patch "tty: pty: Fix race condition between release_one_tty and pty_write" added to tty-next

[<gregkh@linuxfoundation.org>]

This is a note to let you know that I've just added the patch titled

    tty: pty: Fix race condition between release_one_tty and pty_write

to my tty git tree which can be found at
    git://git.kernel.org/pub/scm/linux/kernel/git/gregkh/tty.git
in the tty-next branch.

The patch will show up in the next release of the linux-next tree
(usually sometime within the next 24 hours during the week.)

The patch will also be merged in the next major kernel release
during the merge window.

If you have any questions about this process, please let me know.


From b9ca5f8560af244489b4a1bc1ae88b341f24bc95 Mon Sep 17 00:00:00 2001
From: Sahara <keun-o.park@darkmatter.ae>
Date: Mon, 11 Feb 2019 11:09:15 +0400
Subject: tty: pty: Fix race condition between release_one_tty and pty_write

Especially when a linked tty is used such as pty, the linked tty
port's buf works have not been cancelled while master tty port's
buf work has been cancelled. Since release_one_tty and flush_to_ldisc
run in workqueue threads separately, when pty_cleanup happens and
link tty port is freed, flush_to_ldisc tries to access freed port
and port->itty, eventually it causes a panic.
This patch utilizes the magic value with holding the tty_mutex to
check if the tty->link is valid.

Fixes: 2b022ab7542d ("pty: cancel pty slave port buf's work in tty_release")
Signed-off-by: Sahara <keun-o.park@darkmatter.ae>
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
 drivers/tty/pty.c    | 7 +++++++
 drivers/tty/tty_io.c | 3 +++
 2 files changed, 10 insertions(+)

diff --git a/drivers/tty/pty.c b/drivers/tty/pty.c
index 00099a8439d2..ef72031ab5b9 100644
--- a/drivers/tty/pty.c
+++ b/drivers/tty/pty.c
@@ -116,6 +116,12 @@ static int pty_write(struct tty_struct *tty, const unsigned char *buf, int c)
 	if (tty->stopped)
 		return 0;
 
+	mutex_lock(&tty_mutex);
+	if (to->magic != TTY_MAGIC) {
+		mutex_unlock(&tty_mutex);
+		return -EIO;
+	}
+
 	if (c > 0) {
 		spin_lock_irqsave(&to->port->lock, flags);
 		/* Stuff the data into the input queue of the other end */
@@ -125,6 +131,7 @@ static int pty_write(struct tty_struct *tty, const unsigned char *buf, int c)
 			tty_flip_buffer_push(to->port);
 		spin_unlock_irqrestore(&to->port->lock, flags);
 	}
+	mutex_unlock(&tty_mutex);
 	return c;
 }
 
diff --git a/drivers/tty/tty_io.c b/drivers/tty/tty_io.c
index 5fa250157025..c27777f3b8c4 100644
--- a/drivers/tty/tty_io.c
+++ b/drivers/tty/tty_io.c
@@ -1448,10 +1448,13 @@ static void release_one_tty(struct work_struct *work)
 	struct tty_driver *driver = tty->driver;
 	struct module *owner = driver->owner;
 
+	mutex_lock(&tty_mutex);
 	if (tty->ops->cleanup)
 		tty->ops->cleanup(tty);
 
 	tty->magic = 0;
+	mutex_unlock(&tty_mutex);
+
 	tty_driver_kref_put(driver);
 	module_put(owner);
 
-- 
2.21.0