af_packet: prevent information leak

In 2.6.27, commit 393e52e3 (packet: deliver VLAN TCI to userspace)
added a small information leak.

Add padding field and make sure its zeroed before copy to user.
Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
CC: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/if_packet.h

@@ -62,6 +62,7 @@ struct tpacket_auxdata {
 	__u16		tp_mac;
 	__u16		tp_net;
 	__u16		tp_vlan_tci;
+	__u16		tp_padding;
 };
 
 /* Rx ring - header status */
@@ -101,6 +102,7 @@ struct tpacket2_hdr {
 	__u32		tp_sec;
 	__u32		tp_nsec;
 	__u16		tp_vlan_tci;
+	__u16		tp_padding;
 };
 
 #define TPACKET2_HDRLEN		(TPACKET_ALIGN(sizeof(struct tpacket2_hdr)) + sizeof(struct sockaddr_ll))

net/packet/af_packet.c

@@ -804,6 +804,7 @@ static int tpacket_rcv(struct sk_buff *skb, struct net_device *dev,
 		} else {
 			h.h2->tp_vlan_tci = 0;
 		}
+		h.h2->tp_padding = 0;
 		hdrlen = sizeof(*h.h2);
 		break;
 	default:
@@ -1736,6 +1737,7 @@ static int packet_recvmsg(struct kiocb *iocb, struct socket *sock,
 		} else {
 			aux.tp_vlan_tci = 0;
 		}
+		aux.tp_padding = 0;
 		put_cmsg(msg, SOL_PACKET, PACKET_AUXDATA, sizeof(aux), &aux);
 	}