Re: WARNING in ip_recv_error

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

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.