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

[Daniel Borkmann <daniel@iogearbox.net>]

[ ping to Jesper wrt feedback ]

On 09/26/2018 07:36 AM, Song Liu wrote:
> 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:
>>