On Mon, Jun 15, 2026 at 9:40 PM Michael S. Tsirkin <[email protected]> wrote:
>
> On Mon, Jun 15, 2026 at 09:28:37PM -0700, Xiang Mei wrote:
> > receive_big() bounds the device-announced length by
> > (big_packets_num_skbfrags + 1) * PAGE_SIZE. That is still too loose:
> > add_recvbuf_big() sets sg[1] to start at offset
> > sizeof(struct padded_vnet_hdr) into the first page, so the chain
> > actually carries hdr_len + (PAGE_SIZE - sizeof(padded_vnet_hdr)) +
> > big_packets_num_skbfrags * PAGE_SIZE bytes -- 20 bytes less than the
> > check allows for the common hdr_len == 12 case.
> >
> > A malicious virtio backend can announce a len in that gap. page_to_skb()
> > then walks one frag past the page chain, storing a NULL page->private
> > into skb_shinfo()->frags[MAX_SKB_FRAGS], which is both an out-of-bounds
> > write past the static frag array and a NULL frag handed up the rx path.
> >
> > Bound len by the size add_recvbuf_big() actually advertised.
> >
> > Fixes: 0c716703965f ("virtio-net: fix received length check in big packets")
> > Reported-by: Weiming Shi <[email protected]>
> > Signed-off-by: Xiang Mei <[email protected]>
> > Reviewed-by: Xuan Zhuo <[email protected]>
>
> Acked-by: Michael S. Tsirkin <[email protected]>
>
> > ---
> > v4: use easy to understand math to compute the max_len
> > v3: revoke 2/2 and add Xuan Zhuo's Reviewed-by tag
>
> I still feel 2/2 is good defence in depth but it can be
> pursued separately.
Thanks, Michael. I'll leave 2/2 out of this series.
Appreciate the review.
Xiang
>
> > v2: add additiona check as 2/2
> >
> > drivers/net/virtio_net.c | 7 ++++---
> > 1 file changed, 4 insertions(+), 3 deletions(-)
> >
> > diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> > index f4adcfee7a80..8f4562316aaa 100644
> > --- a/drivers/net/virtio_net.c
> > +++ b/drivers/net/virtio_net.c
> > @@ -1999,15 +1999,16 @@ static struct sk_buff *receive_big(struct
> > net_device *dev,
> > struct virtnet_rq_stats *stats)
> > {
> > struct page *page = buf;
> > + unsigned long max_len = (vi->big_packets_num_skbfrags + 1) *
> > PAGE_SIZE -
> > + sizeof(struct padded_vnet_hdr) + vi->hdr_len;
> > struct sk_buff *skb;
> >
> > /* Make sure that len does not exceed the size allocated in
> > * add_recvbuf_big.
> > */
> > - if (unlikely(len > (vi->big_packets_num_skbfrags + 1) * PAGE_SIZE)) {
> > + if (unlikely(len > max_len)) {
> > pr_debug("%s: rx error: len %u exceeds allocated size %lu\n",
> > - dev->name, len,
> > - (vi->big_packets_num_skbfrags + 1) * PAGE_SIZE);
> > + dev->name, len, max_len);
> > goto err;
> > }
> >
> > --
> > 2.43.0
>
