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Subject[PATCH 4.2 13/61] packet: infer protocol from ethernet header if unset
4.2-stable review patch.  If anyone has any objections, please let me know.


From: Daniel Borkmann <>

[ Upstream commit c72219b75fde768efccf7666342282fab7f9e4e7 ]

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 <>
Signed-off-by: Daniel Borkmann <>
Acked-by: Willem de Bruijn <>
Signed-off-by: David S. Miller <>
Signed-off-by: Greg Kroah-Hartman <>
net/packet/af_packet.c | 11 +++++++++++
1 file changed, 11 insertions(+)

--- a/net/packet/af_packet.c
+++ b/net/packet/af_packet.c
@@ -2219,6 +2219,15 @@ static bool ll_header_truncated(const st
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)
@@ -2300,6 +2309,8 @@ static int tpacket_fill_skb(struct packe
if (unlikely(err))
return err;
+ if (!skb->protocol)
+ tpacket_set_protocol(dev, skb);

data += dev->hard_header_len;
to_write -= dev->hard_header_len;

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