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From:   Willem de Bruijn <willemdebruijn.kernel@gmail.com>
Date:   Fri, 18 May 2018 14:44:22 -0400
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Subject: Re: WARNING in ip_recv_error
To:     David Miller <davem@davemloft.net>
Cc:     Eric Dumazet <eric.dumazet@gmail.com>,
        DaeLyong Jeong <threeearcat@gmail.com>,
        Alexey Kuznetsov <kuznet@ms2.inr.ac.ru>,
        Hideaki YOSHIFUJI <yoshfuji@linux-ipv6.org>,
        Network Development <netdev@vger.kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Byoungyoung Lee <byoungyoung@purdue.edu>,
        Kyungtae Kim <kt0755@gmail.com>, bammanag@purdue.edu,
        Willem de Bruijn <willemb@google.com>
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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.