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putty-source/ssh2connection-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

506 lines
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/*
* Client-specific parts of the SSH-2 connection layer.
*/
#include <assert.h>
#include "putty.h"
#include "ssh.h"
#include "sshbpp.h"
#include "sshppl.h"
#include "sshchan.h"
#include "sshcr.h"
#include "ssh2connection.h"
static ChanopenResult chan_open_x11(
struct ssh2_connection_state *s, SshChannel *sc,
ptrlen peeraddr, int peerport)
{
PacketProtocolLayer *ppl = &s->ppl; /* for ppl_logevent */
char *peeraddr_str;
Channel *ch;
ppl_logevent("Received X11 connect request from %.*s:%d",
PTRLEN_PRINTF(peeraddr), peerport);
if (!s->X11_fwd_enabled && !s->connshare) {
CHANOPEN_RETURN_FAILURE(
SSH2_OPEN_ADMINISTRATIVELY_PROHIBITED,
("X11 forwarding is not enabled"));
}
peeraddr_str = peeraddr.ptr ? mkstr(peeraddr) : NULL;
ch = x11_new_channel(
s->x11authtree, sc, peeraddr_str, peerport, s->connshare != NULL);
sfree(peeraddr_str);
ppl_logevent("Opened X11 forward channel");
CHANOPEN_RETURN_SUCCESS(ch);
}
static ChanopenResult chan_open_forwarded_tcpip(
struct ssh2_connection_state *s, SshChannel *sc,
ptrlen fwdaddr, int fwdport, ptrlen peeraddr, int peerport)
{
PacketProtocolLayer *ppl = &s->ppl; /* for ppl_logevent */
struct ssh_rportfwd pf, *realpf;
Channel *ch;
char *err;
ppl_logevent("Received remote port %.*s:%d open request from %.*s:%d",
PTRLEN_PRINTF(fwdaddr), fwdport,
PTRLEN_PRINTF(peeraddr), peerport);
pf.shost = mkstr(fwdaddr);
pf.sport = fwdport;
realpf = find234(s->rportfwds, &pf, NULL);
sfree(pf.shost);
if (realpf == NULL) {
CHANOPEN_RETURN_FAILURE(
SSH2_OPEN_ADMINISTRATIVELY_PROHIBITED,
("Remote port is not recognised"));
}
if (realpf->share_ctx) {
/*
* This port forwarding is on behalf of a connection-sharing
* downstream.
*/
CHANOPEN_RETURN_DOWNSTREAM(realpf->share_ctx);
}
err = portfwdmgr_connect(
s->portfwdmgr, &ch, realpf->dhost, realpf->dport,
sc, realpf->addressfamily);
ppl_logevent("Attempting to forward remote port to %s:%d",
realpf->dhost, realpf->dport);
if (err != NULL) {
ppl_logevent("Port open failed: %s", err);
sfree(err);
CHANOPEN_RETURN_FAILURE(
SSH2_OPEN_CONNECT_FAILED,
("Port open failed"));
}
ppl_logevent("Forwarded port opened successfully");
CHANOPEN_RETURN_SUCCESS(ch);
}
static ChanopenResult chan_open_auth_agent(
struct ssh2_connection_state *s, SshChannel *sc)
{
if (!ssh_agent_forwarding_permitted(&s->cl)) {
CHANOPEN_RETURN_FAILURE(
SSH2_OPEN_ADMINISTRATIVELY_PROHIBITED,
("Agent forwarding is not enabled"));
}
/*
* 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, sc);
CHANOPEN_RETURN_SUCCESS(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().
*/
CHANOPEN_RETURN_SUCCESS(agentf_new(sc));
}
}
ChanopenResult ssh2_connection_parse_channel_open(
struct ssh2_connection_state *s, ptrlen type,
PktIn *pktin, SshChannel *sc)
{
if (ptrlen_eq_string(type, "x11")) {
ptrlen peeraddr = get_string(pktin);
int peerport = get_uint32(pktin);
return chan_open_x11(s, sc, peeraddr, peerport);
} else if (ptrlen_eq_string(type, "forwarded-tcpip")) {
ptrlen fwdaddr = get_string(pktin);
int fwdport = toint(get_uint32(pktin));
ptrlen peeraddr = get_string(pktin);
int peerport = toint(get_uint32(pktin));
return chan_open_forwarded_tcpip(
s, sc, fwdaddr, fwdport, peeraddr, peerport);
} else if (ptrlen_eq_string(type, "auth-agent@openssh.com")) {
return chan_open_auth_agent(s, sc);
} else {
CHANOPEN_RETURN_FAILURE(
SSH2_OPEN_UNKNOWN_CHANNEL_TYPE,
("Unsupported channel type requested"));
}
}
bool ssh2_connection_parse_global_request(
struct ssh2_connection_state *s, ptrlen type, PktIn *pktin)
{
/*
* We don't know of any global requests that an SSH client needs
* to honour.
*/
return false;
}
PktOut *ssh2_portfwd_chanopen(
struct ssh2_connection_state *s, struct ssh2_channel *c,
const char *hostname, int port,
const char *description, const SocketPeerInfo *peerinfo)
{
PacketProtocolLayer *ppl = &s->ppl; /* for ppl_logevent */
PktOut *pktout;
/*
* In client mode, this function is called by portfwdmgr in
* response to PortListeners that were set up in
* portfwdmgr_config, which means that the hostname and port
* parameters will indicate the host we want to tell the server to
* connect _to_.
*/
ppl_logevent("Opening connection to %s:%d for %s",
hostname, port, description);
pktout = ssh2_chanopen_init(c, "direct-tcpip");
{
char *trimmed_host = host_strduptrim(hostname);
put_stringz(pktout, trimmed_host);
sfree(trimmed_host);
}
put_uint32(pktout, port);
/*
* We make up values for the originator data; partly it's too much
* hassle to keep track, and partly I'm not convinced the server
* should be told details like that about my local network
* configuration. The "originator IP address" is syntactically a
* numeric IP address, and some servers (e.g., Tectia) get upset
* if it doesn't match this syntax.
*/
put_stringz(pktout, "0.0.0.0");
put_uint32(pktout, 0);
return pktout;
}
static int ssh2_rportfwd_cmp(void *av, void *bv)
{
struct ssh_rportfwd *a = (struct ssh_rportfwd *) av;
struct ssh_rportfwd *b = (struct ssh_rportfwd *) bv;
int i;
if ( (i = strcmp(a->shost, b->shost)) != 0)
return i < 0 ? -1 : +1;
if (a->sport > b->sport)
return +1;
if (a->sport < b->sport)
return -1;
return 0;
}
static void ssh2_rportfwd_globreq_response(struct ssh2_connection_state *s,
PktIn *pktin, void *ctx)
{
PacketProtocolLayer *ppl = &s->ppl; /* for ppl_logevent */
struct ssh_rportfwd *rpf = (struct ssh_rportfwd *)ctx;
if (pktin->type == SSH2_MSG_REQUEST_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 *ssh2_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 ssh2_connection_state *s =
container_of(cl, struct ssh2_connection_state, cl);
struct ssh_rportfwd *rpf = snew(struct ssh_rportfwd);
if (!s->rportfwds)
s->rportfwds = newtree234(ssh2_rportfwd_cmp);
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;
rpf->share_ctx = share_ctx;
if (add234(s->rportfwds, rpf) != rpf) {
free_rportfwd(rpf);
return NULL;
}
if (!rpf->share_ctx) {
PktOut *pktout = ssh_bpp_new_pktout(
s->ppl.bpp, SSH2_MSG_GLOBAL_REQUEST);
put_stringz(pktout, "tcpip-forward");
put_bool(pktout, true); /* want reply */
put_stringz(pktout, rpf->shost);
put_uint32(pktout, rpf->sport);
pq_push(s->ppl.out_pq, pktout);
ssh2_queue_global_request_handler(
s, ssh2_rportfwd_globreq_response, rpf);
}
return rpf;
}
void ssh2_rportfwd_remove(ConnectionLayer *cl, struct ssh_rportfwd *rpf)
{
struct ssh2_connection_state *s =
container_of(cl, struct ssh2_connection_state, cl);
if (rpf->share_ctx) {
/*
* We don't manufacture a cancel-tcpip-forward message for
* remote port forwardings being removed on behalf of a
* downstream; we just pass through the one the downstream
* sent to us.
*/
} else {
PktOut *pktout = ssh_bpp_new_pktout(
s->ppl.bpp, SSH2_MSG_GLOBAL_REQUEST);
put_stringz(pktout, "cancel-tcpip-forward");
put_bool(pktout, false); /* _don't_ want reply */
put_stringz(pktout, rpf->shost);
put_uint32(pktout, rpf->sport);
pq_push(s->ppl.out_pq, pktout);
}
assert(s->rportfwds);
struct ssh_rportfwd *realpf = del234(s->rportfwds, rpf);
assert(realpf == rpf);
free_rportfwd(rpf);
}
SshChannel *ssh2_session_open(ConnectionLayer *cl, Channel *chan)
{
struct ssh2_connection_state *s =
container_of(cl, struct ssh2_connection_state, cl);
PacketProtocolLayer *ppl = &s->ppl; /* for ppl_logevent */
struct ssh2_channel *c = snew(struct ssh2_channel);
PktOut *pktout;
c->connlayer = s;
ssh2_channel_init(c);
c->halfopen = true;
c->chan = chan;
ppl_logevent("Opening main session channel");
pktout = ssh2_chanopen_init(c, "session");
pq_push(s->ppl.out_pq, pktout);
return &c->sc;
}
SshChannel *ssh2_serverside_x11_open(
ConnectionLayer *cl, Channel *chan, const SocketPeerInfo *pi)
{
unreachable("Should never be called in the client");
}
SshChannel *ssh2_serverside_agent_open(ConnectionLayer *cl, Channel *chan)
{
unreachable("Should never be called in the client");
}
static void ssh2_channel_response(
struct ssh2_channel *c, PktIn *pkt, void *ctx)
{
/* If pkt==NULL (because this handler has been called in response
* to CHANNEL_CLOSE arriving while the request was still
* outstanding), we treat that the same as CHANNEL_FAILURE. */
chan_request_response(c->chan,
pkt && pkt->type == SSH2_MSG_CHANNEL_SUCCESS);
}
void ssh2channel_start_shell(SshChannel *sc, bool want_reply)
{
struct ssh2_channel *c = container_of(sc, struct ssh2_channel, sc);
struct ssh2_connection_state *s = c->connlayer;
PktOut *pktout = ssh2_chanreq_init(
c, "shell", want_reply ? ssh2_channel_response : NULL, NULL);
pq_push(s->ppl.out_pq, pktout);
}
void ssh2channel_start_command(
SshChannel *sc, bool want_reply, const char *command)
{
struct ssh2_channel *c = container_of(sc, struct ssh2_channel, sc);
struct ssh2_connection_state *s = c->connlayer;
PktOut *pktout = ssh2_chanreq_init(
c, "exec", want_reply ? ssh2_channel_response : NULL, NULL);
put_stringz(pktout, command);
pq_push(s->ppl.out_pq, pktout);
}
bool ssh2channel_start_subsystem(
SshChannel *sc, bool want_reply, const char *subsystem)
{
struct ssh2_channel *c = container_of(sc, struct ssh2_channel, sc);
struct ssh2_connection_state *s = c->connlayer;
PktOut *pktout = ssh2_chanreq_init(
c, "subsystem", want_reply ? ssh2_channel_response : NULL, NULL);
put_stringz(pktout, subsystem);
pq_push(s->ppl.out_pq, pktout);
return true;
}
void ssh2channel_send_exit_status(SshChannel *sc, int status)
{
unreachable("Should never be called in the client");
}
void ssh2channel_send_exit_signal(
SshChannel *sc, ptrlen signame, bool core_dumped, ptrlen msg)
{
unreachable("Should never be called in the client");
}
void ssh2channel_send_exit_signal_numeric(
SshChannel *sc, int signum, bool core_dumped, ptrlen msg)
{
unreachable("Should never be called in the client");
}
void ssh2channel_request_x11_forwarding(
SshChannel *sc, bool want_reply, const char *authproto,
const char *authdata, int screen_number, bool oneshot)
{
struct ssh2_channel *c = container_of(sc, struct ssh2_channel, sc);
struct ssh2_connection_state *s = c->connlayer;
PktOut *pktout = ssh2_chanreq_init(
c, "x11-req", want_reply ? ssh2_channel_response : NULL, NULL);
put_bool(pktout, oneshot);
put_stringz(pktout, authproto);
put_stringz(pktout, authdata);
put_uint32(pktout, screen_number);
pq_push(s->ppl.out_pq, pktout);
}
void ssh2channel_request_agent_forwarding(SshChannel *sc, bool want_reply)
{
struct ssh2_channel *c = container_of(sc, struct ssh2_channel, sc);
struct ssh2_connection_state *s = c->connlayer;
PktOut *pktout = ssh2_chanreq_init(
c, "auth-agent-req@openssh.com",
want_reply ? ssh2_channel_response : NULL, NULL);
pq_push(s->ppl.out_pq, pktout);
}
void ssh2channel_request_pty(
SshChannel *sc, bool want_reply, Conf *conf, int w, int h)
{
struct ssh2_channel *c = container_of(sc, struct ssh2_channel, sc);
struct ssh2_connection_state *s = c->connlayer;
strbuf *modebuf;
PktOut *pktout = ssh2_chanreq_init(
c, "pty-req", want_reply ? ssh2_channel_response : NULL, NULL);
put_stringz(pktout, conf_get_str(conf, CONF_termtype));
put_uint32(pktout, w);
put_uint32(pktout, h);
put_uint32(pktout, 0); /* pixel width */
put_uint32(pktout, 0); /* pixel height */
modebuf = strbuf_new();
write_ttymodes_to_packet(
BinarySink_UPCAST(modebuf), 2,
get_ttymodes_from_conf(s->ppl.seat, conf));
put_stringsb(pktout, modebuf);
pq_push(s->ppl.out_pq, pktout);
}
bool ssh2channel_send_env_var(
SshChannel *sc, bool want_reply, const char *var, const char *value)
{
struct ssh2_channel *c = container_of(sc, struct ssh2_channel, sc);
struct ssh2_connection_state *s = c->connlayer;
PktOut *pktout = ssh2_chanreq_init(
c, "env", want_reply ? ssh2_channel_response : NULL, NULL);
put_stringz(pktout, var);
put_stringz(pktout, value);
pq_push(s->ppl.out_pq, pktout);
return true;
}
bool ssh2channel_send_serial_break(SshChannel *sc, bool want_reply, int length)
{
struct ssh2_channel *c = container_of(sc, struct ssh2_channel, sc);
struct ssh2_connection_state *s = c->connlayer;
PktOut *pktout = ssh2_chanreq_init(
c, "break", want_reply ? ssh2_channel_response : NULL, NULL);
put_uint32(pktout, length);
pq_push(s->ppl.out_pq, pktout);
return true;
}
bool ssh2channel_send_signal(
SshChannel *sc, bool want_reply, const char *signame)
{
struct ssh2_channel *c = container_of(sc, struct ssh2_channel, sc);
struct ssh2_connection_state *s = c->connlayer;
PktOut *pktout = ssh2_chanreq_init(
c, "signal", want_reply ? ssh2_channel_response : NULL, NULL);
put_stringz(pktout, signame);
pq_push(s->ppl.out_pq, pktout);
return true;
}
void ssh2channel_send_terminal_size_change(SshChannel *sc, int w, int h)
{
struct ssh2_channel *c = container_of(sc, struct ssh2_channel, sc);
struct ssh2_connection_state *s = c->connlayer;
PktOut *pktout = ssh2_chanreq_init(c, "window-change", NULL, NULL);
put_uint32(pktout, w);
put_uint32(pktout, h);
put_uint32(pktout, 0); /* pixel width */
put_uint32(pktout, 0); /* pixel height */
pq_push(s->ppl.out_pq, pktout);
}
bool ssh2_connection_need_antispoof_prompt(struct ssh2_connection_state *s)
{
bool success = seat_set_trust_status(s->ppl.seat, false);
return (!success && !ssh_is_bare(s->ppl.ssh));
}
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