Re: WARNING in ip_recv_error

On Fri, May 18, 2018 at 2:44 PM, Willem de Bruijn
[off-list ref] wrote:

On Fri, May 18, 2018 at 1:09 PM, Willem de Bruijn
[off-list ref] wrote:

quoted

On Fri, May 18, 2018 at 11:44 AM, David Miller [off-list ref] wrote:

quoted

From: Eric Dumazet <redacted>
Date: Fri, 18 May 2018 08:30:43 -0700

quoted

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.