[PATCH] Disable AF_UNIX handshake with setsockopt(..., SO_PEERCRED, ...)

Christian Franke Christian.Franke@t-online.de
Mon Oct 13 05:38:00 GMT 2014

Corinna Vinschen wrote:
> On Oct 10 20:04, Corinna Vinschen wrote:
>> On Oct 10 18:36, Christian Franke wrote:
>>> After a nonblocking connect(), postfix calls poll() with pollfd.events =
>>> POLLIN only. If poll() succeeds, it calls recv(). This fails with ENOTCONN
>>> because the state is still connect_pending.
>> Oh.  So it doesn't check if the connect succeeded?  Does it check the
>> poll result for POLLERR or does it explicitely check for revents==POLLIN?
>> Hmm.
>> [...time passes...]
>> It looks like you catched a long-standing bug here.
>> This isn't even AF_LOCAL specific.  The original comment in the
>> write_selected branch is misleading: The AF_LOCAL specific part is just
>> the call to af_local_connect, not setting the connect_state.  There was
>> a previous, longer comment at one point which I shortened for no good
>> reason in 2005:
>>    /* eid credential transaction on successful non-blocking connect.
>>       Since the read bit indicates an error, don't start transaction
>>       if it's set. */
>> However, If I'm not completely mistaken, your patch would only work in
>> the aforementioned scenario if setsockopt(SO_PEERCRED) has been called.
>> Otherwise the handshake would be skipped on the connect side and thus
>> the handshake would fail on the server side.  There's also the problem
>> that read_ready may indicate an error.  And POLLERR is only set if the
>> socket is polled for POLLOUT so a failing connect would go unnoticed.
>> In short, the whole code is written under the assumption that any sane
>> application calling nonblocking connect would always call select/poll to
>> check if connect succeeded in the first place.  Obviously, as postfix
>> shows, this is a wrong assumption.
>> I'm not yet sure how to fix this, but I'll look into this next week.
> I applied a fix which, I think, is much more elegant than the former
> solution.  The af_local_connect call is now called as soon as an
> FD_CONNECT event is generated and read by a call to wait_event.  It
> worked for me, so I have tender hopes that I didn't miss something.
> I also applied your patch on top of this new stuff and I'm just building
> a new developer snapshot for testing.

A quick test of current postfix draft with the snapshot works as 
expected. Thanks.

>    In setsockopt I added a check for
> socket family and type so setsockopt(SO_PEERCRED) only works for
> AF_LOCAL/SOCK_STREAM sockets, just as the entire handshake stuff.

Probably not needed because this check was already in 
af_local_set_no_getpeereid() itself.

>    I
> also added a comment to explain why we do this and a FIXME comment so we
> don't forget we're still looking for a more generic solution for the
> SO_PEERCRED exchange.

Definitely, at least because the current AF_LOCAL emulation has some 
security issues.


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