Commit 6af1799a authored Oct 02, 2019 by Eric Dumazet Committed by David S. Miller Oct 03, 2019

ipv6: drop incoming packets having a v4mapped source address



This began with a syzbot report. syzkaller was injecting
IPv6 TCP SYN packets having a v4mapped source address.

After an unsuccessful 4-tuple lookup, TCP creates a request
socket (SYN_RECV) and calls reqsk_queue_hash_req()

reqsk_queue_hash_req() calls sk_ehashfn(sk)

At this point we have AF_INET6 sockets, and the heuristic
used by sk_ehashfn() to either hash the IPv4 or IPv6 addresses
is to use ipv6_addr_v4mapped(&sk->sk_v6_daddr)

For the particular spoofed packet, we end up hashing V4 addresses
which were not initialized by the TCP IPv6 stack, so KMSAN fired
a warning.

I first fixed sk_ehashfn() to test both source and destination addresses,
but then faced various problems, including user-space programs
like packetdrill that had similar assumptions.

Instead of trying to fix the whole ecosystem, it is better
to admit that we have a dual stack behavior, and that we
can not build linux kernels without V4 stack anyway.

The dual stack API automatically forces the traffic to be IPv4
if v4mapped addresses are used at bind() or connect(), so it makes
no sense to allow IPv6 traffic to use the same v4mapped class.

Fixes: 1da177e4 ("Linux-2.6.12-rc2")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Florian Westphal <fw@strlen.de>
Cc: Hannes Frederic Sowa <hannes@stressinduktion.org>
Reported-by: syzbot <syzkaller@googlegroups.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

parent 134cc4ce

net/ipv6/ip6_input.c

+10 −0

Original line number	Diff line number	Diff line
		@@ -223,6 +223,16 @@ static struct sk_buff ip6_rcv_core(struct sk_buff skb, struct net_device *dev,
		if (ipv6_addr_is_multicast(&hdr->saddr))
		goto err;

		/* While RFC4291 is not explicit about v4mapped addresses
		* in IPv6 headers, it seems clear linux dual-stack
		* model can not deal properly with these.
		* Security models could be fooled by ::ffff:127.0.0.1 for example.
		*
		* https://tools.ietf.org/html/draft-itojun-v6ops-v4mapped-harmful-02
		*/
		if (ipv6_addr_v4mapped(&hdr->saddr))
		goto err;

		skb->transport_header = skb->network_header + sizeof(*hdr);
		IP6CB(skb)->nhoff = offsetof(struct ipv6hdr, nexthdr);

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