netdev - Re: [PATCH net v3] virtio-net: fix received length check in big packets

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Message-ID: <CACGkMEv3KzWjxtBuyga5NDcvBUQzsytu00g-2CwMrQGf0vrqAQ@mail.gmail.com>
Date: Wed, 22 Oct 2025 12:14:07 +0800
From: Jason Wang <jasowang@...hat.com>
To: Bui Quang Minh <minhquangbui99@...il.com>
Cc: netdev@...r.kernel.org, "Michael S. Tsirkin" <mst@...hat.com>, 
	Xuan Zhuo <xuanzhuo@...ux.alibaba.com>, Eugenio Pérez <eperezma@...hat.com>, 
	Andrew Lunn <andrew+netdev@...n.ch>, "David S. Miller" <davem@...emloft.net>, 
	Eric Dumazet <edumazet@...gle.com>, Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>, 
	Gavin Li <gavinl@...dia.com>, Gavi Teitz <gavi@...dia.com>, Parav Pandit <parav@...dia.com>, 
	virtualization@...ts.linux.dev, linux-kernel@...r.kernel.org
Subject: Re: [PATCH net v3] virtio-net: fix received length check in big packets

On Tue, Oct 21, 2025 at 11:45 PM Bui Quang Minh
<minhquangbui99@...il.com> 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

And guest gso features.

>. 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.

It might also be helpful to describe how you can reproduce this issue.

>
> Fixes: 4959aebba8c0 ("virtio-net: use mtu size as buffer length for big packets")

Cc: stable@...r.kernel.org

> Signed-off-by: Bui Quang Minh <minhquangbui99@...il.com>
> ---
> Changes in v3:
> - Convert BUG_ON to WARN_ON_ONCE
> Changes in v2:
> - Remove incorrect give_pages call
> ---
>  drivers/net/virtio_net.c | 18 +++++++++++++-----
>  1 file changed, 13 insertions(+), 5 deletions(-)
>
> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> index a757cbcab87f..e7b33e40ea99 100644
> --- a/drivers/net/virtio_net.c
> +++ b/drivers/net/virtio_net.c
> @@ -852,7 +852,7 @@ 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;
> @@ -916,12 +916,16 @@ 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)) {
> +       if (WARN_ON_ONCE(offset >= PAGE_SIZE))
> +               goto err;

I'd stick to BUG_ON or at least it needs an independent patch.

> +
> +       max_remaining_len = (unsigned int)PAGE_SIZE - offset;
> +       max_remaining_len += vi->big_packets_num_skbfrags * PAGE_SIZE;

Let's add a comment to explain the algorithm here or at least mention
to refer add_recvbuf_big().

> +       if (unlikely(len > max_remaining_len)) {
>                 net_dbg_ratelimited("%s: too much data\n", skb->dev->name);
> -               dev_kfree_skb(skb);
> -               return NULL;
> +               goto err;

It looks like this change is not needed?

>         }
> -       BUG_ON(offset >= PAGE_SIZE);
> +

Unnecessary changes.

>         while (len) {
>                 unsigned int frag_size = min((unsigned)PAGE_SIZE - offset, len);
>                 skb_add_rx_frag(skb, skb_shinfo(skb)->nr_frags, page, offset,
> @@ -941,6 +945,10 @@ static struct sk_buff *page_to_skb(struct virtnet_info *vi,
>                 put_page(page_to_free);
>
>         return skb;
> +
> +err:
> +       dev_kfree_skb(skb);
> +       return NULL;
>  }
>
>  static void virtnet_rq_unmap(struct receive_queue *rq, void *buf, u32 len)
> --
> 2.43.0
>

Thanks