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

[Corinna Vinschen <corinna-cygwin at cygwin dot com>]

On Oct 10 18:36, Christian Franke wrote:
> Corinna Vinschen wrote:
> >I was just looking into applying your patch when I got thinking over the
> >change in select.cc once more.  You're setting the connect_state from
> >connect_pending to connected there when there's something to read on the
> >socket.
> >
> >This puzzles me.  A completed connection attempt should set the
> >write_selected flag (see function peek_socket).
> 
> No, peek_socket() does not change write_selected. It sets write_read if
> write_selected is set.

Uh, sorry, I mistyped.  You're right, of course.

> >   The AF_LOCAL handling
> >in the
> >
> >   if (me->write_selected && me->write_ready)
> >
> >case in set_bits should cover this.  What situation is your special case
> >covering which is not already covered by the write_selected case?
> 
> If only read status is requested via select()/poll(), write_selected is
> always false and the connect_pending=>connected transition is never done.
> 
> 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.


Thanks,
Corinna

-- 
Corinna Vinschen                  Please, send mails regarding Cygwin to
Cygwin Maintainer                 cygwin AT cygwin DOT com
Red Hat

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