On Thu, Oct 15, 2026 at 08:41:39AM -0400, 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 > > 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. > > Assisted-by: LLM > 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]> > --- > include/linux/virtio_net.h | 251 +++++++++++++++++++++++++++++++++++-- > 1 file changed, 241 insertions(+), 10 deletions(-) > > diff --git a/include/linux/virtio_net.h b/include/linux/virtio_net.h > index d6466f96cdd0..357115ff0ec2 100644 > --- a/include/linux/virtio_net.h > +++ b/include/linux/virtio_net.h > @@ -5,7 +5,11 @@ > #include <linux/if_vlan.h> > #include <linux/ip.h> > #include <linux/ipv6.h> > +#include <linux/netdevice.h> > #include <linux/udp.h> > +#include <net/gso.h> > +#include <net/ip.h> > +#include <net/ipv6.h> > #include <net/tcp.h> > #include <uapi/linux/tcp.h> > #include <uapi/linux/virtio_net.h> > @@ -48,6 +52,171 @@ 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. Only the fixed header > + * is guaranteed linear; callers that need the options or the > + * transport header must pull further. > + */ > +static inline struct iphdr *virtio_net_ip4_parse(struct sk_buff *skb, > + unsigned int nhoff, > + unsigned int *thoff) > +{ > + 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; > + > + *thoff = nhoff + iph->ihl * sizeof(u32); > + return iph; > +} > + > +/* 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) > +{ > + if (skb->dev->type == ARPHRD_ETHER) { > + if (!pskb_may_pull(skb, ETH_HLEN)) > + return false; > + *nhoff = ETH_HLEN; > + *proto = eth_hdr(skb)->h_proto; > + if (eth_type_vlan(*proto)) { > + *proto = __vlan_get_protocol(skb, *proto, nhoff); > + if (!*proto) > + return false; > + } > + } else { > + *nhoff = skb_network_offset(skb); > + *proto = skb->protocol; > + } > + 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; > + *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. > + * > + * Return 1 if the checksum was completed in software, else a negative > + * errno. > + */ > +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) > @@ -158,8 +327,6 @@ static inline int __virtio_net_hdr_to_skb(struct sk_buff > *skb, > case SKB_GSO_UDP_L4: > if (!(hdr->flags & VIRTIO_NET_HDR_F_NEEDS_CSUM)) > return -EINVAL; > - if (skb->csum_offset != offsetof(struct udphdr, check)) > - return -EINVAL; > if (skb->len - p_off > gso_size * UDP_MAX_SEGMENTS) > return -EINVAL; > if (gso_type != SKB_GSO_UDP_L4) > @@ -167,10 +334,6 @@ static inline int __virtio_net_hdr_to_skb(struct sk_buff > *skb, > break; > case SKB_GSO_TCPV4: > case SKB_GSO_TCPV6: > - if (skb->ip_summed == CHECKSUM_PARTIAL && > - skb->csum_offset != offsetof(struct tcphdr, check)) > - return -EINVAL; > - > BUILD_BUG_ON(TCP_MIN_GSO_SIZE * GSO_MAX_SEGS < > GSO_MAX_SIZE); > gso_size = max(gso_size, TCP_MIN_GSO_SIZE); > break; > @@ -194,11 +357,36 @@ 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 (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; > + ret = virtio_net_resolve_csum_partial_mismatch(skb); > + > + return ret; > } > > /* This function must be called after virtio_net_hdr_from_skb(). */ > @@ -282,6 +470,35 @@ 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, > @@ -293,6 +510,7 @@ virtio_net_hdr_tnl_to_skb(struct sk_buff *skb, > 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; > > @@ -332,10 +550,13 @@ virtio_net_hdr_tnl_to_skb(struct sk_buff *skb, > inner_th < inner_nh + inner_l3min) > return -EINVAL; > > + if (!virtio_net_tnl_offsets_ok(skb, outer_isv6, outer_th)) > + return -EINVAL; > + > /* 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; > > /* In case of USO, the inner protocol is still unknown and > @@ -353,7 +574,7 @@ virtio_net_hdr_tnl_to_skb(struct sk_buff *skb, > } > > skb_set_inner_protocol(skb, inner_isv6 ? htons(ETH_P_IPV6) : > - htons(ETH_P_IP)); > + htons(ETH_P_IP)); > if (hdr->flags & VIRTIO_NET_HDR_F_UDP_TUNNEL_CSUM) { > if (!tnl_csum_negotiated) > return -EINVAL; > @@ -368,7 +589,17 @@ 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; > - return 0; > + > + /* > + * We know VIRTIO_NET_HDR_F_NEEDS_CSUM was set so > __virtio_net_hdr_to_skb > + * set CHECKSUM_PARTIAL. > + */ > + inner_proto = inner_isv6 ? htons(ETH_P_IPV6) : htons(ETH_P_IP); > + if (virtio_net_csum_partial_ok_at_offset(skb, gso_inner_type, inner_nh, > + inner_proto)) > + return 0; > + > + return virtio_net_resolve_csum_partial_mismatch(skb); > } >
Damn. I went over my testing and it was using qemu from the fedora install, all the tunnels features were not there and were not tested ( That will need more work. I will send v3 in a couple of days. > /* Checksum-related fields validation for the driver */ > -- > MST >
