Re: [PATCH net-next v2 4/7] net: resolve uncommon partial checksums at ingress

["Michael S. Tsirkin" <mst@redhat.com>]

On Thu, Oct 15, 2026 at 08:41:39AM -0400, Michael S. Tsirkin wrote:
> As reported first by Paulos Yibelo <habte.yibelo@gmail.com> (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 <habte.yibelo@gmail.com>
> Reported-by: Chris Arges <carges@cloudflare.com>
> Tested-by: Chris Arges <carges@cloudflare.com>
> Fixes: f43798c27684 ("tun: Allow GSO using virtio_net_hdr")
> Suggested-by: Willem de Bruijn <willemdebruijn.kernel@gmail.com>
> Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
> ---
>  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
>