Re: [PATCH] unix: avoid use-after-free in ep_remove_wait_queue

Jason Baron [off-list ref] writes:

quoted

+
+/* Needs sk unix state lock. After recv_ready indicated not ready,
+ * establish peer_wait connection if still needed.
+ */
+static int unix_dgram_peer_wake_me(struct sock *sk, struct sock *other)
+{
+	int connected;
+
+	connected = unix_dgram_peer_wake_connect(sk, other);
+
+	if (unix_recvq_full(other))
+		return 1;
+
+	if (connected)
+		unix_dgram_peer_wake_disconnect(sk, other);
+
+	return 0;
+}
+

So the comment above this function says 'needs unix state lock', however
the usage in unix_dgram_sendmsg() has the 'other' lock, while the usage
in unix_dgram_poll() has the 'sk' lock. So this looks racy.

That's one thing which is broken with this patch. Judging from a 'quick'
look at the _dgram_sendmsg code, the unix_state_lock(other) will need to
be turned into a unix_state_double_lock(sk, other) and the remaining
code changed accordingly (since all of the checks must be done without
unlocking other). 

There's also something else seriously wrong with the present patch: Some
code in unix_dgram_connect presently (with this change) looks like this:

        /*
         * If it was connected, reconnect.
         */
        if (unix_peer(sk)) {
                struct sock *old_peer = unix_peer(sk);
                unix_peer(sk) = other;

                if (unix_dgram_peer_wake_disconnect(sk, other))
                        wake_up_interruptible_poll(sk_sleep(sk),
                                                   POLLOUT |
                                                   POLLWRNORM |
                                                   POLLWRBAND);

                unix_state_double_unlock(sk, other);

                if (other != old_peer)
                        unix_dgram_disconnected(sk, old_peer);
                sock_put(old_peer);

and trying to disconnect from a peer the socket is just being
connected to is - of course - "flowering tomfoolery" (literal
translation of the German "bluehender Bloedsinn") --- it needs to
disconnect from old_peer instead.

I'll address the suggestion and send an updated patch "later today" (may
become "early tomorrow"). I have some code addressing both issues but
that's part of a release of 'our' kernel fork, ie, 3.2.54-based I'll
need to do 'soon'.