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>
Acked-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

diff --git a/net/packet/af_packet.c b/net/packet/af_packet.c
index 8795b0f..0066da2 100644 (file)
--- 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;