On Tue, Sep 22, 2026 at 09:27:26PM -0400, Willem de Bruijn wrote:
> Willem de Bruijn wrote:
> > Paulos Yibelo wrote:
> > > __virtio_net_hdr_to_skb() checks a minimum network-header length for
> > > CHECKSUM_PARTIAL packets. Its checksum start is relative to skb->data,
> > > but some callers have not established skb->network_header when they
> > > convert the virtio header.
> > >
> > > Pass the data-relative L3 origin explicitly. Ethernet receive paths
> > > parse the frame and nested VLAN headers without changing skb state.
> > > AF_PACKET uses the frame's actual L3 origin even when the socket
> > > protocol is ETH_P_IP and the raw frame carries VLAN tags. Non-Ethernet
> > > AF_PACKET devices retain their established skb network offset.
> > >
> > > Also pass the actual L3 protocol so IPv6 packets use the 40-byte base
> > > header minimum even without TCPv6 GSO. IFF_TUN obtains that protocol
> > > from the packet before skb->protocol is set. Name the Ethernet parser
> > > accordingly, use the same origin for tunnel validation, and propagate
> > > conversion failures in UML.
> > >
> > > The bound remains a minimum; fragmentation paths separately validate
> > > the parsed IPv4 or IPv6 header length before completing a checksum.
> > >
> > > Fixes: 49d14b54a527 ("net: test for not too small csum_start in
> > > virtio_net_hdr_to_skb()")
> > > Fixes: a2fb4bc4e2a6 ("net: implement virtio helpers to handle UDP GSO
> > > tunneling.")
> > > Reported-by: Paulos Yibelo <[email protected]>
> > > Link:
> > > https://lore.kernel.org/netdev/[email protected]/
> > > Cc: [email protected]
> > > Assisted-by: LLM
> > > Signed-off-by: Paulos Yibelo <[email protected]>
> > > ---
> > > arch/um/drivers/vector_transports.c | 13 ++++-
> > > drivers/net/tun_vnet.h | 52 ++++++++++++++++-
> > > drivers/net/virtio_net.c | 10 +++-
> > > include/linux/virtio_net.h | 87 ++++++++++++++++++++++++-----
> > > net/packet/af_packet.c | 24 +++++++-
> > > 5 files changed, 163 insertions(+), 23 deletions(-)
> >
> > The fix may still miss the case IPv4 packets have options.
> >
> > This version is a very large patch.
> >
> > Untested shorter first suggestion by bot, which looks plausible as a
> > starting point for discussion.
>
> Cleaned up some more:
>
> diff --git a/drivers/net/tun_vnet.h b/drivers/net/tun_vnet.h
> index f4c652b1fa44..c24607af2aad 100644
> --- a/drivers/net/tun_vnet.h
> +++ b/drivers/net/tun_vnet.h
> @@ -180,6 +180,9 @@ static inline int tun_vnet_hdr_put(int sz, struct
> iov_iter *iter,
> static inline int tun_vnet_hdr_to_skb(unsigned int flags, struct sk_buff
> *skb,
> const struct virtio_net_hdr *hdr)
> {
> + if ((flags & TUN_TYPE_MASK) == IFF_TUN)
> + skb_reset_network_header(skb);
> +
> return virtio_net_hdr_to_skb(skb, hdr,
> tun_vnet_is_little_endian(flags));
> }
>
> @@ -199,6 +202,9 @@ tun_vnet_hdr_tnl_to_skb(unsigned int flags,
> netdev_features_t features,
> struct sk_buff *skb,
> const struct virtio_net_hdr_v1_hash_tunnel *hdr)
> {
> + if ((flags & TUN_TYPE_MASK) == IFF_TUN)
> + skb_reset_network_header(skb);
> +
>
> diff --git a/include/linux/virtio_net.h b/include/linux/virtio_net.h
> index c381b916c1b5..02c448de0802 100644
> --- a/include/linux/virtio_net.h
> +++ b/include/linux/virtio_net.h
> @@ -48,6 +48,42 @@ static inline int virtio_net_hdr_set_proto(struct
> sk_buff *skb,
> return 0;
> }
>
> +static inline int virtio_net_hdr_nh_min_len(const struct sk_buff *skb,
> + unsigned int nh_min_len)
> +{
> + int thoff = skb_transport_offset(skb);
> + __be16 proto;
> + int nhoff;
> +
> + if (skb_network_header_was_set(skb)) {
> + nhoff = skb_network_offset(skb);
> + proto = skb->protocol;
> + } else {
> + if (unlikely(thoff < ETH_HLEN))
> + return -EINVAL;
> + nhoff = ETH_HLEN;
> + proto = eth_hdr(skb)->h_proto;
> + }
> +
> + if (eth_type_vlan(proto)) {
> + proto = __vlan_get_protocol(skb, proto, &nhoff);
> + if (!proto)
> + return -EINVAL;
> + }
> +
> + if (proto == htons(ETH_P_IP)) {
> + const struct iphdr *iph = (void *)(skb->data + nhoff);
> +
> + if (unlikely(thoff < nhoff + sizeof(*iph)))
> + return -EINVAL;
> + nh_min_len = max_t(u32, iph->ihl * 4, sizeof(*iph));
> + } else if (proto == htons(ETH_P_IPV6)) {
> + nh_min_len = sizeof(struct ipv6hdr);
> + }
> +
> + return nhoff + nh_min_len;
> +}
> +
> static inline int __virtio_net_hdr_to_skb(struct sk_buff *skb,
> const struct virtio_net_hdr *hdr,
> bool little_endian, u8 hdr_gso_type)
> @@ -98,13 +134,15 @@ static inline int __virtio_net_hdr_to_skb(struct
> sk_buff *skb,
> u32 start = __virtio16_to_cpu(little_endian, hdr->csum_start);
> u32 off = __virtio16_to_cpu(little_endian, hdr->csum_offset);
> u32 needed = start + max_t(u32, thlen, off + sizeof(__sum16));
> + int min_thoff;
>
> if (!pskb_may_pull(skb, needed))
> return -EINVAL;
>
> if (!skb_partial_csum_set(skb, start, off))
> return -EINVAL;
> - if (skb_transport_offset(skb) < nh_min_len)
> + min_thoff = virtio_net_hdr_nh_min_len(skb, nh_min_len);
> + if (min_thoff < 0 || skb_transport_offset(skb) < min_thoff)
> return -EINVAL;
Certainly looks much better. But I'd like to ask, generally:
doesn't the net stack need to protect against weird packets?
It seems likely that not all drivers validate headers defensively,
and incoming packets can easily become outgoing ones.
So do we even need virtio specific validation, or is it enough to
validate everything in the net stack, where we are poking at the
header anyway?
Or maybe it's more a defense in depth thing?
My worries:
- more poking at the header, more cache misses, where we really
do not need that
- future protocol extensions that now will require surgery in
virtio, instead of just being passed through to the host
What do others think?
--
MST
