Re: [bpf-next PATCH 1/3] net: fix generic XDP to handle if eth header was mangled

[Song Liu <liu.song.a23@gmail.com>]

On Tue, Sep 25, 2018 at 7:26 AM Jesper Dangaard Brouer
<brouer@redhat.com> wrote:
>
> XDP can modify (and resize) the Ethernet header in the packet.
>
> There is a bug in generic-XDP, because skb->protocol and skb->pkt_type
> are setup before reaching (netif_receive_)generic_xdp.
>
> This bug was hit when XDP were popping VLAN headers (changing
> eth->h_proto), as skb->protocol still contains VLAN-indication
> (ETH_P_8021Q) causing invocation of skb_vlan_untag(skb), which corrupt
> the packet (basically popping the VLAN again).
>
> This patch catch if XDP changed eth header in such a way, that SKB
> fields needs to be updated.
>
> Fixes: d445516966dc ("net: xdp: support xdp generic on virtual devices")
> Signed-off-by: Jesper Dangaard Brouer <brouer@redhat.com>
> ---
>  net/core/dev.c |   14 ++++++++++++++
>  1 file changed, 14 insertions(+)
>
> diff --git a/net/core/dev.c b/net/core/dev.c
> index ca78dc5a79a3..db6d89f536cb 100644
> --- a/net/core/dev.c
> +++ b/net/core/dev.c
> @@ -4258,6 +4258,9 @@ static u32 netif_receive_generic_xdp(struct sk_buff *skb,
>         struct netdev_rx_queue *rxqueue;
>         void *orig_data, *orig_data_end;
>         u32 metalen, act = XDP_DROP;
> +       __be16 orig_eth_type;
> +       struct ethhdr *eth;
> +       bool orig_bcast;
>         int hlen, off;
>         u32 mac_len;
>
> @@ -4298,6 +4301,9 @@ static u32 netif_receive_generic_xdp(struct sk_buff *skb,
>         xdp->data_hard_start = skb->data - skb_headroom(skb);
>         orig_data_end = xdp->data_end;
>         orig_data = xdp->data;
> +       eth = (struct ethhdr *)xdp->data;
> +       orig_bcast = is_multicast_ether_addr_64bits(eth->h_dest);
> +       orig_eth_type = eth->h_proto;
>
>         rxqueue = netif_get_rxqueue(skb);
>         xdp->rxq = &rxqueue->xdp_rxq;
> @@ -4321,6 +4327,14 @@ static u32 netif_receive_generic_xdp(struct sk_buff *skb,
>
>         }
>
> +       /* check if XDP changed eth hdr such SKB needs update */
> +       eth = (struct ethhdr *)xdp->data;
> +       if ((orig_eth_type != eth->h_proto) ||
> +           (orig_bcast != is_multicast_ether_addr_64bits(eth->h_dest))) {

Is the actions below always correct for the condition above? Do we need
to confirm the SKB is updated properly?

> +               __skb_push(skb, mac_len);
> +               skb->protocol = eth_type_trans(skb, skb->dev);
> +       }
> +
>         switch (act) {
>         case XDP_REDIRECT:
>         case XDP_TX:
>