Re: [PATCH net v4] virtio-net: fix len check in receive_big()

[Bui Quang Minh <minhquangbui99@gmail.com>]

On 6/16/26 11:28, Xiang Mei wrote:
> receive_big() bounds the device-announced length by
> (big_packets_num_skbfrags + 1) * PAGE_SIZE.  That is still too loose:
> add_recvbuf_big() sets sg[1] to start at offset
> sizeof(struct padded_vnet_hdr) into the first page, so the chain
> actually carries hdr_len + (PAGE_SIZE - sizeof(padded_vnet_hdr)) +
> big_packets_num_skbfrags * PAGE_SIZE bytes -- 20 bytes less than the
> check allows for the common hdr_len == 12 case.
>
> A malicious virtio backend can announce a len in that gap.  page_to_skb()
> then walks one frag past the page chain, storing a NULL page->private
> into skb_shinfo()->frags[MAX_SKB_FRAGS], which is both an out-of-bounds
> write past the static frag array and a NULL frag handed up the rx path.
>
> Bound len by the size add_recvbuf_big() actually advertised.
>
> Fixes: 0c716703965f ("virtio-net: fix received length check in big packets")

I think we should Cc: stable@vger.kernel.org

> Reported-by: Weiming Shi <bestswngs@gmail.com>
> Signed-off-by: Xiang Mei <xmei5@asu.edu>
> Reviewed-by: Xuan Zhuo <xuanzhuo@linux.alibaba.com>
> ---
> v4: use easy to understand math to compute the max_len
> v3: revoke 2/2 and add Xuan Zhuo's Reviewed-by tag
> v2: add additiona check as 2/2
>
>   drivers/net/virtio_net.c | 7 ++++---
>   1 file changed, 4 insertions(+), 3 deletions(-)
>
> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> index f4adcfee7a80..8f4562316aaa 100644
> --- a/drivers/net/virtio_net.c
> +++ b/drivers/net/virtio_net.c
> @@ -1999,15 +1999,16 @@ static struct sk_buff *receive_big(struct net_device *dev,
>   				   struct virtnet_rq_stats *stats)
>   {
>   	struct page *page = buf;
> +	unsigned long max_len = (vi->big_packets_num_skbfrags + 1) * PAGE_SIZE -
> +				sizeof(struct padded_vnet_hdr) + vi->hdr_len;
>   	struct sk_buff *skb;
>   
>   	/* Make sure that len does not exceed the size allocated in
>   	 * add_recvbuf_big.
>   	 */
> -	if (unlikely(len > (vi->big_packets_num_skbfrags + 1) * PAGE_SIZE)) {
> +	if (unlikely(len > max_len)) {
>   		pr_debug("%s: rx error: len %u exceeds allocated size %lu\n",
> -			 dev->name, len,
> -			 (vi->big_packets_num_skbfrags + 1) * PAGE_SIZE);
> +			 dev->name, len, max_len);
>   		goto err;
>   	}
>   

Reviewed-by: Bui Quang Minh <minhquangbui99@gmail.com>

Thanks,
Quang Minh.