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>
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>
    Reviewed-by: Willem de Bruijn <willemb>
    Signed-off-by: David S. Miller <davem>
+++


Current workaround is to disable UFO offloading on guest VMs