ipv6: drop non loopback packets claiming to originate from ::1

We lack a saddr check for ::1. This causes security issues e.g. with acls
permitting connections from ::1 because of assumption that these originate
from local machine.

Assuming a source address of ::1 is local seems reasonable.
RFC4291 doesn't allow such a source address either, so drop such packets.

Reported-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Florian Westphal <fw@strlen.de>
Acked-by: Eric Dumazet <edumazet@google.com>
Acked-by: Hannes Frederic Sowa <hannes@stressinduktion.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv6/ip6_input.c

@@ -122,11 +122,14 @@ int ipv6_rcv(struct sk_buff *skb, struct net_device *dev, struct packet_type *pt
 			max_t(unsigned short, 1, skb_shinfo(skb)->gso_segs));
 	/*
 	 * RFC4291 2.5.3
+	 * The loopback address must not be used as the source address in IPv6
+	 * packets that are sent outside of a single node. [..]
 	 * A packet received on an interface with a destination address
 	 * of loopback must be dropped.
 	 */
-	if (!(dev->flags & IFF_LOOPBACK) &&
-	    ipv6_addr_loopback(&hdr->daddr))
+	if ((ipv6_addr_loopback(&hdr->saddr) ||
+	     ipv6_addr_loopback(&hdr->daddr)) &&
+	     !(dev->flags & IFF_LOOPBACK))
 		goto err;
 
 	/* RFC4291 Errata ID: 3480