Re: [PATCH] vdpa/mlx5: fix param validation in mlx5_vdpa_get_config()

[Michael S. Tsirkin]

On Mon, Feb 08, 2021 at 05:17:41PM +0100, Stefano Garzarella wrote:
> It's legal to have 'offset + len' equal to
> sizeof(struct virtio_net_config), since 'ndev->config' is a
> 'struct virtio_net_config', so we can safely copy its content under
> this condition.
> 
> Fixes: 1a86b377aa21 ("vdpa/mlx5: Add VDPA driver for supported mlx5 devices")
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Stefano Garzarella <sgarzare@xxxxxxxxxx>
> ---
>  drivers/vdpa/mlx5/net/mlx5_vnet.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/vdpa/mlx5/net/mlx5_vnet.c b/drivers/vdpa/mlx5/net/mlx5_vnet.c
> index dc88559a8d49..10e9b09932eb 100644
> --- a/drivers/vdpa/mlx5/net/mlx5_vnet.c
> +++ b/drivers/vdpa/mlx5/net/mlx5_vnet.c
> @@ -1820,7 +1820,7 @@ static void mlx5_vdpa_get_config(struct vdpa_device *vdev, unsigned int offset,
>  	struct mlx5_vdpa_dev *mvdev = to_mvdev(vdev);
>  	struct mlx5_vdpa_net *ndev = to_mlx5_vdpa_ndev(mvdev);
>  
> -	if (offset + len < sizeof(struct virtio_net_config))
> +	if (offset + len <= sizeof(struct virtio_net_config))
>  		memcpy(buf, (u8 *)&ndev->config + offset, len);
>  }

Actually first I am not sure we need these checks at all.
vhost_vdpa_config_validate already validates the values, right?

Second, what will happen when we extend the struct and then
run new userspace on an old kernel? Looks like it will just
fail right? So what is the plan? I think we should
allow a bigger size, and return the copied config size to userspace.


> -- 
> 2.29.2