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    SubjectRe: WARNING in ip_recv_error
    On Sun, May 20, 2018 at 7:13 PM, Willem de Bruijn
    <willemdebruijn.kernel@gmail.com> wrote:
    > On Fri, May 18, 2018 at 2:59 PM, Willem de Bruijn
    > <willemdebruijn.kernel@gmail.com> wrote:
    >> On Fri, May 18, 2018 at 2:46 PM, Willem de Bruijn
    >> <willemdebruijn.kernel@gmail.com> wrote:
    >>> On Fri, May 18, 2018 at 2:44 PM, Willem de Bruijn
    >>> <willemdebruijn.kernel@gmail.com> wrote:
    >>>> On Fri, May 18, 2018 at 1:09 PM, Willem de Bruijn
    >>>> <willemdebruijn.kernel@gmail.com> wrote:
    >>>>> On Fri, May 18, 2018 at 11:44 AM, David Miller <davem@davemloft.net> wrote:
    >>>>>> From: Eric Dumazet <eric.dumazet@gmail.com>
    >>>>>> Date: Fri, 18 May 2018 08:30:43 -0700
    >>>>>>
    >>>>>>> We probably need to revert Willem patch (7ce875e5ecb8562fd44040f69bda96c999e38bbc)
    >>>>>>
    >>>>>> Is it really valid to reach ip_recv_err with an ipv6 socket?
    >>>>>
    >>>>> I guess the issue is that setsockopt IPV6_ADDRFORM is not an
    >>>>> atomic operation, so that the socket is neither fully ipv4 nor fully
    >>>>> ipv6 by the time it reaches ip_recv_error.
    >>>>>
    >>>>> sk->sk_socket->ops = &inet_dgram_ops;
    >>>>> < HERE >
    >>>>> sk->sk_family = PF_INET;
    >>>>>
    >>>>> Even calling inet_recv_error to demux would not necessarily help.
    >>>>>
    >>>>> Safest would be to look up by skb->protocol, similar to what
    >>>>> ipv6_recv_error does to handle v4-mapped-v6.
    >>>>>
    >>>>> Or to make that function safe with PF_INET and swap the order
    >>>>> of the above two operations.
    >>>>>
    >>>>> All sound needlessly complicated for this rare socket option, but
    >>>>> I don't have a better idea yet. Dropping on the floor is not nice,
    >>>>> either.
    >>>>
    >>>> Ensuring that ip_recv_error correctly handles packets from either
    >>>> socket and removing the warning should indeed be good.
    >>>>
    >>>> It is robust against v4-mapped packets from an AF_INET6 socket,
    >>>> but see caveat on reconnect below.
    >>>>
    >>>> The code between ipv6_recv_error for v4-mapped addresses and
    >>>> ip_recv_error is essentially the same, the main difference being
    >>>> whether to return network headers as sockaddr_in with SOL_IP
    >>>> or sockaddr_in6 with SOL_IPV6.
    >>>>
    >>>> There are very few other locations in the stack that explicitly test
    >>>> sk_family in this way and thus would be vulnerable to races with
    >>>> IPV6_ADDRFORM.
    >>>>
    >>>> I'm not sure whether it is possible for a udpv6 socket to queue a
    >>>> real ipv6 packet on the error queue, disconnect, connect to an
    >>>> ipv4 address, call IPV6_ADDRFORM and then call ip_recv_error
    >>>> on a true ipv6 packet. That would return buggy data, e.g., in
    >>>> msg_name.
    >>>
    >>> In do_ipv6_setsockopt IPV6_ADDRFORM we can test that the
    >>> error queue is empty, and then take its lock for the duration of the
    >>> operation.
    >>
    >> Actually, no reason to hold the lock. This setsockopt holds the socket
    >> lock, which connect would need, too. So testing that the queue
    >> is empty after testing that it is connected to a v4 address is
    >> sufficient to ensure that no ipv6 packets are queued for reception.
    >>
    >> diff --git a/net/ipv6/ipv6_sockglue.c b/net/ipv6/ipv6_sockglue.c
    >> index 4d780c7f0130..a975d6311341 100644
    >> --- a/net/ipv6/ipv6_sockglue.c
    >> +++ b/net/ipv6/ipv6_sockglue.c
    >> @@ -199,6 +199,11 @@ static int do_ipv6_setsockopt(struct sock *sk,
    >> int level, int optname,
    >>
    >> if (ipv6_only_sock(sk) ||
    >> !ipv6_addr_v4mapped(&sk->sk_v6_daddr)) {
    >> retv = -EADDRNOTAVAIL;
    >> break;
    >> }
    >>
    >> + if (!skb_queue_empty(&sk->sk_error_queue)) {
    >> + retv = -EBUSY;
    >> + break;
    >> + }
    >> +
    >> fl6_free_socklist(sk);
    >> __ipv6_sock_mc_close(sk);
    >>
    >> After this it should be safe to remove the warning in ip_recv_error.
    >
    > Hmm.. nope.
    >
    > This ensures that the socket cannot produce any new true v6 packets.
    > But it does not guarantee that they are not already in the system, e.g.
    > queued in tc, and will find their way to the error queue later.
    >
    > We'll have to just be able to handle ipv6 packets in ip_recv_error.
    > Since IPV6_ADDRFORM is used to pass to legacy v4-only
    > processes and those likely are only confused by SOL_IPV6
    > error messages, it is probably best to just drop them and perhaps
    > WARN_ONCE.

    Even more fun, this is not limited to the error queue.

    I can queue a v6 packet for reception on a socket, connect to a v4
    address, call IPV6_ADDRFORM and then a regular recvfrom will
    return a partial v6 address as AF_INET.

    We definitely do not want to have to add a check

    if (skb->protocol == htons(ETH_P_IPV6)) {
    kfree_skb(skb);
    goto try_again;
    }

    to the normal recvmsg path.

    An alternative may be to tighten the check on when to allow
    IPV6_ADDRFORM. Not only return EBUSY if a packet is pending,
    but also if any sk_{rmem, omem, wmem}_alloc is non-zero. Only,
    these tightened constraints could break a legacy application.

    Either way, this race is somewhat tangential to the one that
    RaceFuzzer found. The sk changes that IPV6_ADDRFORM makes
    to sk_prot, sk_socket->ops and sk_family are not atomic and will
    not be. They need not be, because no other code assumes this
    consistency.

    So I'll start by removing the warning as Eric suggested.

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