On Wed, Sep 23, 2026 at 12:46:42PM +0200, Eric Dumazet wrote: > On Wed, Sep 23, 2026 at 12:21 PM Michael S. Tsirkin <[email protected]> wrote: > > > > 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 does, but csum_start/csum_offset are not "weird packet" material, > they are skb metadata, not bytes on the wire. > > For essentially every skb in the kernel, this metadata is produced by > the kernel itself, > from headers it has just built or just parsed, and the rest of the > stack (GSO, skb_checksum_help(), > fragmentation, netfilter, and every driver doing TX csum offload) > consumes it as an invariant. > The only producers of attacker/guest controlled CHECKSUM_PARTIAL metadata are > the virtio_net_hdr_to_skb() callers: af_packet, tun/tap, virtio_net, > and the UML vector driver. > > So this is not "virtio specific validation", this is input validation > at the one trust boundary > where the invariant can be violated.
Thanks for the explanation Eric! > > > It seems likely that not all drivers validate headers defensively, > > and incoming packets can easily become outgoing ones. > > Packets coming from a real NIC are CHECKSUM_UNNECESSARY, > CHECKSUM_COMPLETE or CHECKSUM_NONE. > They do not carry a remote-provided csum_start. > (Remote checksum offload is the rare exception, and there the offsets > are computed by the stack from headers it just parsed.) > > Incoming packets becoming outgoing ones is precisely the problem here: > a virtio_net RX skb with VIRTIO_NET_HDR_F_NEEDS_CSUM becomes > an skb that can be bridged/forwarded/fragmented and then handed to a real NIC. Well: $ git grep 'skb->csum_start\ =' drivers/net/ethernet/hisilicon/hns3/hns3_enet.c: skb->csum_start = (unsigned char *)th - skb->head; drivers/net/ethernet/mellanox/mlx5/core/en_rx.c: skb->csum_start = (unsigned char *)uh - skb->head; drivers/net/ethernet/mellanox/mlx5/core/en_rx.c: skb->csum_start = (unsigned char *)uh - skb->head; drivers/net/ethernet/mellanox/mlx5/core/en_rx.c: skb->csum_start = (unsigned char *)tcp - skb->head; drivers/net/ethernet/mellanox/mlx5/core/en_rx.c: skb->csum_start = (unsigned char *)tcp - skb->head; drivers/net/ethernet/stmicro/stmmac/stmmac_selftests.c: skb->csum_start = skb_transport_header(skb) - skb->head; What did I miss? > > > 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? > > The core stack cannot afford it. > > 1) Drivers program skb->csum_start / skb->csum_offset straight into > the TX descriptor, and the NIC will happily write two bytes wherever > it was told, e.g. into the IP header of the frame we put on the wire. > Auditing/adding checks in every driver is not realistic, and drivers > must not pay for it. > > 2) Catching this in the core would mean re-parsing L2/L3 in > dev_hard_start_xmit() > (or in every place that eventually looks at the transport header) for > the 99.99+% > of packets that were built by the stack and are known to be consistent. > That is far more expensive than one check at injection time. > > 3) Failing at injection returns -EINVAL to the sendmsg()/writev() caller, > which is the correct and testable behavior. Failing later means dropping > the packet deep in the xmit path, usually with a splat: the commit being fixed > here (49d14b54a527) exists exactly because such a packet reached > skb_checksum_help() from ip_do_fragment() and hit the "offset (-6) >= > skb_headlen() (14)" WARN. > > > > > 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 > > I do not expect anything measurable. > > af_packet and tun: we have just copied that header from user space, it is in > L1. > virtio_net: we call eth_type_trans() right after, and GRO parses L3/L4 > immediately. > > The whole block is under the CHECKSUM_PARTIAL condition, where we already > do pskb_may_pull() and skb_partial_csum_set(), i.e. we already touch > this cache line. > Reading iph->ihl from a cache line we are about to read anyway is noise. > > > - future protocol extensions that now will require surgery in > > virtio, instead of just being passed through to the host > > Fair, and this is an argument about how the check is written, not > about whether it exists. > The rule should be: > > Only tighten the bound for the protocols we already parse (IPv4/IPv6), > and keep the existing generic minimum for anything else. > > Then an unknown ethertype simply keeps flowing, no surgery is needed. > Willem's version does that. A new protocol would only be impacted if it wanted > csum_start to point inside what we consider the L3 header, and such a packet > would not survive the rest of the stack anyway. > > So: not defense in depth, but validation at the trust boundary, where > it is cheapest > and where we can still report the error to the producer. > > > > > What do others think? > > > > -- > > MST > >
