Re: [PATCH net] ipv6: Fix SO_REUSEPORT UDP socket with implicit sk_ipv6only

[David Miller <davem@davemloft.net>]

From: Martin KaFai Lau <kafai@fb.com>
Date: Wed, 24 Jan 2018 23:15:27 -0800

> If a sk_v6_rcv_saddr is !IPV6_ADDR_ANY and !IPV6_ADDR_MAPPED, it
> implicitly implies it is an ipv6only socket.  However, in inet6_bind(),
> this addr_type checking and setting sk->sk_ipv6only to 1 are only done
> after sk->sk_prot->get_port(sk, snum) has been completed successfully.
> 
> This inconsistency between sk_v6_rcv_saddr and sk_ipv6only confuses
> the 'get_port()'.
> 
> In particular, when binding SO_REUSEPORT UDP sockets,
> udp_reuseport_add_sock(sk,...) is called.  udp_reuseport_add_sock()
> checks "ipv6_only_sock(sk2) == ipv6_only_sock(sk)" before adding sk to
> sk2->sk_reuseport_cb.  In this case, ipv6_only_sock(sk2) could be
> 1 while ipv6_only_sock(sk) is still 0 here.  The end result is,
> reuseport_alloc(sk) is called instead of adding sk to the existing
> sk2->sk_reuseport_cb.
> 
> It can be reproduced by binding two SO_REUSEPORT UDP sockets on an
> IPv6 address (!ANY and !MAPPED).  Only one of the socket will
> receive packet.
> 
> The fix is to set the implicit sk_ipv6only before calling get_port().
> The original sk_ipv6only has to be saved such that it can be restored
> in case get_port() failed.  The situation is similar to the
> inet_reset_saddr(sk) after get_port() has failed.
> 
> Thanks to Calvin Owens <calvinowens@fb.com> who created an easy
> reproduction which leads to a fix.
> 
> Fixes: e32ea7e74727 ("soreuseport: fast reuseport UDP socket selection")
> Signed-off-by: Martin KaFai Lau <kafai@fb.com>

Applied and queued up for -stable, thanks Martin.