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tty: Fix ->pgrp locking in tiocspgrp()

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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: 47f86834bb ("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>
This commit is contained in:
Jann Horn 2020-12-03 02:25:04 +01:00 committed by Greg Kroah-Hartman
parent 418baf2c28
commit 54ffccbf05
1 changed files with 2 additions and 2 deletions
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4
drivers/tty/tty_jobctrl.c
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@ -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;