On Fri, Oct 30, 2026 at 03:21:57PM -0400, Willem de Bruijn wrote: > Michael S. Tsirkin wrote: > > As reported first by Paulos Yibelo <[email protected]> (after which > > others, credited below, found more ways the bug can affect users) a > > virtio net header can place CHECKSUM_PARTIAL at any in-packet offset. > > If that state is retained, later checksum completion can modify the > > packet after protocol processing has made decisions from it. > > > > Let's fix that: let's validate csum_start and csum_offset and only keep > > CHECKSUM_PARTIAL when csum_start and csum_offset match a parsed TCP or > > UDP header, while handling VLAN headers, IPv4 options and IPv6 extension > > headers. Complete other non-GSO checksums immediately, before any > > protocol processing runs. > > > > We can't easily "complete the checksum" for GSO we don't recognize - GSO > > assumes knowledge of the protocol. We could segment ignoring the > > checksum then do whatever csum_start and csum_offset say, but that > > seems like an overkill for something that shouldn't happen. > > > > There's some risk here: > > - non-gso packets which can't be parsed get checksum calculated in > > software, adding performance overhead > > - gso packets which can't be parsed get dropped > > > > Note that for zero copy, userspace can change the data after the > > checksum has been calculated. This is fundamental to zero copy by > > design, and the only outcome is a bad checksum and a packet drop. > > > > To mitigate the risks, the following patch will add a sysctl knob to > > bypass the resolution. This is reasonable if the setup is such that > > there is no protocol processing at all. > > > > UDP tunnel headers get the same handling on the inner header. > > > > Packets without VIRTIO_NET_HDR_F_NEEDS_CSUM are unaffected. > > > > Note a fundamental limitation of this approach: an nf/BPF program > > that tweaks the payload of a packet in flight may rely on the > > checksum being fixed up later, assuming the packet carries > > CHECKSUM_PARTIAL. > > > > For metadata that fails validation the checksum > > is now completed at input, so such edits leave a stale checksum > > and the packet is dropped at the receiver. Protocols whose > > transport header we cannot parse (anything but TCP and UDP) are > > affected even when the metadata is self-consistent. > > > > The sysctl added in the next patch is the escape hatch for these setups, > > but it is a very blunt tool. > > > > Reported-by: Paulos Yibelo <[email protected]> > > Reported-by: Chris Arges <[email protected]> > > Tested-by: Chris Arges <[email protected]> > > Fixes: f43798c27684 ("tun: Allow GSO using virtio_net_hdr") > > Suggested-by: Willem de Bruijn <[email protected]> > > Signed-off-by: Michael S. Tsirkin <[email protected]> > > > +static inline bool > > +virtio_net_l4_start(struct sk_buff *skb, unsigned int nhoff, __be16 proto, > > + unsigned int *thoff, int *ip_proto) > > +{ > > + if (proto == htons(ETH_P_IP)) { > > + const struct iphdr *iph = virtio_net_ip4_parse(skb, nhoff, > > thoff); > > + > > + if (!iph || ip_is_fragment(iph)) > > + return false; > > + *ip_proto = iph->protocol; > > + return true; > > + } > > + > > + if (proto == htons(ETH_P_IPV6)) { > > + const struct ipv6hdr *iph; > > + int flags = 0; > > + > > + if (!pskb_may_pull(skb, nhoff + sizeof(*iph))) > > + return false; > > + iph = (const void *)(skb->data + nhoff); > > + if (iph->version != 6) > > + return false; > > + *thoff = nhoff + sizeof(*iph); > > + *ip_proto = iph->nexthdr; > > + if (*ip_proto != IPPROTO_TCP && *ip_proto != IPPROTO_UDP) { > > + *thoff = nhoff; > > + /* Find the transport header behind any > > + * extension headers, AH included. > > + */ > > + *ip_proto = ipv6_find_hdr(skb, thoff, -1, NULL, > > + &flags); > > + if (*ip_proto < 0 || flags & IP6_FH_F_FRAG) > > + return false; > > Minor, not cause for respinning on its own. > > `flags` should include `IP6_FH_F_AUTH` so `ipv6_find_hdr()` stops at > `NEXTHDR_AUTH` instead of skipping it: > > - `CHECKSUM_PARTIAL` and GSO behind IPsec AH are invalid: AH authenticates > the inner L4 header including its checksum field > > > + } > > + return true; > > + } > > + > > + return false; > > +} > > + > > > @@ -368,6 +660,21 @@ virtio_net_hdr_tnl_to_skb(struct sk_buff *skb, > > skb->inner_mac_header = inner_nh + skb_headroom(skb); > > skb->transport_header = outer_th + skb_headroom(skb); > > skb->encapsulation = 1; > > + > > + if (!skb_is_gso(skb)) { > > + /* Too small to segment: complete the checksum in place > > + * instead of dropping - senders do tag small packets > > + * with GSO headers, e.g. packetdrill sets gso_type and > > + * gso_size with --mss, whatever the packet size. > > + * The outer UDP checksum is not calculated, but a zero > > + * one is fine: UDP over IPv4 allows a disabled > > + * checksum, and so do tunnels over IPv6 where the > > + * receiver opts in (RFC 6935, RFC 6936). > > + */ > > + skb_shinfo(skb)->gso_type &= ~(SKB_GSO_UDP_TUNNEL_CSUM | > > + SKB_GSO_UDP_TUNNEL); > > Maybe this carve-out for non-gso packet is best moved to its own > patch, as that is where the remaining issues (to merging the rest) > seem to be. > > 1. Moving `!tnl_csum_negotiated` below > `virtio_net_csum_partial_ok_at_offset()` > accepts packets with an unnegotiated `VIRTIO_NET_HDR_F_UDP_TUNNEL_CSUM` > flag whenever inner validation falls back to > `__virtio_net_resolve_csum_partial_mismatch()`. > 2. `__virtio_net_resolve_csum_partial_mismatch()` calls > `skb_checksum_help()`, > which completes only the inner checksum at `hdr->csum_start` and sets > `skb->ip_summed = CHECKSUM_NONE`, leaving the outer UDP `uh->check` > with > an uncompleted pseudo-header checksum seed. > > Despite the comment, this block neither completes any checksum in place > nor > is the outer UDP checksum zero when `VIRTIO_NET_HDR_F_UDP_TUNNEL_CSUM` is > set: > > - With `VIRTIO_NET_HDR_F_UDP_TUNNEL_CSUM`, the outer `uh->check` carries > the > non-zero pseudo-header checksum seed (as expected by > `__skb_udp_tunnel_segment()`), not `0`.
Oh yea i kept tweaking this and forgot to update the ocmment. > - Clearing `SKB_GSO_UDP_TUNNEL_CSUM` on a `!skb_is_gso(skb)` packet leaves > `skb->ip_summed == CHECKSUM_PARTIAL` at `inner_th` with nothing > completing > the outer `uh->check` on transmit/forwarding (`validate_xmit_skb()` only > completes/offloads the single inner checksum, emitting a corrupt outer > UDP > checksum). Even zeroing `uh->check` would be invalid over IPv6 unless > the > receiver explicitly opted into RFC 6936 (`UDP_NO_CHECK6_RX`). Indeed. > > + } > > + > > return 0; > > } > > > > -- > > MST > > >
