From 882e038d2cd276163a8fc7bbeffda59ae0924471 Mon Sep 17 00:00:00 2001 From: Jann Horn Date: Thu, 3 Dec 2020 02:25:04 +0100 Subject: [PATCH] tty: Fix ->pgrp locking in tiocspgrp() commit 54ffccbf053b5b6ca4f6e45094b942fab92a25fc upstream. tiocspgrp() takes two tty_struct pointers: One to the tty that userspace passed to ioctl() (`tty`) and one to the TTY being changed (`real_tty`). These pointers are different when ioctl() is called with a master fd. To properly lock real_tty->pgrp, we must take real_tty->ctrl_lock. This bug makes it possible for racing ioctl(TIOCSPGRP, ...) calls on both sides of a PTY pair to corrupt the refcount of `struct pid`, leading to use-after-free errors. Fixes: 47f86834bbd4 ("redo locking of tty->pgrp") CC: stable@kernel.org Signed-off-by: Jann Horn Reviewed-by: Jiri Slaby Signed-off-by: Greg Kroah-Hartman --- drivers/tty/tty_jobctrl.c | 4 ++-- 1 file changed, 2 insertions(+), 2 deletions(-) diff --git a/drivers/tty/tty_jobctrl.c b/drivers/tty/tty_jobctrl.c index e7032309ee87..33dcd4eb92da 100644 --- a/drivers/tty/tty_jobctrl.c +++ b/drivers/tty/tty_jobctrl.c @@ -493,10 +493,10 @@ static int tiocspgrp(struct tty_struct *tty, struct tty_struct *real_tty, pid_t if (session_of_pgrp(pgrp) != task_session(current)) goto out_unlock; retval = 0; - spin_lock_irq(&tty->ctrl_lock); + spin_lock_irq(&real_tty->ctrl_lock); put_pid(real_tty->pgrp); real_tty->pgrp = get_pid(pgrp); - spin_unlock_irq(&tty->ctrl_lock); + spin_unlock_irq(&real_tty->ctrl_lock); out_unlock: rcu_read_unlock(); return retval; -- 2.20.1