Re: [PATCH net v2 2/2] virtio-net: harden page_to_skb() big-packet frag loop

[Xuan Zhuo]

On Wed, 10 Jun 2026 16:29:36 -0700, Xiang Mei <xmei5@xxxxxxx> wrote:
> This is a robustness hardening patch. The slow-path frag loop in
> page_to_skb() walks the page chain via page->private until the
> device-reported len is consumed, implicitly trusting that len fits the
> chain. It does not stop when the chain is exhausted (page becomes NULL
> at the tail), nor when nr_frags reaches the end of the static
> skb_shinfo()->frags[MAX_SKB_FRAGS] array.
>
> Both bounds are needed: the chain length is big_packets_num_skbfrags + 1
> pages, which for an MTU-driven configuration can be well below
> MAX_SKB_FRAGS, so neither guard implies the other.
>
> Make the loop self-defending so it no longer relies on the caller having
> validated len: stop once the chain is exhausted, and never index past
> MAX_SKB_FRAGS. No functional change for well-formed input.

At this point, we are assuming that len represents the correct packet length. If
there is a bug in the validation, it can be fixed, just like in your previous
patch. Indeed, not checking nr_frags is also based on the overall design.
However, I do not recommend adding this kind of enhancement. If we follow
this logic, we would end up adding similar code in many other places, which
doesn't make much sense.

Thanks.

>
> Signed-off-by: Xiang Mei <xmei5@xxxxxxx>
> ---
> v2: robustness patch
>
>  drivers/net/virtio_net.c | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> index afe73eda1491..518c22fa1b68 100644
> --- a/drivers/net/virtio_net.c
> +++ b/drivers/net/virtio_net.c
> @@ -906,8 +906,11 @@ static struct sk_buff *page_to_skb(struct virtnet_info *vi,
>  	}
>
>  	BUG_ON(offset >= PAGE_SIZE);
> -	while (len) {
> +	while (len && page) {
>  		unsigned int frag_size = min((unsigned)PAGE_SIZE - offset, len);
> +
> +		if (unlikely(skb_shinfo(skb)->nr_frags >= MAX_SKB_FRAGS))
> +			break;
>  		skb_add_rx_frag(skb, skb_shinfo(skb)->nr_frags, page, offset,
>  				frag_size, truesize);
>  		len -= frag_size;
> --
> 2.43.0
>