[PATCH 1/2] serdev: fix race between tty-port unregister and in-flight callbacks

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

From: Joshua Rogers <linux@joshua.hu>

serdev_tty_port_unregister() clears port->client_data and frees the
controller without synchronizing with in-flight flip buffer work.
This can cause NULL pointer dereferences or use-after-free if
ttyport_receive_buf() or ttyport_write_wakeup() runs concurrently.

Add cancel_work_sync() to drain pending buffer work before clearing
state, and add NULL checks for client_data in both callbacks as
secondary hardening.

Assisted-by: AISLE:Snapshot
Cc: stable <stable@kernel.org>
Signed-off-by: Joshua Rogers <linux@joshua.hu>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
 drivers/tty/serdev/serdev-ttyport.c | 15 +++++++++++++--
 1 file changed, 13 insertions(+), 2 deletions(-)

diff --git a/drivers/tty/serdev/serdev-ttyport.c b/drivers/tty/serdev/serdev-ttyport.c
index bab1b143b8a6..48ce5b3f8308 100644
--- a/drivers/tty/serdev/serdev-ttyport.c
+++ b/drivers/tty/serdev/serdev-ttyport.c
@@ -26,9 +26,14 @@ static size_t ttyport_receive_buf(struct tty_port *port, const u8 *cp,
 				  const u8 *fp, size_t count)
 {
 	struct serdev_controller *ctrl = port->client_data;
-	struct serport *serport = serdev_controller_get_drvdata(ctrl);
+	struct serport *serport;
 	size_t ret;
 
+	if (!ctrl)
+		return 0;
+
+	serport = serdev_controller_get_drvdata(ctrl);
+
 	if (!test_bit(SERPORT_ACTIVE, &serport->flags))
 		return 0;
 
@@ -46,9 +51,14 @@ static size_t ttyport_receive_buf(struct tty_port *port, const u8 *cp,
 static void ttyport_write_wakeup(struct tty_port *port)
 {
 	struct serdev_controller *ctrl = port->client_data;
-	struct serport *serport = serdev_controller_get_drvdata(ctrl);
+	struct serport *serport;
 	struct tty_struct *tty;
 
+	if (!ctrl)
+		return;
+
+	serport = serdev_controller_get_drvdata(ctrl);
+
 	tty = tty_port_tty_get(port);
 	if (!tty)
 		return;
@@ -312,6 +322,7 @@ int serdev_tty_port_unregister(struct tty_port *port)
 		return -ENODEV;
 
 	serdev_controller_remove(ctrl);
+	cancel_work_sync(&port->buf.work);
 	port->client_data = NULL;
 	port->client_ops = &tty_port_default_client_ops;
 	serdev_controller_put(ctrl);

-- 
2.55.0