[PATCH v2] virtio_console: do not free control-out buffers on remove

[Jia Jia <physicalmtea@gmail.com>]

__send_control_msg() publishes &portdev->cpkt as the control-out
virtqueue cookie. remove_vqs() walks every virtqueue and passes leftover
cookies to free_buf(), which treats them as struct port_buffer and
reads sgpages.

If a control message is still on c_ovq when the device is unbound,
free_buf() reads past the ports_device object.

KASAN reported slab-out-of-bounds in free_buf():

	free_buf
	remove_vqs
	virtcons_remove
	unbind_store

The object was the ports_device allocated in virtcons_probe().

Drain c_ovq without freeing. The packet lives in portdev and is released
with it.

Fixes: a7a69ec0d8e4 ("virtio_console: free buffers after reset")
Signed-off-by: Jia Jia <physicalmtea@gmail.com>
---
Changes in v2:
- Guard c_ovq handling with use_multiport() because c_ovq and
  c_ovq_lock are only initialized for multiport devices.

 drivers/char/virtio_console.c | 21 ++++++++++++++++++---
 1 file changed, 18 insertions(+), 3 deletions(-)

diff --git a/drivers/char/virtio_console.c b/drivers/char/virtio_console.c
index 9a33217c68d9..8e0d10d91763 100644
--- a/drivers/char/virtio_console.c
+++ b/drivers/char/virtio_console.c
@@ -1884,12 +1884,27 @@ static const struct file_operations portdev_fops = {
 static void remove_vqs(struct ports_device *portdev)
 {
 	struct virtqueue *vq;
+	bool multiport = use_multiport(portdev);
 
 	virtio_device_for_each_vq(portdev->vdev, vq) {
 		struct port_buffer *buf;
+		unsigned int len;
 
-		flush_bufs(vq, true);
-		while ((buf = virtqueue_detach_unused_buf(vq)))
-			free_buf(buf, true);
+		/*
+		 * c_ovq cookies are &portdev->cpkt, not port_buffer.
+		 * Detach them but do not free_buf().
+		 */
+		if (multiport && vq == portdev->c_ovq) {
+			spin_lock(&portdev->c_ovq_lock);
+			while (virtqueue_get_buf(vq, &len))
+				;
+			while (virtqueue_detach_unused_buf(vq))
+				;
+			spin_unlock(&portdev->c_ovq_lock);
+		} else {
+			flush_bufs(vq, true);
+			while ((buf = virtqueue_detach_unused_buf(vq)))
+				free_buf(buf, true);
+		}
 		cond_resched();
 	}
 	portdev->vdev->config->del_vqs(portdev->vdev);
-- 
2.34.1