Re: [PATCH net v2] virtio-net: fix received length check in big packets

[Paolo Abeni <pabeni@redhat.com>]

On 7/8/25 4:42 PM, Bui Quang Minh wrote:
> Since commit 4959aebba8c0 ("virtio-net: use mtu size as buffer length
> for big packets"), the allocated size for big packets is not
> MAX_SKB_FRAGS * PAGE_SIZE anymore but depends on negotiated MTU. The
> number of allocated frags for big packets is stored in
> vi->big_packets_num_skbfrags. This commit fixes the received length
> check corresponding to that change. The current incorrect check can lead
> to NULL page pointer dereference in the below while loop when erroneous
> length is received.
> 
> Fixes: 4959aebba8c0 ("virtio-net: use mtu size as buffer length for big packets")
> Signed-off-by: Bui Quang Minh <minhquangbui99@gmail.com>
> ---
> Changes in v2:
> - Remove incorrect give_pages call
> ---
>  drivers/net/virtio_net.c | 9 ++++++---
>  1 file changed, 6 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> index 5d674eb9a0f2..3a7f435c95ae 100644
> --- a/drivers/net/virtio_net.c
> +++ b/drivers/net/virtio_net.c
> @@ -823,7 +823,7 @@ static struct sk_buff *page_to_skb(struct virtnet_info *vi,
>  {
>  	struct sk_buff *skb;
>  	struct virtio_net_common_hdr *hdr;
> -	unsigned int copy, hdr_len, hdr_padded_len;
> +	unsigned int copy, hdr_len, hdr_padded_len, max_remaining_len;
>  	struct page *page_to_free = NULL;
>  	int tailroom, shinfo_size;
>  	char *p, *hdr_p, *buf;
> @@ -887,12 +887,15 @@ static struct sk_buff *page_to_skb(struct virtnet_info *vi,
>  	 * tries to receive more than is possible. This is usually
>  	 * the case of a broken device.
>  	 */
> -	if (unlikely(len > MAX_SKB_FRAGS * PAGE_SIZE)) {
> +	BUG_ON(offset >= PAGE_SIZE);

Minor nit (not intended to block this patch): since you are touching
this, you could consider replacing the BUG_ON() with a:

 if (WARN_ON_ONCE()) <goto error path>.

/P