From 08fd4b69e84364677a10e519ccb25b71710ee686 Mon Sep 17 00:00:00 2001 From: Tobias Stoeckmann Date: Thu, 23 Feb 2017 09:47:29 -0600 Subject: [PATCH] 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. --- src/su.c | 19 +++++++++++++++++-- 1 file changed, 17 insertions(+), 2 deletions(-) --- a/src/su.c +++ b/src/su.c @@ -363,20 +363,35 @@ static void prepare_pam_close_session (v /* 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); (void) kill (-pid_child, caught); (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); }