su: properly clear child PID If su is compiled with PAM support, it is possible for any local user to send SIGKILL to other processes with root privileges. There are only two conditions. First, the user must be able to perform su with a successful login. This does NOT have to be the root user, even using su with the same id is enough, e.g. "su $(whoami)". Second, SIGKILL can only be sent to processes which were executed after the su process. It is not possible to send SIGKILL to processes which were already running. I consider this as a security vulnerability, because I was able to write a proof of concept which unlocked a screen saver of another user this way. diff --git a/src/su.c b/src/su.c index f20d230..d86aa86 100644 --- a/src/su.c +++ b/src/su.c @@ -379,11 +379,13 @@ static void prepare_pam_close_session (void) /* wake child when resumed */ kill (pid, SIGCONT); stop = false; + } else { + pid_child = 0; } } while (!stop); } - if (0 != caught) { + if (0 != caught && 0 != pid_child) { (void) fputs ("\n", stderr); (void) fputs (_("Session terminated, terminating shell..."), stderr); @@ -393,9 +395,22 @@ static void prepare_pam_close_session (void) snprintf (wait_msg, sizeof wait_msg, _(" ...waiting for child to terminate.\n")); (void) signal (SIGALRM, kill_child); + (void) signal (SIGCHLD, catch_signals); (void) alarm (2); - (void) wait (&status); + sigemptyset (&ourset); + if ((sigaddset (&ourset, SIGALRM) != 0) + || (sigprocmask (SIG_BLOCK, &ourset, NULL) != 0)) { + fprintf (stderr, _("%s: signal masking malfunction\n"), Prog); + kill_child (0); + } else { + while (0 == waitpid (pid_child, &status, WNOHANG)) { + sigsuspend (&ourset); + } + pid_child = 0; + (void) sigprocmask (SIG_UNBLOCK, &ourset, NULL); + } + (void) fputs (_(" ...terminated.\n"), stderr); }