Re: [PATCH v4 0/3] net: unix: fix use-after-free

[Rainer Weikusat <rweikusat@mobileactivedefense.com>]

David Miller <davem@davemloft.net> writes:
> From: Jason Baron <jbaron@akamai.com>
> Date: Fri,  9 Oct 2015 00:15:59 -0400
>
>> These patches are against mainline, I can re-base to net-next, please
>> let me know.
>> 
>> They have been tested against: https://lkml.org/lkml/2015/9/13/195,
>> which causes the use-after-free quite quickly and here:
>> https://lkml.org/lkml/2015/10/2/693.
>
> I'd like to understand how patches that don't even compile can be
> "tested"?
>
> net/unix/af_unix.c: In function ‘unix_dgram_writable’:
> net/unix/af_unix.c:2480:3: error: ‘other_full’ undeclared (first use in this function)
> net/unix/af_unix.c:2480:3: note: each undeclared identifier is reported only once for each function it appears in
>
> Could you explain how that works, I'm having a hard time understanding
> this?

This is basicallly a workaround for the problem that it's not possible
to tell epoll to let go of a certain wait queue: Instead of registering
the peer_wait queue via sock_poll_wait, a wait_queue_t under control of
the af_unix.c code is linked onto it which relays a wake up on the
peer_wait queue to the 'ordinary' wait queue associated with the polled
socket via custom wake function. But (at least the code I looked it) it
enqueues a unix socket on connect which has certain side effects (in
particular, /dev/log will have a seriously large wait queue of entirely
uninterested peers) and in many cases, this is simply not necessary, as
the additional peer_wait event is only interesting in case a peer of a
fan-in socket (like /dev/log) happens to be waiting for writeabilty via
poll/ select/ epoll/ ...

Since the wait queue handling code is now under control of the af_unix.c
code, it can remove itself from the peer_wait queue prior to dropping
its reference to a peer on disconnect or on detecting a dead peer in
unix_dgram_sendmsg.