[PATCH] tty: pty: Fix race condition between release_one_tty and pty_write

[kpark3469@gmail.com]

From: Sahara <keun-o.park@darkmatter.ae>

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>
---
 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 00099a84..ef72031 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 23c6fd2..6c4a206 100644
--- a/drivers/tty/tty_io.c
+++ b/drivers/tty/tty_io.c
@@ -1446,10 +1446,13 @@ 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);
 
-- 
2.7.4