tty: Fix ->pgrp locking in tiocspgrp()

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 <jannh@google.com>
Reviewed-by: Jiri Slaby <jirislaby@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/drivers/tty/tty_jobctrl.c b/drivers/tty/tty_jobctrl.c
index 28a23a0..baadeea 100644 (file)
--- a/drivers/tty/tty_jobctrl.c
+++ b/drivers/tty/tty_jobctrl.c
@@ -494,10 +494,10 @@ 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;