Re: [PATCH v1 2/6] virtio console: Harden port adding

Michael S. Tsirkin
2023-01-20T13:00:39Z

On Thu, Jan 19, 2023 at 03:57:17PM +0200, Alexander Shishkin wrote:
> From: Andi Kleen <ak@linux.intel.com>
>
> The ADD_PORT operation reads and sanity checks the port id multiple
> times from the untrusted host. This is not safe because a malicious
> host could change it between reads.
>
> Read the port id only once and cache it for subsequent uses.
>
> Signed-off-by: Andi Kleen <ak@linux.intel.com>
> Signed-off-by: Alexander Shishkin <alexander.shishkin@linux.intel.com>
> Cc: Amit Shah <amit@kernel.org>
> Cc: Arnd Bergmann <arnd@arndb.de>
> Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

I suspect anyone worried about this kind of thing already uses a bounce
buffer. No? The patch itself makes the code more readable, except maybe
for the READ_ONCE thing.

> ---
> drivers/char/virtio_console.c | 10 ++++++----
> 1 file changed, 6 insertions(+), 4 deletions(-)
>
> diff --git a/drivers/char/virtio_console.c b/drivers/char/virtio_console.c
> index f4fd5fe7cd3a..6599c2956ba4 100644
> --- a/drivers/char/virtio_console.c
> +++ b/drivers/char/virtio_console.c
> @@ -1563,10 +1563,13 @@ static void handle_control_message(struct virtio_device *vdev,
> struct port *port;
> size_t name_size;
> int err;
> + unsigned id;
>
> cpkt = (struct virtio_console_control *)(buf->buf + buf->offset);
>
> - port = find_port_by_id(portdev, virtio32_to_cpu(vdev, cpkt->id));
> + /* Make sure the host cannot change id under us */
> + id = virtio32_to_cpu(vdev, READ_ONCE(cpkt->id));
> + port = find_port_by_id(portdev, id);
> if (!port &&
> cpkt->event != cpu_to_virtio16(vdev, VIRTIO_CONSOLE_PORT_ADD)) {
> /* No valid header at start of buffer. Drop it. */
> @@ -1583,15 +1586,14 @@ static void handle_control_message(struct virtio_device *vdev,
> send_control_msg(port, VIRTIO_CONSOLE_PORT_READY, 1);
> break;
> }
> - if (virtio32_to_cpu(vdev, cpkt->id) >=
> - portdev->max_nr_ports) {
> + if (id >= portdev->max_nr_ports) {
> dev_warn(&portdev->vdev->dev,
> "Request for adding port with "
> "out-of-bound id %u, max. supported id: %u\n",
> cpkt->id, portdev->max_nr_ports - 1);
> break;
> }
> - add_port(portdev, virtio32_to_cpu(vdev, cpkt->id));
> + add_port(portdev, id);
> break;
> case VIRTIO_CONSOLE_PORT_REMOVE:
> unplug_port(port);
> --
> 2.39.0