[PATCH 2/2] serdev: use tty_port_tty_get() in ttyport_write_buf() to prevent UAF

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

From: Joshua Rogers <linux@joshua.hu>

ttyport_write_buf() snapshots serport->tty as a raw pointer without
taking a reference, while ttyport_close() can concurrently release the
tty via tty_release_struct(), leading to a use-after-free. Use
tty_port_tty_get() to obtain a reference-counted tty pointer, matching
the pattern already used by ttyport_write_wakeup().

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 | 12 ++++++++++--
 1 file changed, 10 insertions(+), 2 deletions(-)

diff --git a/drivers/tty/serdev/serdev-ttyport.c b/drivers/tty/serdev/serdev-ttyport.c
index 48ce5b3f8308..4d37c8130dd7 100644
--- a/drivers/tty/serdev/serdev-ttyport.c
+++ b/drivers/tty/serdev/serdev-ttyport.c
@@ -85,13 +85,21 @@ static const struct tty_port_client_operations client_ops = {
 static ssize_t ttyport_write_buf(struct serdev_controller *ctrl, const u8 *data, size_t len)
 {
 	struct serport *serport = serdev_controller_get_drvdata(ctrl);
-	struct tty_struct *tty = serport->tty;
+	struct tty_struct *tty;
+	ssize_t ret;
 
 	if (!test_bit(SERPORT_ACTIVE, &serport->flags))
 		return 0;
 
+	tty = tty_port_tty_get(serport->port);
+	if (!tty)
+		return 0;
+
 	set_bit(TTY_DO_WRITE_WAKEUP, &tty->flags);
-	return tty->ops->write(serport->tty, data, len);
+	ret = tty->ops->write(tty, data, len);
+	tty_kref_put(tty);
+
+	return ret;
 }
 
 static void ttyport_write_flush(struct serdev_controller *ctrl)

-- 
2.55.0