tree b76cc63d9a65d462384045bcea6e75187fb26d6a
parent 87f04358c2fe1bbf8b25790748db095c5f72906e
author Jann Horn <jannh@google.com> 1606958704 +0100
committer Luca Weiss <luca.weiss@fairphone.com> 1626347892 +0200

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

(cherry picked from commit 742f3062298ac1ae1d28de31b1f946f93db1eba1)