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Plink: default to sanitising non-tty console output.

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If Plink's standard output and/or standard error points at a Windows
console or a Unix tty device, and if Plink was not configured to
request a remote pty (and hence to send a terminal-type string), then
we apply the new control-character stripping facility.

The idea is to be a mild defence against malicious remote processes
sending confusing escape sequences through the standard error channel
when Plink is being used as a transport for something like git: it's
OK to have actual sensible error messages come back from the server,
but when you run a git command, you didn't really intend to give the
remote server the implicit licence to write _all over_ your local
terminal display. At the same time, in that scenario, the standard
_output_ of Plink is left completely alone, on the grounds that git
will be expecting it to be 8-bit clean. (And Plink can tell that
because it's redirected away from the console.)

For interactive login sessions using Plink, this behaviour is
disabled, on the grounds that once you've sent a terminal-type string
it's assumed that you were _expecting_ the server to use it to know
what escape sequences to send to you.

So it should be transparent for all the use cases I've so far thought
of. But in case it's not, there's a family of new command-line options
like -no-sanitise-stdout and -sanitise-stderr that you can use to
forcibly override the autodetection of whether to do it.

This all applies the same way to both Unix and Windows Plink.
This commit is contained in:
Simon Tatham
2019-02-20 07:03:57 +00:00
parent 462b8c7b84
commit 91cf47dd0d
4 changed files with 159 additions and 15 deletions
Download Patch File Download Diff File Expand all files Collapse all files

66
unix/uxplink.c
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@ -323,6 +323,10 @@ void cleanup_termios(void)
}
bufchain stdout_data, stderr_data;
bufchain_sink stdout_bcs, stderr_bcs;
StripCtrlChars *stdout_scc, *stderr_scc;
BinarySink *stdout_bs, *stderr_bs;
enum { EOF_NO, EOF_PENDING, EOF_SENT } outgoingeof;
size_t try_output(bool is_stderr)
@ -357,14 +361,12 @@ size_t try_output(bool is_stderr)
static size_t plink_output(
Seat *seat, bool is_stderr, const void *data, size_t len)
{
if (is_stderr) {
bufchain_add(&stderr_data, data, len);
return try_output(true);
} else {
assert(outgoingeof == EOF_NO);
bufchain_add(&stdout_data, data, len);
return try_output(false);
}
assert(is_stderr || outgoingeof == EOF_NO);
BinarySink *bs = is_stderr ? stderr_bs : stdout_bs;
put_data(bs, data, len);
return try_output(is_stderr);
}
static bool plink_eof(Seat *seat)
@ -529,6 +531,10 @@ static void usage(void)
printf(" -share enable use of connection sharing\n");
printf(" -hostkey aa:bb:cc:...\n");
printf(" manually specify a host key (may be repeated)\n");
printf(" -sanitise-stderr, -sanitise-stdout, "
"-no-sanitise-stderr, -no-sanitise-stdout\n");
printf(" do/don't strip control chars from standard "
"output/error\n");
printf(" -m file read remote command(s) from file\n");
printf(" -s remote command is an SSH subsystem (SSH-2 only)\n");
printf(" -N don't start a shell/command (SSH-2 only)\n");
@ -565,6 +571,7 @@ int main(int argc, char **argv)
int i, fdsize, fdstate;
int exitcode;
bool errors;
enum TriState sanitise_stdout = AUTO, sanitise_stderr = AUTO;
bool use_subsystem = false;
bool just_test_share_exists = false;
unsigned long now;
@ -582,6 +589,10 @@ int main(int argc, char **argv)
bufchain_init(&stdout_data);
bufchain_init(&stderr_data);
bufchain_sink_init(&stdout_bcs, &stdout_data);
bufchain_sink_init(&stderr_bcs, &stderr_data);
stdout_bs = BinarySink_UPCAST(&stdout_bcs);
stderr_bs = BinarySink_UPCAST(&stderr_bcs);
outgoingeof = EOF_NO;
flags = FLAG_STDERR_TTY;
@ -655,6 +666,18 @@ int main(int argc, char **argv)
} else if (!strcmp(p, "-fuzznet")) {
conf_set_int(conf, CONF_proxy_type, PROXY_FUZZ);
conf_set_str(conf, CONF_proxy_telnet_command, "%host");
} else if (!strcmp(p, "-sanitise-stdout") ||
!strcmp(p, "-sanitize-stdout")) {
sanitise_stdout = FORCE_ON;
} else if (!strcmp(p, "-no-sanitise-stdout") ||
!strcmp(p, "-no-sanitize-stdout")) {
sanitise_stdout = FORCE_OFF;
} else if (!strcmp(p, "-sanitise-stderr") ||
!strcmp(p, "-sanitize-stderr")) {
sanitise_stderr = FORCE_ON;
} else if (!strcmp(p, "-no-sanitise-stderr") ||
!strcmp(p, "-no-sanitize-stderr")) {
sanitise_stderr = FORCE_OFF;
} else if (*p != '-') {
char *command;
int cmdlen, cmdsize;
@ -766,6 +789,33 @@ int main(int argc, char **argv)
conf_set_int(conf, CONF_height, size.ws_row);
}
/*
* Decide whether to sanitise control sequences out of standard
* output and standard error.
*
* If we weren't given a command-line override, we do this if (a)
* the fd in question is pointing at a terminal, and (b) we aren't
* trying to allocate a terminal as part of the session.
*
* (Rationale: the risk of control sequences is that they cause
* confusion when sent to a local terminal, so if there isn't one,
* no problem. Also, if we allocate a remote terminal, then we
* sent a terminal type, i.e. we told it what kind of escape
* sequences we _like_, i.e. we were expecting to receive some.)
*/
if (sanitise_stdout == FORCE_ON ||
(sanitise_stdout == AUTO && isatty(STDOUT_FILENO) &&
conf_get_bool(conf, CONF_nopty))) {
stdout_scc = stripctrl_new(stdout_bs, true, L'\0');
stdout_bs = BinarySink_UPCAST(stdout_scc);
}
if (sanitise_stderr == FORCE_ON ||
(sanitise_stderr == AUTO && isatty(STDERR_FILENO) &&
conf_get_bool(conf, CONF_nopty))) {
stderr_scc = stripctrl_new(stderr_bs, true, L'\0');
stderr_bs = BinarySink_UPCAST(stderr_scc);
}
sk_init();
uxsel_init();