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Avoid falling prey to tiocsti #1660

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29 changes: 28 additions & 1 deletion src/su.c
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Original file line number Diff line number Diff line change
Expand Up @@ -56,6 +56,7 @@
#ifdef USE_PAM
#include "pam_defs.h"
#endif /* USE_PAM */
#include "io/fgets/fgets.h"
#include "pwauth.h"
#include "prototypes.h"
#include "shadowlog.h"
Expand Down Expand Up @@ -996,6 +997,29 @@ static void set_environment (struct passwd *pw)

}

/*
* See the following kernel commit:
* commit 83efeeeb3d04b22aaed1df99bc70a48fe9d22c4d
* Author: Kees Cook <kees@kernel.org>
* Date: Sat Oct 22 11:29:49 2022 -0700
* Subject: tty: Allow TIOCSTI to be disabled
*/
Comment on lines +1000 to +1006

@alejandro-colomar alejandro-colomar Jun 28, 2026 •
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How about compacting that?:

// See linux.git 83efeeeb3d04 (2022-10-22; "tty: Allow TIOCSTI to be disabled")

static bool legacy_tiocsti_is_disabled(void) {

@alejandro-colomar alejandro-colomar Jun 28, 2026

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Please move the brace to a new line (and optionally the type, but at least the brace).

char buf[3];
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alejandro-colomar marked this conversation as resolved.
FILE *fp;
void *ret;

fp = fopen("/proc/sys/dev/tty/legacy_tiocsti", "r");
if (NULL == fp)
return false;
ret = fgets_a(buf, fp);
fclose(fp);
if (ret == NULL)
return false;

return buf[0] == '0';

@alejandro-colomar alejandro-colomar Jun 28, 2026

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I'd like to avoid all direct operations on bytes of strings. The good thing about str*() functions is that you can fortify them with _FORTIFY_SOURCE, and other niceties. They're also easier to grep(1).

I suggest using one of the following alternatives:

return streq(buf, "0\n");

return !!strspn(buf, "0");

@alejandro-colomar alejandro-colomar Jun 28, 2026 •
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I prefer the former, because it also implicitly validates the rest of the string.

Maybe it would be interesting to validate and remove the newline explicitly separately:

if (stpspn(buf, "\n") == NULL)
	return false;

return streq(buf, "0");

This makes the streq() call more readable, while still validating the entire line.

}

/*
* su - switch user id
*
Expand All @@ -1010,6 +1034,7 @@ int main (int argc, char **argv)
{
const char *cp;
struct passwd *pw = NULL;
bool need_pty_prot;

#ifdef USE_PAM
int ret;
Expand All @@ -1023,6 +1048,8 @@ int main (int argc, char **argv)

save_caller_context();

need_pty_prot = caller_is_root && !legacy_tiocsti_is_disabled();

OPENLOG (Prog);

process_flags (argc, argv);
Expand Down Expand Up @@ -1152,7 +1179,7 @@ int main (int argc, char **argv)

set_environment (pw);

if (!doshell) {
if (!doshell || need_pty_prot) {
/* There is no need for a controlling terminal.
* This avoids the callee to inject commands on
* the caller's tty. */
Expand Down
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