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

[Willem de Bruijn <willemdebruijn.kernel@gmail.com>]

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>

>  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;
> -

Maybe keep this and the above check, to keep things exactly as is when the
sysctl is disabled.

Removing these checks weakens validation vs the present state.