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su: Prevent stack overflow in check_perms

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This is no real world security fix.

The overflow could occur if too many layered subsystems are encountered
because the function check_perms calls itself recursively.

It would already take a misconfigured system for this to achieve it.

Use an iterative approach by calling the do_check_perms in a loop
instead of calling itself recursively.

As a side note: At least GCC 13 optimized this code and already uses
a jmp in its assembler code. I could only see the stack overflow by
activating address sanitizer which prevented the optimization.

Co-developed-by: Serge Hallyn <serge@hallyn.com>
Signed-off-by: Samanta Navarro <ferivoz@riseup.net>
This commit is contained in:
Samanta Navarro 2023-05-23 11:55:09 +00:00 committed by Serge Hallyn
parent 4ad359ccc6
commit a116e20c76
1 changed files with 19 additions and 3 deletions
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22
src/su.c
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@ -115,6 +115,7 @@ static bool iswheel (const char *);
static bool restricted_shell (const char *shellname);
NORETURN static void su_failure (const char *tty, bool su_to_root);
static /*@only@*/struct passwd * check_perms (void);
static /*@only@*/struct passwd * do_check_perms (void);
#ifdef USE_PAM
static void check_perms_pam (const struct passwd *pw);
#else /* !USE_PAM */
@ -618,10 +619,25 @@ static void check_perms_nopam (const struct passwd *pw)
/*
* check_perms - check permissions to switch to the user 'name'
*
* In case of subsystem login, the user is first authenticated in the
* caller's root subsystem, and then in the user's target subsystem.
* The user is authenticated in all subsystems iterately.
*/
static /*@only@*/struct passwd * check_perms (void)
{
struct passwd *pw = NULL;
while (pw == NULL)
pw = do_check_perms();
return pw;
}
/*
* do_check_perms - check permissions to switch to the user 'name'
*
* The user is authenticated in current subsystem, if any. Returns
* NULL if permissions have to be checked in next subsystem, in
* which case the subsystem has already been entered.
*/
static /*@only@*/struct passwd * do_check_perms (void)
{
#ifdef USE_PAM
const void *tmp_name;
@ -692,7 +708,7 @@ static /*@only@*/struct passwd * check_perms (void)
endpwent (); /* close the old password databases */
endspent ();
pw_free (pw);
return check_perms (); /* authenticate in the subsystem */
return NULL; /* authenticate in the subsystem */
}
return pw;