Re: [PATCH v1 5/6] virtio_net: Guard against buffer length overflow in xdp_linearize_page()

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

On Thu, Jan 19, 2023 at 03:57:20PM +0200, Alexander Shishkin wrote:
> When reassembling incoming buffers to an xdp_page, there is a potential
> integer overflow in the buffer size test and trigger and out of bounds
> memcpy().
> 
> Fix this by reordering the test so that both sides are of the same
> signedness.
> 
> Signed-off-by: Alexander Shishkin <alexander.shishkin@linux.intel.com>
> Cc: Alexei Starovoitov <ast@kernel.org>
> Cc: Daniel Borkmann <daniel@iogearbox.net>
> Cc: Jesper Dangaard Brouer <hawk@kernel.org>
> Cc: John Fastabend <john.fastabend@gmail.com>
> Cc: David S. Miller <davem@davemloft.net>
> Cc: Eric Dumazet <edumazet@google.com>
> Cc: Jakub Kicinski <kuba@kernel.org>
> Cc: Paolo Abeni <pabeni@redhat.com>
> ---
>  drivers/net/virtio_net.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> index 7723b2a49d8e..dfa51dd95f63 100644
> --- a/drivers/net/virtio_net.c
> +++ b/drivers/net/virtio_net.c
> @@ -751,8 +751,10 @@ static struct page *xdp_linearize_page(struct receive_queue *rq,
>  
>  		/* guard against a misconfigured or uncooperative backend that
>  		 * is sending packet larger than the MTU.
> +		 * At the same time, make sure that an especially uncooperative
> +		 * backend can't overflow the test by supplying a large buflen.
>  		 */
> -		if ((page_off + buflen + tailroom) > PAGE_SIZE) {
> +		if (buflen > PAGE_SIZE - page_off - tailroom) {
>  			put_page(p);
>  			goto err_buf;
>  		}

I feel the issue should be addressed in virtqueue_get_buf.
In fact, when using DMA API, we already keep track of the
length in vring_desc_extra.

So, isn't this just the question of passing the length and
validating it e.g. in vring_unmap_one_split?
We can also use the index_nospec trick since otherwise
there could be speculation concerns.

> -- 
> 2.39.0