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    Subject[PATCH 5.10 099/154] net: virtio_net_hdr_to_skb: count transport header in UFO
    Date
    From: Jonathan Davies <jonathan.davies@nutanix.com>

    [ Upstream commit cf9acc90c80ecbee00334aa85d92f4e74014bcff ]

    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>
    Signed-off-by: Sasha Levin <sashal@kernel.org>
    ---
    include/linux/virtio_net.h | 7 ++++++-
    1 file changed, 6 insertions(+), 1 deletion(-)

    diff --git a/include/linux/virtio_net.h b/include/linux/virtio_net.h
    index b465f8f3e554f..04e87f4b9417c 100644
    --- a/include/linux/virtio_net.h
    +++ b/include/linux/virtio_net.h
    @@ -120,10 +120,15 @@ retry:

    if (hdr->gso_type != VIRTIO_NET_HDR_GSO_NONE) {
    u16 gso_size = __virtio16_to_cpu(little_endian, hdr->gso_size);
    + unsigned int nh_off = p_off;
    struct skb_shared_info *shinfo = skb_shinfo(skb);

    + /* UFO may not include transport header in gso_size. */
    + if (gso_type & SKB_GSO_UDP)
    + nh_off -= thlen;
    +
    /* Too small packets are not really GSO ones. */
    - if (skb->len - p_off > gso_size) {
    + if (skb->len - nh_off > gso_size) {
    shinfo->gso_size = gso_size;
    shinfo->gso_type = gso_type;

    --
    2.33.0


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