Re: [PATCH 27/32] tcp: NAT IPv4-mapped IPv6 addresses like IPv4 addresses

[Stefano Brivio <sbrivio@redhat.com>]

On Wed, 16 Nov 2022 15:42:07 +1100
David Gibson <david@gibson.dropbear.id.au> wrote:

> passt usually doesn't NAT, but it does do so for the remapping of the
> gateway address to refer to the host.  Currently we perform this NAT with
> slightly different rules on both IPv4 addresses and IPv6 addresses, but not
> on IPv4-mapped IPv6 addresses.  This means we won't correctly handle the
> case of an IPv4 connection over an IPv6 socket, which is possible on Linux
> (and probably other platforms).

By the way, I really think it's just Linux, I can't think of other
examples.

> Refactor tcp_conn_from_sock() to perform the NAT after converting either
> address family into an inany_addr, so IPv4 and and IPv4-mapped addresses
> have the same representation.
> 
> With two new helpers this lets us remove the IPv4 and IPv6 specific paths
> from tcp_conn_from_sock().
> 
> Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
> ---
>  inany.h | 30 ++++++++++++++++++++++++++--
>  tcp.c   | 62 ++++++++++++++++++++++++---------------------------------
>  2 files changed, 54 insertions(+), 38 deletions(-)
> 
> diff --git a/inany.h b/inany.h
> index 4e53da9..a677aa7 100644
> --- a/inany.h
> +++ b/inany.h
> @@ -30,11 +30,11 @@ union inany_addr {
>   *
>   * Return: IPv4 address if @addr is IPv4, NULL otherwise
>   */
> -static inline const struct in_addr *inany_v4(const union inany_addr *addr)
> +static inline struct in_addr *inany_v4(const union inany_addr *addr)

There must be a reason, but I can't understand why this change is
needed here.

>  {
>  	if (!IN6_IS_ADDR_V4MAPPED(&addr->a6))
>  		return NULL;
> -	return &addr->_v4mapped.a4;
> +	return (struct in_addr *)&addr->_v4mapped.a4;
>  }
>  
>  /** inany_equals - Compare two IPv[46] addresses
> @@ -66,3 +66,29 @@ static inline void inany_from_af(union inany_addr *aa, int af, const void *addr)
>  		assert(0);
>  	}
>  }
> +
> +/** inany_from_sockaddr - Extract IPv[46] address and port number from sockaddr
> + * @a:		Pointer to store IPv[46] address

This is aa below, I'm not sure why.

> + * @port:	Pointer to store port number, host order
> + * @addr:	struct sockaddr_in (IPv4) or struct sockaddr_in6 (IPv6)

This became sa_ (needless to say, addr would make more sense).

> + */
> +static inline void inany_from_sockaddr(union inany_addr *aa, in_port_t *port,
> +				       const void *sa_)
> +{
> +	const struct sockaddr *sa = (const struct sockaddr *)sa_;
> +
> +	if (sa->sa_family == AF_INET6) {
> +		struct sockaddr_in6 *sa6 = (struct sockaddr_in6 *)sa;
> +
> +		inany_from_af(aa, AF_INET6, &sa6->sin6_addr);
> +		*port = ntohs(sa6->sin6_port);
> +	} else if (sa->sa_family == AF_INET) {
> +		struct sockaddr_in *sa4 = (struct sockaddr_in *)sa;
> +
> +		inany_from_af(aa, AF_INET, &sa4->sin_addr);
> +		*port = ntohs(sa4->sin_port);
> +	} else {
> +		/* Not valid to call with other address families */
> +		assert(0);
> +	}
> +}
> diff --git a/tcp.c b/tcp.c
> index b05ed6c..fca5df4 100644
> --- a/tcp.c
> +++ b/tcp.c
> @@ -2724,6 +2724,29 @@ static void tcp_connect_finish(struct ctx *c, struct tcp_tap_conn *conn)
>  	conn_flag(c, conn, ACK_FROM_TAP_DUE);
>  }
>  
> +static void tcp_snat_inbound(const struct ctx *c, union inany_addr *addr)

What this does is kind of obvious, still a comment would be nice.

> +{
> +	struct in_addr *addr4 = inany_v4(addr);
> +
> +	if (addr4) {
> +		if (IN4_IS_ADDR_LOOPBACK(addr4) ||
> +		    IN4_IS_ADDR_UNSPECIFIED(addr4) ||
> +		    IN4_ARE_ADDR_EQUAL(addr4, &c->ip4.addr_seen))
> +			*addr4 = c->ip4.gw;
> +	} else {
> +		struct in6_addr *addr6 = &addr->a6;
> +
> +		if (IN6_IS_ADDR_LOOPBACK(addr6) ||
> +		    IN6_ARE_ADDR_EQUAL(addr6, &c->ip6.addr_seen) ||
> +		    IN6_ARE_ADDR_EQUAL(addr6, &c->ip6.addr)) {
> +			if (IN6_IS_ADDR_LINKLOCAL(&c->ip6.gw))
> +				*addr6 = c->ip6.gw;
> +			else
> +				*addr6 = c->ip6.addr_ll;
> +		}
> +	}
> +}
> +
>  /**
>   * tcp_tap_conn_from_sock() - Initialize state for non-spliced connection
>   * @c:		Execution context
> @@ -2744,43 +2767,10 @@ static void tcp_tap_conn_from_sock(struct ctx *c, union epoll_ref ref,
>  	conn->ws_to_tap = conn->ws_from_tap = 0;
>  	conn_event(c, conn, SOCK_ACCEPTED);
>  
> -	if (sa->sa_family == AF_INET6) {
> -		struct sockaddr_in6 sa6;
> -
> -		memcpy(&sa6, sa, sizeof(sa6));
> -
> -		if (IN6_IS_ADDR_LOOPBACK(&sa6.sin6_addr) ||
> -		    IN6_ARE_ADDR_EQUAL(&sa6.sin6_addr, &c->ip6.addr_seen) ||
> -		    IN6_ARE_ADDR_EQUAL(&sa6.sin6_addr, &c->ip6.addr)) {
> -			struct in6_addr *src;
> +	inany_from_sockaddr(&conn->addr, &conn->sock_port, sa);
> +	conn->tap_port = ref.r.p.tcp.tcp.index;
>  
> -			if (IN6_IS_ADDR_LINKLOCAL(&c->ip6.gw))
> -				src = &c->ip6.gw;
> -			else
> -				src = &c->ip6.addr_ll;
> -
> -			memcpy(&sa6.sin6_addr, src, sizeof(*src));
> -		}
> -
> -		inany_from_af(&conn->addr, AF_INET6, &sa6.sin6_addr);
> -
> -		conn->sock_port = ntohs(sa6.sin6_port);
> -		conn->tap_port = ref.r.p.tcp.tcp.index;
> -	} else {
> -		struct sockaddr_in sa4;
> -
> -		memcpy(&sa4, sa, sizeof(sa4));
> -
> -		if (IN4_IS_ADDR_LOOPBACK(&sa4.sin_addr) ||
> -		    IN4_IS_ADDR_UNSPECIFIED(&sa4.sin_addr) ||
> -		    IN4_ARE_ADDR_EQUAL(&sa4.sin_addr, &c->ip4.addr_seen))
> -			sa4.sin_addr = c->ip4.gw;
> -
> -		inany_from_af(&conn->addr, AF_INET, &sa4.sin_addr);
> -
> -		conn->sock_port = ntohs(sa4.sin_port);
> -		conn->tap_port = ref.r.p.tcp.tcp.index;
> -	}
> +	tcp_snat_inbound(c, &conn->addr);
>  
>  	tcp_seq_init(c, conn, now);
>  	tcp_hash_insert(c, conn);

-- 
Stefano