Re: [PATCH] virtio: console: check the control packet size before parsing it

["Michael S. Tsirkin" <mst@redhat.com>]

On Tue, Nov 03, 2026 at 04:41:56PM +0800, Guanglei Zhu wrote:
> The control queue handler hands whatever the host wrote to the
> control queue straight to the parser:
> 
> 	buf->len = min_t(size_t, len, buf->size);
> 	buf->offset = 0;
> 
> 	handle_control_message(vq->vdev, portdev, buf);
> 
> handle_control_message() casts the start of the buffer to struct
> virtio_console_control and reads the id, event and value fields
> without ever looking at buf->len. A control packet shorter than the
> structure

That's a broken device.

> makes the driver parse whatever bytes were left in the
> reused buffer as a control header, and the uninitialized event then
> drives the port state machine.

So what?

> Drop control packets that are too short to hold the header.

And then what?

> Fixes: 17634ba25544 ("virtio: console: Add a new MULTIPORT feature, support for generic ports")
> Cc: stable@vger.kernel.org

Not clear why this is needed on stable.


> Signed-off-by: Guanglei Zhu <zhugl3@xiaopeng.com>
> ---
>  drivers/char/virtio_console.c | 6 ++++++
>  1 file changed, 6 insertions(+)
> 
> diff --git a/drivers/char/virtio_console.c b/drivers/char/virtio_console.c
> index 019bcae81af5..5e2a4403dbf8 100644
> --- a/drivers/char/virtio_console.c
> +++ b/drivers/char/virtio_console.c
> @@ -1597,6 +1597,12 @@ static void handle_control_message(struct virtio_device *vdev,
>  	size_t name_size;
>  	int err;
>  
> +	if (buf->len < sizeof(struct virtio_console_control)) {
> +		dev_dbg(&portdev->vdev->dev,
> +			"Invalid control packet size %zu\n", buf->len);
> +		return;
> +	}
> +
>  	cpkt = (struct virtio_console_control *)(buf->buf + buf->offset);
>  
>  	port = find_port_by_id(portdev, virtio32_to_cpu(vdev, cpkt->id));
> -- 
> 2.50.1
>