Subject: Re: [PATCH net v6] virtio-net: fix received length check in big packets

From: "Michael S. Tsirkin" <[email-masked]>
Date: Tue, 28 Oct 2025 10:41:42 -0400

On Tue, Oct 28, 2025 at 09:31:16PM +0700, Bui Quang Minh wrote:
> Since commit 4959aebba8c0 ("virtio-net: use mtu size as buffer length
> for big packets"), when guest gso is off, 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.
> 
> Because the host announced buffer length can be malicious (e.g. the host
> vhost_net driver's get_rx_bufs is modified to announce incorrect
> length), we need a check in virtio_net receive path. Currently, the
> check is not adapted to the new change which can lead to NULL page
> pointer dereference in the below while loop when receiving length that
> is larger than the allocated one.
> 
> This commit fixes the received length check corresponding to the new
> change.
> 
> Fixes: 4959aebba8c0 ("virtio-net: use mtu size as buffer length for big packets")
> Cc: [email-masked]
> Signed-off-by: Bui Quang Minh <[email-masked]>
> ---
> Changes in v6:
> - Fix the length check
> - Link to v5: https://lore.kernel.org/netdev/[email-masked]/
> Changes in v5:
> - Move the length check to receive_big
> - Link to v4: https://lore.kernel.org/netdev/[email-masked]/
> Changes in v4:
> - Remove unrelated changes, add more comments
> - Link to v3: https://lore.kernel.org/netdev/[email-masked]/
> Changes in v3:
> - Convert BUG_ON to WARN_ON_ONCE
> - Link to v2: https://lore.kernel.org/netdev/[email-masked]/
> Changes in v2:
> - Remove incorrect give_pages call
> - Link to v1: https://lore.kernel.org/netdev/[email-masked]/
> ---
>  drivers/net/virtio_net.c | 25 ++++++++++++-------------
>  1 file changed, 12 insertions(+), 13 deletions(-)
> 
> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> index a757cbcab87f..461ad1019c37 100644
> --- a/drivers/net/virtio_net.c
> +++ b/drivers/net/virtio_net.c
> @@ -910,17 +910,6 @@ static struct sk_buff *page_to_skb(struct virtnet_info *vi,
>  		goto ok;
>  	}
>  
> -	/*
> -	 * Verify that we can indeed put this data into a skb.
> -	 * This is here to handle cases when the device erroneously
> -	 * tries to receive more than is possible. This is usually
> -	 * the case of a broken device.
> -	 */
> -	if (unlikely(len > MAX_SKB_FRAGS * PAGE_SIZE)) {
> -		net_dbg_ratelimited("%s: too much data\n", skb->dev->name);
> -		dev_kfree_skb(skb);
> -		return NULL;
> -	}
>  	BUG_ON(offset >= PAGE_SIZE);
>  	while (len) {
>  		unsigned int frag_size = min((unsigned)PAGE_SIZE - offset, len);
> @@ -2107,9 +2096,19 @@ static struct sk_buff *receive_big(struct net_device *dev,
>  				   struct virtnet_rq_stats *stats)
>  {
>  	struct page *page = buf;
> -	struct sk_buff *skb =
> -		page_to_skb(vi, rq, page, 0, len, PAGE_SIZE, 0);
> +	struct sk_buff *skb;
> +
> +	/* Make sure that len does not exceed the allocated size in
> +	 * add_recvbuf_big.

you mean "the size allocated in add_recvbuf_big"

> +	 */
> +	if (unlikely(len > (vi->big_packets_num_skbfrags + 1) * PAGE_SIZE)) {
> +		pr_debug("%s: rx error: len %u exceeds allocate size %lu\n",

allocated?

> +			 dev->name, len,
> +			 (vi->big_packets_num_skbfrags + 1) * PAGE_SIZE);
> +		goto err;
> +	}
>  
> +	skb = page_to_skb(vi, rq, page, 0, len, PAGE_SIZE, 0);
>  	u64_stats_add(&stats->bytes, len - vi->hdr_len);
>  	if (unlikely(!skb))
>  		goto err;
> -- 
> 2.43.0