From 551e205f6dfa469f4f32a166ee3fb691201d27a7 Mon Sep 17 00:00:00 2001 From: Konstantin Belousov Date: Thu, 17 Dec 2020 19:51:39 +0000 Subject: [PATCH] Fix a race in tty_signal_sessleader() with unlocked read of s_leader. Since we do not own the session lock, a parallel killjobc() might reset s_leader to NULL after we checked it. Read s_leader only once and ensure that compiler is not allowed to reload. While there, make access to t_session somewhat more pretty by using local variable. PR: 251915 Submitted by: Jakub Piecuch MFC after: 1 week --- sys/kern/tty.c | 11 +++++++++-- 1 file changed, 9 insertions(+), 2 deletions(-) diff --git a/sys/kern/tty.c b/sys/kern/tty.c index 7526638b921..8d4d25a4ac0 100644 --- a/sys/kern/tty.c +++ b/sys/kern/tty.c @@ -1474,6 +1474,7 @@ void tty_signal_sessleader(struct tty *tp, int sig) { struct proc *p; + struct session *s; tty_assert_locked(tp); MPASS(sig >= 1 && sig < NSIG); @@ -1482,8 +1483,14 @@ tty_signal_sessleader(struct tty *tp, int sig) tp->t_flags &= ~TF_STOPPED; tp->t_termios.c_lflag &= ~FLUSHO; - if (tp->t_session != NULL && tp->t_session->s_leader != NULL) { - p = tp->t_session->s_leader; + /* + * Load s_leader exactly once to avoid race where s_leader is + * set to NULL by a concurrent invocation of killjobc() by the + * session leader. Note that we are not holding t_session's + * lock for the read. + */ + if ((s = tp->t_session) != NULL && + (p = atomic_load_ptr(&s->s_leader)) != NULL) { PROC_LOCK(p); kern_psignal(p, sig); PROC_UNLOCK(p); -- 2.45.0