Subject: Re: [PATCH v2 1/3] unix: fix use-after-free in unix_dgram_poll()

From: Rainer Weikusat <[email-masked]>
Date: Mon, 05 Oct 2015 17:31:06 +0100

Jason Baron <[email-masked]> writes:
> The unix_dgram_poll() routine calls sock_poll_wait() not only for the wait
> queue associated with the socket s that we are poll'ing against, but also calls
> sock_poll_wait() for a remote peer socket p, if it is connected. Thus,
> if we call poll()/select()/epoll() for the socket s, there are then
> a couple of code paths in which the remote peer socket p and its associated
> peer_wait queue can be freed before poll()/select()/epoll() have a chance
> to remove themselves from the remote peer socket.
>
> The way that remote peer socket can be freed are:
>
> 1. If s calls connect() to a connect to a new socket other than p, it will
> drop its reference on p, and thus a close() on p will free it.
>
> 2. If we call close on p(), then a subsequent sendmsg() from s, will drop
> the final reference to p, allowing it to be freed.

Here's a more simple idea which _might_ work. The underlying problem
seems to be that the second sock_poll_wait introduces a covert reference
to the peer socket which isn't accounted for. The basic idea behind this
is to execute an additional sock_hold for the peer whenever the
sock_poll_wait is called for it and store it (the struct sock *) in a
new struct unix_sock member. Upon entering unix_dgram_poll, if the
member is not NULL, it's cleared and a sock_put for its former value is
done. The 'poll peer not NULL -> sock_put it' code is also added to the
destructor, although I'm unsure if this is really necessary. The patch
below also includes the additional SOCK_DEAD test suggested by Martin as
that seems generally sensible to me.

NB: This has survived both Martin's and my test programs for a number
of executions/ longer periods of time than was common before without
generating list corruption warnings. The patch below is against 'my'
3.2.54 and is here provided as a suggestion in the hope that it will be
useful for someting, not as patch submission, as I spent less time
thinking through this than I should ideally have but despite of this,
it's another 2.5 hours of my life spent on something completely
different than what I should be working on at the moment.

--------------
diff -pru linux-2-6/include/net/af_unix.h linux-2-6.p/include/net/af_unix.h
--- linux-2-6/include/net/af_unix.h	2014-01-20 21:52:53.000000000 +0000
+++ linux-2-6.p/include/net/af_unix.h	2015-10-05 15:11:20.270958297 +0100
@@ -50,6 +50,7 @@ struct unix_sock {
 	struct vfsmount		*mnt;
 	struct mutex		readlock;
 	struct sock		*peer;
+	struct sock		*poll_peer;
 	struct sock		*other;
 	struct list_head	link;
 	atomic_long_t		inflight;
diff -pru linux-2-6/net/unix/af_unix.c linux-2-6.p/net/unix/af_unix.c
--- linux-2-6/net/unix/af_unix.c	2014-01-22 16:51:52.000000000 +0000
+++ linux-2-6.p/net/unix/af_unix.c	2015-10-05 17:05:28.358273567 +0100
@@ -361,6 +361,9 @@ static void unix_sock_destructor(struct
 	if (u->addr)
 		unix_release_addr(u->addr);
 
+	if (u->poll_peer)
+		sock_put(u->poll_peer);
+
 	atomic_long_dec(&unix_nr_socks);
 	local_bh_disable();
 	sock_prot_inuse_add(sock_net(sk), sk->sk_prot, -1);
@@ -625,6 +628,7 @@ static struct sock *unix_create1(struct
 	u	  = unix_sk(sk);
 	u->dentry = NULL;
 	u->mnt	  = NULL;
+	u->poll_peer = NULL;
 	spin_lock_init(&u->lock);
 	atomic_long_set(&u->inflight, 0);
 	INIT_LIST_HEAD(&u->link);
@@ -2135,8 +2139,16 @@ static unsigned int unix_poll(struct fil
 static unsigned int unix_dgram_poll(struct file *file, struct socket *sock,
 				    poll_table *wait)
 {
-	struct sock *sk = sock->sk, *other;
-	unsigned int mask, writable;
+	struct sock *sk = sock->sk, *other, *pp;
+	struct unix_sock *u;
+	unsigned int mask, writable, dead;
+
+	u = unix_sk(sk);
+	pp = u->poll_peer;
+	if (pp) {
+		u->poll_peer = NULL;
+		sock_put(pp);
+	}
 
 	sock_poll_wait(file, sk_sleep(sk), wait);
 	mask = 0;
@@ -2170,7 +2182,20 @@ static unsigned int unix_dgram_poll(stru
 	other = unix_peer_get(sk);
 	if (other) {
 		if (unix_peer(other) != sk) {
-			sock_poll_wait(file, &unix_sk(other)->peer_wait, wait);
+			unix_state_lock(other);
+
+			dead = sock_flag(other, SOCK_DEAD);
+			if (!dead)
+				sock_poll_wait(file, &unix_sk(other)->peer_wait,
+					       wait);
+
+			unix_state_unlock(other);
+
+			if (!dead) {
+				u->poll_peer = other;
+				sock_hold(other);
+			}
+
 			if (unix_recvq_full(other))
 				writable = 0;
 		}