Subject: Re: [PATCH] unix: avoid use-after-free in ep_remove_wait_queue (w/ Fixes:)

From: Rainer Weikusat <[email-masked]>
Date: Tue, 17 Nov 2015 22:48:52 +0000

Rainer Weikusat <[email-masked]> writes:

[...]

> This leaves only the option of a somewhat incorrect solution and what is
> or isn't acceptable in this respect is somewhat difficult to decide. The
> basic options would be

[...]
> 	- retry sending a limited number of times, eg, once, before
>           returning EAGAIN, on the grounds that this is nicer to the
>           application and that redoing all the stuff up to the _lock in
>           dgram_sendmsg can possibly/ likely be avoided

Since it's better to have a specific example of something: Here's
another 'code sketch' of this option (hopefully with less errors this
time, there's an int restart = 0 above):

	if (unix_peer(other) != sk && unix_recvq_full(other)) {
		int need_wakeup;

[...]

		need_wakeup = 0;
		err = 0;
		unix_state_unlock(other);
		unix_state_lock(sk);

		if (unix_peer(sk) == other) {
			if (++restart == 2) {
				need_wakeup = unix_dgram_peer_wake_connect(sk, other) &&
					      sk_receive_queue_len(other) == 0;
				err = -EAGAIN;
			} else if (unix_dgram_peer_wake_me(sk, other))
				err = -EAGAIN;
		} else
			err = -EAGAIN;

		unix_state_unlock(sk);

		if (err || !restart) {
			if (need_wakeup)
				wake_up_interruptible_poll(sk_sleep(sk),
							   POLLOUT |
							   POLLWRNORM |
							   POLLWRBAND);
			
			goto out_free;
		}
		
		goto restart;
	}

I don't particularly like that, either, and to me, the best option seems
to be to return the spurious EAGAIN if taking both locks unconditionally
is not an option as that's the simplest choice.