Re: [PATCH v2 net] net: virtio_net_hdr_to_skb: count transport header in UFO

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

On Tue, Nov 16, 2021 at 6:43 PM Jonathan Davies
<jonathan.davies@nutanix.com> wrote:
>
> 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: 6dd912f8 ("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>

> ---
> Changes in v2:
>  - refactor to use variable for readability
> ---
>  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 b465f8f..04e87f4b 100644
> --- a/include/linux/virtio_net.h
> +++ b/include/linux/virtio_net.h
> @@ -120,10 +120,15 @@ static inline int virtio_net_hdr_to_skb(struct sk_buff *skb,
>
>         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;

Subtracting from an unsigned int always has the chance of negative overflow.

This case is safe, as all three paths that lead here have a p_off >= thlen.

I just noticed a more obscure fourth path:

        if (hdr->gso_type != VIRTIO_NET_HDR_GSO_NONE) {
                switch (hdr->gso_type & ~VIRTIO_NET_HDR_GSO_ECN) {

We do not explicitly check against hdr->gso_type ==
VIRTIO_NET_HDR_GSO_ECN. An obviously bogus value. That leaves p_off 0.
But it also leaves th_len 0, so it is safe.

Negative overflow is also safe in this case.
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