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

["Michael S. Tsirkin" <mst@redhat.com>]

On Thu, Jun 11, 2026 at 10:18:49AM +0800, Xuan Zhuo wrote:
> On Wed, 10 Jun 2026 16:29:36 -0700, Xiang Mei <xmei5@asu.edu> 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.

i don't get it, and then what?

> >
> > 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.


I will be frank, I'm never sure where the confidential computing guys
draw the line.

Are speculative things of concern, for example?


> >
> > Signed-off-by: Xiang Mei <xmei5@asu.edu>
> > ---
> > 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) {


don't see why we would check page

> >  		unsigned int frag_size = min((unsigned)PAGE_SIZE - offset, len);
> > +
> > +		if (unlikely(skb_shinfo(skb)->nr_frags >= MAX_SKB_FRAGS))
> > +			break;

so do we want BUG_ON here maybe?

> >  		skb_add_rx_frag(skb, skb_shinfo(skb)->nr_frags, page, offset,
> >  				frag_size, truesize);
> >  		len -= frag_size;
> > --
> > 2.43.0
> >