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  1. RHEL
  2. RHEL-6030

backport "net: virtio_net_hdr_to_skb: count transport header in UFO"

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    • kernel-4.18.0-553.el8_10
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    • ZStream, CustomerScenariosInitiative
    • sst_networking_core
    • ssg_networking
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      Description of problem:

      With UDP segmentation offload enabled on Guests, big packets gets dropped before reaching Guest VM.

      Version-Release number of selected component (if applicable):
      kernel-4.18.0-372.19.1.el8_6

      How reproducible:

      Always

      Below upstream commit is solving this issue and is tested by customer and also locally.

      The test kernel was provided to customer over RHEL kernel 4.18.0-372.19.1.el8_6

      +++
      commit cf9acc90c80ecbee00334aa85d92f4e74014bcff
      Author: Jonathan Davies <jonathan.davies@nutanix.com>
      Date: Tue Nov 16 17:42:42 2021 +0000

      net: virtio_net_hdr_to_skb: count transport header in UFO

      virtio_net_hdr_to_skb does not set the skb's gso_size and gso_type
      correctly for UFO packets received via virtio-net that are a little over
      the GSO size. This can lead to problems elsewhere in the networking
      stack, e.g. ovs_vport_send dropping over-sized packets if gso_size is
      not set.

      This is due to the comparison

      if (skb->len - p_off > gso_size)

      not properly accounting for the transport layer header.

      p_off includes the size of the transport layer header (thlen), so
      skb->len - p_off is the size of the TCP/UDP payload.

      gso_size is read from the virtio-net header. For UFO, fragmentation
      happens at the IP level so does not need to include the UDP header.

      Hence the calculation could be comparing a TCP/UDP payload length with
      an IP payload length, causing legitimate virtio-net packets to have
      lack gso_type/gso_size information.

      Example: a UDP packet with payload size 1473 has IP payload size 1481.
      If the guest used UFO, it is not fragmented and the virtio-net header's
      flags indicate that it is a GSO frame (VIRTIO_NET_HDR_GSO_UDP), with
      gso_size = 1480 for an MTU of 1500. skb->len will be 1515 and p_off
      will be 42, so skb->len - p_off = 1473. Hence the comparison fails, and
      shinfo->gso_size and gso_type are not set as they should be.

      Instead, add the UDP header length before comparing to gso_size when
      using UFO. In this way, it is the size of the IP payload that is
      compared to gso_size.

      Fixes: 6dd912f82680 ("net: check untrusted gso_size at kernel entry")
      Signed-off-by: Jonathan Davies <jonathan.davies@nutanix.com>
      Reviewed-by: Willem de Bruijn <willemb@google.com>
      Signed-off-by: David S. Miller <davem@davemloft.net>
      +++

      Current workaround is to disable UFO offloading on guest VMs

            lulu-redhat-name Cindy Lu
            rhn-support-surkumar Suresh Kumar
            Jason Wang
            Cindy Lu Cindy Lu
            Lei Yang Lei Yang
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              Created:
              Updated:
              Resolved: