Re: [PATCH v2] virtio-blk: limit seg_max to a safe value

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

On Mon, May 24, 2021 at 04:40:20PM +0100, Stefan Hajnoczi wrote:
> The struct virtio_blk_config seg_max value is read from the device and
> incremented by 2 to account for the request header and status byte
> descriptors added by the driver.
> 
> In preparation for supporting untrusted virtio-blk devices, protect
> against integer overflow and limit the value to a safe maximum.
> 
> Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
> ---
> v2:
>  * Limit to a virtio-specific value instead of using SG_MAX_SEGMENTS
>    [Christoph]
> ---
>  drivers/block/virtio_blk.c | 8 +++++++-
>  1 file changed, 7 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/block/virtio_blk.c b/drivers/block/virtio_blk.c
> index b9fa3ef5b57c..1635d4289202 100644
> --- a/drivers/block/virtio_blk.c
> +++ b/drivers/block/virtio_blk.c
> @@ -21,6 +21,9 @@
>  #define VQ_NAME_LEN 16
>  #define MAX_DISCARD_SEGMENTS 256u
>  
> +/* The maximum number of sg elements that fit into a virtqueue */
> +#define VIRTIO_BLK_MAX_SG_ELEMS 32768

32768 is what it says for split queues but for packed queues
it says 2^15.

So 0x8000 then?

> +
>  static int major;
>  static DEFINE_IDA(vd_index_ida);
>  
> @@ -728,7 +731,10 @@ static int virtblk_probe(struct virtio_device *vdev)
>  	if (err || !sg_elems)
>  		sg_elems = 1;
>  
> -	/* We need an extra sg elements at head and tail. */

s/an extra/extra/

> +	/* Prevent integer overflows and honor max vq size */
> +	sg_elems = min_t(u32, sg_elems, VIRTIO_BLK_MAX_SG_ELEMS - 2);
> +
> +	/* We need extra sg elements at head and tail. */
>  	sg_elems += 2;
>  	vdev->priv = vblk = kmalloc(sizeof(*vblk), GFP_KERNEL);
>  	if (!vblk) {
> -- 
> 2.31.1
>