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

[Xiang Mei]

On Wed, Jun 10, 2026 at 7:41 PM Xuan Zhuo <xuanzhuo@xxxxxxxxxxxxxxxxx> wrote:
>
> On Wed, 10 Jun 2026 19:24:03 -0700, Xiang Mei <xmei5@xxxxxxx> wrote:
> > Thanks for the review. I agree with that as I replied at the end of
> > v1. If we obsolete 2/2 but keep 1/2, is it okay to just leave it as
> > is?
>
> You should post a new version.
>
Thanks for the tips! V3 has been sent out.

Xiang
> Thanks.
>
> >
> > Xiang
> >
> > On Wed, Jun 10, 2026 at 7:19 PM Xuan Zhuo <xuanzhuo@xxxxxxxxxxxxxxxxx> wrote:
> > >
> > > 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
> > > >