Re: WARNING in ip_recv_error

[Willem de Bruijn]

On Fri, May 18, 2018 at 2:44 PM, Willem de Bruijn
<willemdebruijn.kernel@xxxxxxxxx> wrote:
> On Fri, May 18, 2018 at 1:09 PM, Willem de Bruijn
> <willemdebruijn.kernel@xxxxxxxxx> wrote:
>> On Fri, May 18, 2018 at 11:44 AM, David Miller <davem@xxxxxxxxxxxxx> wrote:
>>> From: Eric Dumazet <eric.dumazet@xxxxxxxxx>
>>> 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.