Re: [PATCH net] ipv6: weaken the v4mapped source check

[Mat Martineau <mathew.j.martineau@linux.intel.com>]

On Wed, 17 Mar 2021, Jakub Kicinski wrote:

> This reverts commit 6af1799aaf3f1bc8defedddfa00df3192445bbf3.
>
> Commit 6af1799aaf3f ("ipv6: drop incoming packets having a v4mapped
> source address") introduced an input check against v4mapped addresses.
> Use of such addresses on the wire is indeed questionable and not
> allowed on public Internet. As the commit pointed out
>
>  https://tools.ietf.org/html/draft-itojun-v6ops-v4mapped-harmful-02
>
> lists potential issues.
>
> Unfortunately there are applications which use v4mapped addresses,
> and breaking them is a clear regression. For example v4mapped
> addresses (or any semi-valid addresses, really) may be used
> for uni-direction event streams or packet export.
>
> Since the issue which sparked the addition of the check was with
> TCP and request_socks in particular push the check down to TCPv6
> and DCCP. This restores the ability to receive UDPv6 packets with
> v4mapped address as the source.
>
> Keep using the IPSTATS_MIB_INHDRERRORS statistic to minimize the
> user-visible changes.
>
> Fixes: 6af1799aaf3f ("ipv6: drop incoming packets having a v4mapped source address")
> Reported-by: Sunyi Shao <sunyishao@fb.com>
> Signed-off-by: Jakub Kicinski <kuba@kernel.org>
> ---
> net/dccp/ipv6.c      |  5 +++++
> net/ipv6/ip6_input.c | 10 ----------
> net/ipv6/tcp_ipv6.c  |  5 +++++
> net/mptcp/subflow.c  |  5 +++++
> 4 files changed, 15 insertions(+), 10 deletions(-)

Jakub -

Thanks for keeping the MPTCP code in sync. The IPv6 and v4mapped MPTCP 
selftests still pass. For the MPTCP content:

Acked-by: Mat Martineau <mathew.j.martineau@linux.intel.com>


>
> diff --git a/net/dccp/ipv6.c b/net/dccp/ipv6.c
> index 1f73603913f5..2be5c69824f9 100644
> --- a/net/dccp/ipv6.c
> +++ b/net/dccp/ipv6.c
> @@ -319,6 +319,11 @@ static int dccp_v6_conn_request(struct sock *sk, struct sk_buff *skb)
> 	if (!ipv6_unicast_destination(skb))
> 		return 0;	/* discard, don't send a reset here */
>
> +	if (ipv6_addr_v4mapped(&ipv6_hdr(skb)->saddr)) {
> +		__IP6_INC_STATS(sock_net(sk), NULL, IPSTATS_MIB_INHDRERRORS);
> +		return 0;
> +	}
> +
> 	if (dccp_bad_service_code(sk, service)) {
> 		dcb->dccpd_reset_code = DCCP_RESET_CODE_BAD_SERVICE_CODE;
> 		goto drop;
> diff --git a/net/ipv6/ip6_input.c b/net/ipv6/ip6_input.c
> index e9d2a4a409aa..80256717868e 100644
> --- a/net/ipv6/ip6_input.c
> +++ b/net/ipv6/ip6_input.c
> @@ -245,16 +245,6 @@ 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);
>
> diff --git a/net/ipv6/tcp_ipv6.c b/net/ipv6/tcp_ipv6.c
> index bd44ded7e50c..d0f007741e8e 100644
> --- a/net/ipv6/tcp_ipv6.c
> +++ b/net/ipv6/tcp_ipv6.c
> @@ -1175,6 +1175,11 @@ static int tcp_v6_conn_request(struct sock *sk, struct sk_buff *skb)
> 	if (!ipv6_unicast_destination(skb))
> 		goto drop;
>
> +	if (ipv6_addr_v4mapped(&ipv6_hdr(skb)->saddr)) {
> +		__IP6_INC_STATS(sock_net(sk), NULL, IPSTATS_MIB_INHDRERRORS);
> +		return 0;
> +	}
> +
> 	return tcp_conn_request(&tcp6_request_sock_ops,
> 				&tcp_request_sock_ipv6_ops, sk, skb);
>
> diff --git a/net/mptcp/subflow.c b/net/mptcp/subflow.c
> index 3d47d670e665..d17d39ccdf34 100644
> --- a/net/mptcp/subflow.c
> +++ b/net/mptcp/subflow.c
> @@ -477,6 +477,11 @@ static int subflow_v6_conn_request(struct sock *sk, struct sk_buff *skb)
> 	if (!ipv6_unicast_destination(skb))
> 		goto drop;
>
> +	if (ipv6_addr_v4mapped(&ipv6_hdr(skb)->saddr)) {
> +		__IP6_INC_STATS(sock_net(sk), NULL, IPSTATS_MIB_INHDRERRORS);
> +		return 0;
> +	}
> +
> 	return tcp_conn_request(&mptcp_subflow_request_sock_ops,
> 				&subflow_request_sock_ipv6_ops, sk, skb);
>
> -- 
> 2.30.2

--
Mat Martineau
Intel