Re: WARNING in ip_recv_error

[Willem de Bruijn <willemdebruijn.kernel@gmail.com>]

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.