linux-kernel - [PATCH 4.9 28/45] tty: Fix ->pgrp locking in tiocspgrp()

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Message-Id: <20201210142603.739750087@linuxfoundation.org>
Date:   Thu, 10 Dec 2020 15:26:42 +0100
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org, stable@...nel.org,
        Jann Horn <jannh@...gle.com>, Jiri Slaby <jirislaby@...nel.org>
Subject: [PATCH 4.9 28/45] tty: Fix ->pgrp locking in tiocspgrp()

From: Jann Horn <jannh@...gle.com>

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@...nel.org
Signed-off-by: Jann Horn <jannh@...gle.com>
Reviewed-by: Jiri Slaby <jirislaby@...nel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>

---
 drivers/tty/tty_io.c |    4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

--- a/drivers/tty/tty_io.c
+++ b/drivers/tty/tty_io.c
@@ -2645,10 +2645,10 @@ static int tiocspgrp(struct tty_struct *
 	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;