af-packet: fix - avoid reading stale data

Currently we flush tp_status and then flush the remainder of the header+payload.
tp_status should be flushed in the end to avoid stale data being read by user-space.

Incorrectly re-ordered barriers in v1.
Signed-off-by: NChetan Loke <loke.chetan@gmail.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/packet/af_packet.c

@@ -1129,7 +1129,6 @@ static int tpacket_rcv(struct sk_buff *skb, struct net_device *dev,
 	else
 		sll->sll_ifindex = dev->ifindex;
 
-	__packet_set_status(po, h.raw, status);
 	smp_mb();
 #if ARCH_IMPLEMENTS_FLUSH_DCACHE_PAGE == 1
 	{
@@ -1138,8 +1137,10 @@ static int tpacket_rcv(struct sk_buff *skb, struct net_device *dev,
 		end = (u8 *)PAGE_ALIGN((unsigned long)h.raw + macoff + snaplen);
 		for (start = h.raw; start < end; start += PAGE_SIZE)
 			flush_dcache_page(pgv_to_page(start));
+		smp_wmb();
 	}
 #endif
+	__packet_set_status(po, h.raw, status);
 
 	sk->sk_data_ready(sk, 0);