[net,v3,4/5] packet: infer protocol from ethernet header if unset

In case no struct sockaddr_ll has been passed to packet
socket's sendmsg() when doing a TX_RING flush run, then
skb->protocol is set to po->num instead, which is the protocol
passed via socket(2)/bind(2).

Applications only xmitting can go the path of allocating the
socket as socket(PF_PACKET, <mode>, 0) and do a bind(2) on the
TX_RING with sll_protocol of 0. That way, register_prot_hook()
is neither called on creation nor on bind time, which saves
cycles when there's no interest in capturing anyway.

That leaves us however with po->num 0 instead and therefore
the TX_RING flush run sets skb->protocol to 0 as well. Eric
reported that this leads to problems when using tools like
trafgen over bonding device. I.e. the bonding's hash function
could invoke the kernel's flow dissector, which depends on
skb->protocol being properly set. In the current situation, all
the traffic is then directed to a single slave.

Fix it up by inferring skb->protocol from the Ethernet header
when not set and we have ARPHRD_ETHER device type. This is only
done in case of SOCK_RAW and where we have a dev->hard_header_len
length. In case of ARPHRD_ETHER devices, this is guaranteed to
cover ETH_HLEN, and therefore being accessed on the skb after
the skb_store_bits().

Reported-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
---
 net/packet/af_packet.c | 11 +++++++++++
 1 file changed, 11 insertions(+)

On Wed, Nov 11, 2015 at 5:25 PM, Daniel Borkmann <daniel@iogearbox.net> wrote:
> In case no struct sockaddr_ll has been passed to packet
> socket's sendmsg() when doing a TX_RING flush run, then
> skb->protocol is set to po->num instead, which is the protocol
> passed via socket(2)/bind(2).
>
> Applications only xmitting can go the path of allocating the
> socket as socket(PF_PACKET, <mode>, 0) and do a bind(2) on the
> TX_RING with sll_protocol of 0. That way, register_prot_hook()
> is neither called on creation nor on bind time, which saves
> cycles when there's no interest in capturing anyway.
>
> That leaves us however with po->num 0 instead and therefore
> the TX_RING flush run sets skb->protocol to 0 as well. Eric
> reported that this leads to problems when using tools like
> trafgen over bonding device.
> I.e. the bonding's hash function
> could invoke the kernel's flow dissector, which depends on
> skb->protocol being properly set. In the current situation, all
> the traffic is then directed to a single slave.
>
> Fix it up by inferring skb->protocol from the Ethernet header
> when not set and we have ARPHRD_ETHER device type. This is only
> done in case of SOCK_RAW and where we have a dev->hard_header_len
> length. In case of ARPHRD_ETHER devices, this is guaranteed to
> cover ETH_HLEN, and therefore being accessed on the skb after
> the skb_store_bits().
>
> Reported-by: Eric Dumazet <edumazet@google.com>
> Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>

Acked-by: Willem de Bruijn <willemb@google.com>
> ---
>  net/packet/af_packet.c | 11 +++++++++++
>  1 file changed, 11 insertions(+)
>
> diff --git a/net/packet/af_packet.c b/net/packet/af_packet.c
> index 8795b0f..0066da2 100644
> --- a/net/packet/af_packet.c
> +++ b/net/packet/af_packet.c
> @@ -2338,6 +2338,15 @@ static bool ll_header_truncated(const struct net_device *dev, int len)
>         return false;
>  }
>
> +static void tpacket_set_protocol(const struct net_device *dev,
> +                                struct sk_buff *skb)
> +{
> +       if (dev->type == ARPHRD_ETHER) {
> +               skb_reset_mac_header(skb);
> +               skb->protocol = eth_hdr(skb)->h_proto;
> +       }
> +}
> +
>  static int tpacket_fill_skb(struct packet_sock *po, struct sk_buff *skb,
>                 void *frame, struct net_device *dev, int size_max,
>                 __be16 proto, unsigned char *addr, int hlen)
> @@ -2419,6 +2428,8 @@ static int tpacket_fill_skb(struct packet_sock *po, struct sk_buff *skb,
>                                 dev->hard_header_len);
>                 if (unlikely(err))
>                         return err;
> +               if (!skb->protocol)
> +                       tpacket_set_protocol(dev, skb);
>
>                 data += dev->hard_header_len;
>                 to_write -= dev->hard_header_len;
> --
> 1.9.3
>
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