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Fri, 30 Oct 2026 14:28:54 -0700 (PDT) Received: from 20HS2G4 ([2a09:bac6:bf21:15d7::22d:91]) by smtp.gmail.com with ESMTPSA id 5614622812f47-5640fd93451sm2339531b6e.3.2026.10.30.14.28.52 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 30 Oct 2026 14:28:54 -0700 (PDT) Date: Fri, 30 Oct 2026 16:28:50 -0500 From: Chris Arges To: "Michael S. Tsirkin" Cc: linux-kernel@vger.kernel.org, Willem de Bruijn , =?utf-8?B?QmrDtnJuIFTDtnBlbA==?= , Phil Sutter , Jason Wang , Eugenio =?utf-8?B?UMOpcmV6?= , Xuan Zhuo , Andrew Lunn , "David S. Miller" , Eric Dumazet , Jakub Kicinski , Paolo Abeni , netdev@vger.kernel.org, virtualization@lists.linux.dev, Max Krasnyansky , Rusty Russell , Paulos Yibelo Subject: Re: [PATCH RFC v7 4/5] net: resolve uncommon partial checksums at ingress Message-ID: References: <93d0e37c99711bebdf60c120af03010455c2f2e2.1793276116.git.mst@redhat.com> X-Mailing-List: virtualization@lists.linux.dev List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <93d0e37c99711bebdf60c120af03010455c2f2e2.1793276116.git.mst@redhat.com> On 2026-10-29 14:53:34, Michael S. Tsirkin wrote: > As reported first by Paulos Yibelo (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. > v7: Tested-by: Chris Arges > Reported-by: Paulos Yibelo > Reported-by: Chris Arges > Tested-by: Chris Arges > Fixes: f43798c27684 ("tun: Allow GSO using virtio_net_hdr") > Suggested-by: Willem de Bruijn > Signed-off-by: Michael S. Tsirkin > --- > > Notes (changelog): > v4->v5: > caller changes moved to "account for ingress-resolved > virtio net hdr state" (Willem de Bruijn) > a packet too small to be GSO: ignore the tunnel GSO request, > complete wild metadata in place (sashiko) > document the zero copy interaction (sashiko) > honor a preset skb->protocol on ethernet: the stack uses it, > except on loopback which re-parses the frame (sashiko) > v5->v6: > read both skb->protocol and the frame ethertype: do not > validate when they disagree, instead of preferring the > preset protocol; same check after the IP version inference > where there is no L2 header (Willem de Bruijn) > pull the IPv4 options in the helper as introduced, with the > transport header offset computed before the pull > (Willem de Bruijn) > fold the trivial protocol calculation cleanup, formerly a > separate patch, in (Willem de Bruijn) > > include/linux/virtio_net.h | 315 ++++++++++++++++++++++++++++++++++++- > 1 file changed, 311 insertions(+), 4 deletions(-) > > diff --git a/include/linux/virtio_net.h b/include/linux/virtio_net.h > index 013c6c7e2dea..54048759cc6e 100644 > --- a/include/linux/virtio_net.h > +++ b/include/linux/virtio_net.h > @@ -5,7 +5,11 @@ > #include > #include > #include > +#include > #include > +#include > +#include > +#include > #include > #include > #include > @@ -48,6 +52,216 @@ static inline int virtio_net_hdr_set_proto(struct sk_buff *skb, > return 0; > } > > +/* TODO: nothing virtio specific here, move to ip.h */ > + > +/* > + * Pull and minimally validate the IPv4 header at @nhoff. > + * Returns the header and stores the transport header offset in > + * @thoff, or NULL if truncated or malformed. The whole header, > + * options included, is guaranteed linear on return. > + */ > +static inline struct iphdr *virtio_net_ip4_parse(struct sk_buff *skb, > + unsigned int nhoff, > + unsigned int *thoff) > +{ > + unsigned int ihl; > + struct iphdr *iph; > + > + if (!pskb_may_pull(skb, nhoff + sizeof(struct iphdr))) > + return NULL; > + > + iph = (struct iphdr *)(skb->data + nhoff); > + if (iph->version != 4 || > + iph->ihl < sizeof(struct iphdr) / sizeof(u32)) > + return NULL; > + > + ihl = iph->ihl * sizeof(u32); > + *thoff = nhoff + ihl; > + /* Pull the options as well: the transport header offset > + * returned above must be within the linear data. iph goes > + * stale across the pull. > + */ > + if (!pskb_may_pull(skb, nhoff + ihl)) > + return NULL; > + > + return (struct iphdr *)(skb->data + nhoff); > +} > + > +/* Where does the L3 header start, and what protocol is it? > + * Handles Ethernet and VLAN headers; other devices use the > + * network header offset the caller set up. > + */ > +static inline bool > +virtio_net_l3_start(struct sk_buff *skb, int *nhoff, __be16 *proto) > +{ > + /* skb->dev should always be set here already, > + * but let's not crash when it is not. > + */ > + if (WARN_ON_ONCE(!skb->dev)) > + return false; > + > + if (skb->dev->type == ARPHRD_ETHER || > + skb->dev->type == ARPHRD_LOOPBACK) { > + __be16 h_proto; > + > + if (!pskb_may_pull(skb, ETH_HLEN)) > + return false; > + h_proto = eth_hdr(skb)->h_proto; > + /* skb->protocol is a sender claim, and parts of the > + * stack parse the packet per it: when it disagrees with > + * the frame we do not know which view the packet will > + * be parsed as, so do not validate. > + */ > + if (skb->protocol && skb->protocol != h_proto) > + return false; > + *nhoff = ETH_HLEN; > + *proto = h_proto; > + if (eth_type_vlan(*proto)) { > + *proto = __vlan_get_protocol(skb, *proto, nhoff); > + if (!*proto) > + return false; > + } > + } else { > + const struct iphdr *iph; > + > + *nhoff = skb_network_offset(skb); > + /* An unset network header shows up as an offset > + * outside of the packet. > + */ > + if (*nhoff < 0 || (unsigned int)*nhoff >= skb->len) > + return false; > + /* No L2 header to cross-check with: take the protocol > + * from the IP version at the network header. > + */ > + if (!pskb_may_pull(skb, *nhoff + 1)) > + return false; > + iph = (const struct iphdr *)(skb->data + *nhoff); > + if (iph->version == 4) > + *proto = htons(ETH_P_IP); > + else if (iph->version == 6) > + *proto = htons(ETH_P_IPV6); > + else > + return false; > + if (skb->protocol && skb->protocol != *proto) > + return false; > + } > + return true; > +} > + > +/* Walk the L3 header at @nhoff: where does its transport header > + * start, and which protocol is it? Returns false if the header > + * is truncated, malformed or fragmented. > + */ > +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; > + } > + return true; > + } > + > + return false; > +} > + > +/* Does checksum metadata match a known TCP or UDP header? */ > +static inline bool > +virtio_net_csum_partial_ok_at_offset(struct sk_buff *skb, u8 hdr_gso_type, > + unsigned int nhoff, __be16 proto) > +{ > + unsigned int thoff; > + int ip_proto; > + > + if (!virtio_net_l4_start(skb, nhoff, proto, &thoff, &ip_proto)) > + return false; > + > + if (skb_checksum_start_offset(skb) != thoff) > + return false; > + > + switch (ip_proto) { > + case IPPROTO_TCP: > + if (skb->csum_offset != offsetof(struct tcphdr, check) || > + !pskb_may_pull(skb, thoff + sizeof(struct tcphdr))) > + return false; > + break; > + case IPPROTO_UDP: > + if (skb->csum_offset != offsetof(struct udphdr, check) || > + !pskb_may_pull(skb, thoff + sizeof(struct udphdr))) > + return false; > + break; > + default: > + return false; > + } > + > + switch (hdr_gso_type & ~VIRTIO_NET_HDR_GSO_ECN) { > + case VIRTIO_NET_HDR_GSO_NONE: > + return true; > + case VIRTIO_NET_HDR_GSO_TCPV4: > + return proto == htons(ETH_P_IP) && ip_proto == IPPROTO_TCP; > + case VIRTIO_NET_HDR_GSO_TCPV6: > + return proto == htons(ETH_P_IPV6) && ip_proto == IPPROTO_TCP; > + case VIRTIO_NET_HDR_GSO_UDP: > + case VIRTIO_NET_HDR_GSO_UDP_L4: > + return ip_proto == IPPROTO_UDP; > + default: > + return false; > + } > +} > + > +static inline bool > +virtio_net_csum_partial_ok(struct sk_buff *skb, u8 hdr_gso_type) > +{ > + int nhoff; > + __be16 proto; > + > + if (!virtio_net_l3_start(skb, &nhoff, &proto)) > + return false; > + > + return virtio_net_csum_partial_ok_at_offset(skb, hdr_gso_type, nhoff, > + proto); > +} > + > +/* > + * Handle a packet whose checksum metadata does not match the packet: > + * GSO packets are dropped, as segmentation needs one checksum per > + * segment; others are completed in software. > + */ > +static inline int __virtio_net_resolve_csum_partial_mismatch(struct sk_buff *skb) > +{ > + if (skb_is_gso(skb)) > + return -EINVAL; > + > + return skb_checksum_help(skb) ?: 1; > +} > + > 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) > @@ -194,11 +408,35 @@ static inline int __virtio_net_hdr_to_skb(struct sk_buff *skb, > return 0; > } > > +/* > + * Return value: > + * 0 - success > + * >0 - checksum was completed in software (slow path) > + * <0 - failure > + */ > static inline int virtio_net_hdr_to_skb(struct sk_buff *skb, > const struct virtio_net_hdr *hdr, > bool little_endian) > { > - return __virtio_net_hdr_to_skb(skb, hdr, little_endian, hdr->gso_type); > + typeof(skb->transport_header) old_transport = skb->transport_header; > + int ret = __virtio_net_hdr_to_skb(skb, hdr, little_endian, > + hdr->gso_type); > + > + if (ret < 0) > + return ret; > + > + if (likely(skb->ip_summed != CHECKSUM_PARTIAL || > + virtio_net_csum_partial_ok(skb, hdr->gso_type))) > + return 0; > + > + /* > + * Here we do not trust the checksum metadata. > + * __virtio_net_hdr_to_skb set the transport header to csum_start > + * from virtio hdr, restore it to a known good value. > + */ > + skb->transport_header = old_transport; > + > + return __virtio_net_resolve_csum_partial_mismatch(skb); > } > > /* This function must be called after virtio_net_hdr_from_skb(). */ > @@ -282,6 +520,35 @@ static inline unsigned int virtio_l3min(bool is_ipv6) > return is_ipv6 ? sizeof(struct ipv6hdr) : sizeof(struct iphdr); > } > > +/* Do the tunnel offsets match the outer headers in the frame? > + * The L3 header the tunnel claims must be the one the frame > + * carries, with UDP following it at @outer_th. > + */ > +static inline bool > +virtio_net_tnl_offsets_ok(struct sk_buff *skb, bool outer_isv6, > + unsigned int outer_th) > +{ > + int nhoff; > + __be16 proto; > + unsigned int thoff; > + int ip_proto; > + > + if (!virtio_net_l3_start(skb, &nhoff, &proto)) > + return false; > + if (outer_isv6 != (proto == htons(ETH_P_IPV6))) > + return false; > + if (!virtio_net_l4_start(skb, nhoff, proto, &thoff, &ip_proto)) > + return false; > + > + return ip_proto == IPPROTO_UDP && outer_th == thoff; > +} > + > +/* > + * Return value: > + * 0 - success > + * >0 - checksum was completed in software (slow path) > + * <0 - failure > + */ > static inline int > virtio_net_hdr_tnl_to_skb(struct sk_buff *skb, > const struct virtio_net_hdr_v1_hash_tunnel *vhdr, > @@ -290,9 +557,11 @@ virtio_net_hdr_tnl_to_skb(struct sk_buff *skb, > bool little_endian) > { > const struct virtio_net_hdr *hdr = (const struct virtio_net_hdr *)vhdr; > + typeof(skb->transport_header) old_transport = skb->transport_header; > unsigned int inner_nh, outer_th, inner_th; > unsigned int inner_l3min, outer_l3min; > u8 gso_inner_type, gso_tunnel_type; > + __be16 inner_proto; > bool outer_isv6, inner_isv6; > int ret; > > @@ -335,9 +604,12 @@ virtio_net_hdr_tnl_to_skb(struct sk_buff *skb, > /* Let the basic parsing deal with plain GSO features. */ > ret = __virtio_net_hdr_to_skb(skb, hdr, true, > hdr->gso_type & ~gso_tunnel_type); > - if (ret) > + if (ret < 0) > return ret; > > + if (!virtio_net_tnl_offsets_ok(skb, outer_isv6, outer_th)) > + return -EINVAL; > + > /* In case of USO, the inner protocol is still unknown and > * `inner_isv6` is just a guess, additional parsing is needed. > * The previous validation ensures that accessing an ipv4 inner > @@ -352,9 +624,29 @@ virtio_net_hdr_tnl_to_skb(struct sk_buff *skb, > return -EINVAL; > } > > - skb_set_inner_protocol(skb, inner_isv6 ? htons(ETH_P_IPV6) : > - htons(ETH_P_IP)); > + inner_proto = inner_isv6 ? htons(ETH_P_IPV6) : htons(ETH_P_IP); > + > + /* > + * We know VIRTIO_NET_HDR_F_NEEDS_CSUM was set so __virtio_net_hdr_to_skb > + * set CHECKSUM_PARTIAL. > + */ > + if (unlikely(!virtio_net_csum_partial_ok_at_offset(skb, gso_inner_type, > + inner_nh, > + inner_proto))) { > + /* __virtio_net_hdr_to_skb set the transport header to > + * csum_start, restore it to a known good value. > + */ > + skb->transport_header = old_transport; > + > + return __virtio_net_resolve_csum_partial_mismatch(skb); > + } > + > + skb_set_inner_protocol(skb, inner_proto); > if (hdr->flags & VIRTIO_NET_HDR_F_UDP_TUNNEL_CSUM) { > + /* Checked here, after the validation: a packet completed > + * in software never uses the feature, so an unnegotiated > + * request only matters for metadata good enough to act on. > + */ > if (!tnl_csum_negotiated) > return -EINVAL; > > @@ -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); > + } > + > return 0; > } > > -- > MST >