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

[Jason Wang <jasowang@redhat.com>]

On Mon, Jul 7, 2025 at 11:48 AM Jason Wang <jasowang@redhat.com> wrote:
>
> On Sun, Jul 6, 2025 at 10:12 PM Bui Quang Minh <minhquangbui99@gmail.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. 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>
> > ---
> >  drivers/net/virtio_net.c | 10 +++++++---
> >  1 file changed, 7 insertions(+), 3 deletions(-)
> >
> > diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> > index 5d674eb9a0f2..ead1cd2fb8af 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 +887,16 @@ 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)) {
> > +       BUG_ON(offset >= PAGE_SIZE);
> > +       max_remaining_len = (unsigned int)PAGE_SIZE - offset;
> > +       max_remaining_len += vi->big_packets_num_skbfrags * PAGE_SIZE;
> > +       if (unlikely(len > max_remaining_len)) {
> >                 net_dbg_ratelimited("%s: too much data\n", skb->dev->name);
> >                 dev_kfree_skb(skb);
> > +               give_pages(rq, page);
>
> Should this be an independent fix?

Btw, the page_to_skb() is really kind of messed up right now that it
is used by both big and mergeable. We may need to consider splitting
the logic in the future.

Thanks

>
> >                 return NULL;
> >         }
> > -       BUG_ON(offset >= PAGE_SIZE);
> > +
> >         while (len) {
> >                 unsigned int frag_size = min((unsigned)PAGE_SIZE - offset, len);
> >                 skb_add_rx_frag(skb, skb_shinfo(skb)->nr_frags, page, offset,
> > --
> > 2.43.0
> >
>
> Thanks