sc);
}
pktout = ssh_bpp_new_pktout(
s->ppl.bpp, SSH1_MSG_CHANNEL_OPEN_CONFIRMATION);
put_uint32(pktout, c->remoteid);
put_uint32(pktout, c->localid);
pq_push(s->ppl.out_pq, pktout);
}
return true;
case SSH1_MSG_PORT_OPEN:
remid = get_uint32(pktin);
host = get_string(pktin);
port = toint(get_uint32(pktin));
pf.dhost = mkstr(host);
pf.dport = port;
pfp = find234(s->rportfwds, &pf, NULL);
if (!pfp) {
ppl_logevent("Rejected remote port open request for %s:%d",
pf.dhost, port);
pktout = ssh_bpp_new_pktout(
s->ppl.bpp, SSH1_MSG_CHANNEL_OPEN_FAILURE);
put_uint32(pktout, remid);
pq_push(s->ppl.out_pq, pktout);
} else {
char *err;
c = snew(struct ssh1_channel);
c->connlayer = s;
ppl_logevent("Received remote port open request for %s:%d",
pf.dhost, port);
err = portfwdmgr_connect(
s->portfwdmgr, &c->chan, pf.dhost, port,
&c->sc, pfp->addressfamily);
if (err) {
ppl_logevent("Port open failed: %s", err);
sfree(err);
ssh1_channel_free(c);
pktout = ssh_bpp_new_pktout(
s->ppl.bpp, SSH1_MSG_CHANNEL_OPEN_FAILURE);
put_uint32(pktout, remid);
pq_push(s->ppl.out_pq, pktout);
} else {
ssh1_channel_init(c);
c->remoteid = remid;
c->halfopen = false;
pktout = ssh_bpp_new_pktout(
s->ppl.bpp, SSH1_MSG_CHANNEL_OPEN_CONFIRMATION);
put_uint32(pktout, c->remoteid);
put_uint32(pktout, c->localid);
pq_push(s->ppl.out_pq, pktout);
ppl_logevent("Forwarded port opened successfully");
}
}
sfree(pf.dhost);
return true;
case SSH1_SMSG_STDOUT_DATA:
case SSH1_SMSG_STDERR_DATA:
data = get_string(pktin);
if (!get_err(pktin)) {
int bufsize = seat_output(
s->ppl.seat, pktin->type == SSH1_SMSG_STDERR_DATA,
data.ptr, data.len);
if (!s->stdout_throttling && bufsize > SSH1_BUFFER_LIMIT) {
s->stdout_throttling = true;
ssh_throttle_conn(s->ppl.ssh, +1);
}
}
return true;
case SSH1_SMSG_EXIT_STATUS: {
int exitcode = get_uint32(pktin);
ppl_logevent("Server sent command exit status %d", exitcode);
ssh_got_exitcode(s->ppl.ssh, exitcode);
s->session_terminated = true;
return true;
}
default:
return false;
}
}
static void ssh1mainchan_succfail_wantreply(struct ssh1_connection_state *s,
bool success, void *ctx)
{
chan_request_response(s->mainchan_chan, success);
}
static void ssh1mainchan_succfail_nowantreply(struct ssh1_connection_state *s,
bool success, void *ctx)
{
}
static void ssh1mainchan_queue_response(struct ssh1_connection_state *s,
bool want_reply, bool trivial)
{
sf_handler_fn_t handler = (want_reply ? ssh1mainchan_succfail_wantreply :
ssh1mainchan_succfail_nowantreply);
ssh1_queue_succfail_handler(s, handler, NULL, trivial);
}
static void ssh1mainchan_request_x11_forwarding(
SshChannel *sc, bool want_reply, const char *authproto,
const char *authdata, int screen_number, bool oneshot)
{
struct ssh1_connection_state *s =
container_of(sc, struct ssh1_connection_state, mainchan_sc);
PktOut *pktout;
pktout = ssh_bpp_new_pktout(s->ppl.bpp, SSH1_CMSG_X11_REQUEST_FORWARDING);
put_stringz(pktout, authproto);
put_stringz(pktout, authdata);
if (s->local_protoflags & SSH1_PROTOFLAG_SCREEN_NUMBER)
put_uint32(pktout, screen_number);
pq_push(s->ppl.out_pq, pktout);
ssh1mainchan_queue_response(s, want_reply, false);
}
static void ssh1mainchan_request_agent_forwarding(
SshChannel *sc, bool want_reply)
{
struct ssh1_connection_state *s =
container_of(sc, struct ssh1_connection_state, mainchan_sc);
PktOut *pktout;
pktout = ssh_bpp_new_pktout(
s->ppl.bpp, SSH1_CMSG_AGENT_REQUEST_FORWARDING);
pq_push(s->ppl.out_pq, pktout);
ssh1mainchan_queue_response(s, want_reply, false);
}
static void ssh1mainchan_request_pty(
SshChannel *sc, bool want_reply, Conf *conf, int w, int h)
{
struct ssh1_connection_state *s =
container_of(sc, struct ssh1_connection_state, mainchan_sc);
PktOut *pktout;
pktout = ssh_bpp_new_pktout(s->ppl.bpp, SSH1_CMSG_REQUEST_PTY);
put_stringz(pktout, conf_get_str(s->conf, CONF_termtype));
put_uint32(pktout, h);
put_uint32(pktout, w);
put_uint32(pktout, 0); /* width in pixels */
put_uint32(pktout, 0); /* height in pixels */
write_ttymodes_to_packet(
BinarySink_UPCAST(pktout), 1,
get_ttymodes_from_conf(s->ppl.seat, conf));
pq_push(s->ppl.out_pq, pktout);
ssh1mainchan_queue_response(s, want_reply, false);
}
static bool ssh1mainchan_send_env_var(
SshChannel *sc, bool want_reply, const char *var, const char *value)
{
return false; /* SSH-1 doesn't support this at all */
}
static void ssh1mainchan_start_shell(SshChannel *sc, bool want_reply)
{
struct ssh1_connection_state *s =
container_of(sc, struct ssh1_connection_state, mainchan_sc);
PktOut *pktout;
pktout = ssh_bpp_new_pktout(s->ppl.bpp, SSH1_CMSG_EXEC_SHELL);
pq_push(s->ppl.out_pq, pktout);
ssh1mainchan_queue_response(s, want_reply, true);
}
static void ssh1mainchan_start_command(
SshChannel *sc, bool want_reply, const char *command)
{
struct ssh1_connection_state *s =
container_of(sc, struct ssh1_connection_state, mainchan_sc);
PktOut *pktout;
pktout = ssh_bpp_new_pktout(s->ppl.bpp, SSH1_CMSG_EXEC_CMD);
put_stringz(pktout, command);
pq_push(s->ppl.out_pq, pktout);
ssh1mainchan_queue_response(s, want_reply, true);
}
static bool ssh1mainchan_start_subsystem(
SshChannel *sc, bool want_reply, const char *subsystem)
{
return false; /* SSH-1 doesn't support this at all */
}
static bool ssh1mainchan_send_serial_break(
SshChannel *sc, bool want_reply, int length)
{
return false; /* SSH-1 doesn't support this at all */
}
static bool ssh1mainchan_send_signal(
SshChannel *sc, bool want_reply, const char *signame)
{
return false; /* SSH-1 doesn't support this at all */
}
static void ssh1mainchan_send_terminal_size_change(
SshChannel *sc, int w, int h)
{
struct ssh1_connection_state *s =
container_of(sc, struct ssh1_connection_state, mainchan_sc);
PktOut *pktout;
pktout = ssh_bpp_new_pktout(s->ppl.bpp, SSH1_CMSG_WINDOW_SIZE);
put_uint32(pktout, h);
put_uint32(pktout, w);
put_uint32(pktout, 0); /* width in pixels */
put_uint32(pktout, 0); /* height in pixels */
pq_push(s->ppl.out_pq, pktout);
}
static void ssh1mainchan_hint_channel_is_simple(SshChannel *sc)
{
}
static size_t ssh1mainchan_write(
SshChannel *sc, bool is_stderr, const void *data, size_t len)
{
struct ssh1_connection_state *s =
container_of(sc, struct ssh1_connection_state, mainchan_sc);
PktOut *pktout;
pktout = ssh_bpp_new_pktout(s->ppl.bpp, SSH1_CMSG_STDIN_DATA);
put_string(pktout, data, len);
pq_push(s->ppl.out_pq, pktout);
return 0;
}
static void ssh1mainchan_write_eof(SshChannel *sc)
{
struct ssh1_connection_state *s =
container_of(sc, struct ssh1_connection_state, mainchan_sc);
PktOut *pktout;
pktout = ssh_bpp_new_pktout(s->ppl.bpp, SSH1_CMSG_EOF);
pq_push(s->ppl.out_pq, pktout);
}
static const SshChannelVtable ssh1mainchan_vtable = {
.write = ssh1mainchan_write,
.write_eof = ssh1mainchan_write_eof,
.request_x11_forwarding = ssh1mainchan_request_x11_forwarding,
.request_agent_forwarding = ssh1mainchan_request_agent_forwarding,
.request_pty = ssh1mainchan_request_pty,
.send_env_var = ssh1mainchan_send_env_var,
.start_shell = ssh1mainchan_start_shell,
.start_command = ssh1mainchan_start_command,
.start_subsystem = ssh1mainchan_start_subsystem,
.send_serial_break = ssh1mainchan_send_serial_break,
.send_signal = ssh1mainchan_send_signal,
.send_terminal_size_change = ssh1mainchan_send_terminal_size_change,
.hint_channel_is_simple = ssh1mainchan_hint_channel_is_simple,
/* other methods are NULL */
};
static void ssh1_session_confirm_callback(void *vctx)
{
struct ssh1_connection_state *s = (struct ssh1_connection_state *)vctx;
chan_open_confirmation(s->mainchan_chan);
}
SshChannel *ssh1_session_open(ConnectionLayer *cl, Channel *chan)
{
struct ssh1_connection_state *s =
container_of(cl, struct ssh1_connection_state, cl);
s->mainchan_sc.vt = &ssh1mainchan_vtable;
s->mainchan_sc.cl = &s->cl;
s->mainchan_chan = chan;
queue_toplevel_callback(ssh1_session_confirm_callback, s);
return &s->mainchan_sc;
}
static void ssh1_rportfwd_response(struct ssh1_connection_state *s,
bool success, void *ctx)
{
PacketProtocolLayer *ppl = &s->ppl; /* for ppl_logevent */
struct ssh_rportfwd *rpf = (struct ssh_rportfwd *)ctx;
if (success) {
ppl_logevent("Remote port forwarding from %s enabled",
rpf->log_description);
} else {
ppl_logevent("Remote port forwarding from %s refused",
rpf->log_description);
struct ssh_rportfwd *realpf = del234(s->rportfwds, rpf);
assert(realpf == rpf);
portfwdmgr_close(s->portfwdmgr, rpf->pfr);
free_rportfwd(rpf);
}
}
struct ssh_rportfwd *ssh1_rportfwd_alloc(
ConnectionLayer *cl,
const char *shost, int sport, const char *dhost, int dport,
int addressfamily, const char *log_description, PortFwdRecord *pfr,
ssh_sharing_connstate *share_ctx)
{
struct ssh1_connection_state *s =
container_of(cl, struct ssh1_connection_state, cl);
struct ssh_rportfwd *rpf = snew(struct ssh_rportfwd);
rpf->shost = dupstr(shost);
rpf->sport = sport;
rpf->dhost = dupstr(dhost);
rpf->dport = dport;
rpf->addressfamily = addressfamily;
rpf->log_description = dupstr(log_description);
rpf->pfr = pfr;
if (add234(s->rportfwds, rpf) != rpf) {
free_rportfwd(rpf);
return NULL;
}
PktOut *pktout = ssh_bpp_new_pktout(
s->ppl.bpp, SSH1_CMSG_PORT_FORWARD_REQUEST);
put_uint32(pktout, rpf->sport);
put_stringz(pktout, rpf->dhost);
put_uint32(pktout, rpf->dport);
pq_push(s->ppl.out_pq, pktout);
ssh1_queue_succfail_handler(s, ssh1_rportfwd_response, rpf, false);
return rpf;
}
SshChannel *ssh1_serverside_x11_open(
ConnectionLayer *cl, Channel *chan, const SocketPeerInfo *pi)
{
unreachable("Should never be called in the client");
}
SshChannel *ssh1_serverside_agent_open(ConnectionLayer *cl, Channel *chan)
{
unreachable("Should never be called in the client");
}
bool ssh1_connection_need_antispoof_prompt(struct ssh1_connection_state *s)
{
return !seat_set_trust_status(s->ppl.seat, false);
}
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