[Devel] [PATCH RHEL7 COMMIT] ms/unix: avoid use-after-free in ep_remove_wait_queue

Konstantin Khorenko khorenko at virtuozzo.com
Sat Jan 23 06:04:36 PST 2016


The commit is pushed to "branch-rh7-3.10.0-327.3.1-vz7.10.x-ovz" and will appear at https://src.openvz.org/scm/ovz/vzkernel.git
after rh7-3.10.0-327.3.1.vz7.10.3
------>
commit 20ce0cf008e738d191c6198caa493afe9c3ab8ea
Author: Rainer Weikusat <rweikusat at mobileactivedefense.com>
Date:   Fri Nov 20 22:07:23 2015 +0000

    ms/unix: avoid use-after-free in ep_remove_wait_queue
    
    Rainer Weikusat <rweikusat at mobileactivedefense.com> writes:
    An AF_UNIX datagram socket being the client in an n:1 association with
    some server socket is only allowed to send messages to the server if the
    receive queue of this socket contains at most sk_max_ack_backlog
    datagrams. This implies that prospective writers might be forced to go
    to sleep despite none of the message presently enqueued on the server
    receive queue were sent by them. In order to ensure that these will be
    woken up once space becomes again available, the present unix_dgram_poll
    routine does a second sock_poll_wait call with the peer_wait wait queue
    of the server socket as queue argument (unix_dgram_recvmsg does a wake
    up on this queue after a datagram was received). This is inherently
    problematic because the server socket is only guaranteed to remain alive
    for as long as the client still holds a reference to it. In case the
    connection is dissolved via connect or by the dead peer detection logic
    in unix_dgram_sendmsg, the server socket may be freed despite "the
    polling mechanism" (in particular, epoll) still has a pointer to the
    corresponding peer_wait queue. There's no way to forcibly deregister a
    wait queue with epoll.
    
    Based on an idea by Jason Baron, the patch below changes the code such
    that a wait_queue_t belonging to the client socket is enqueued on the
    peer_wait queue of the server whenever the peer receive queue full
    condition is detected by either a sendmsg or a poll. A wake up on the
    peer queue is then relayed to the ordinary wait queue of the client
    socket via wake function. The connection to the peer wait queue is again
    dissolved if either a wake up is about to be relayed or the client
    socket reconnects or a dead peer is detected or the client socket is
    itself closed. This enables removing the second sock_poll_wait from
    unix_dgram_poll, thus avoiding the use-after-free, while still ensuring
    that no blocked writer sleeps forever.
    
    Signed-off-by: Rainer Weikusat <rweikusat at mobileactivedefense.com>
    Fixes: ec0d215f9420 ("af_unix: fix 'poll for write'/connected DGRAM sockets")
    Reviewed-by: Jason Baron <jbaron at akamai.com>
    Signed-off-by: David S. Miller <davem at davemloft.net>
    
    ========================================================
    This is a fix for CVE-2013-7446: use after free triggered from inside container
    
    https://access.redhat.com/security/cve/CVE-2013-7446
    https://bugzilla.redhat.com/show_bug.cgi?id=1282688
    
    RedHat does not apply the patch due to unknown reason.
    
    In mainline issue was finally fixed by
    commit 7d267278a9ece963d77eefec61630223fce08c6c
    ("unix: avoid use-after-free in ep_remove_wait_queue").
    
    Signed-off-by: Vasily Averin <vvs at virtuozzo.com>
---
 include/net/af_unix.h |   1 +
 net/unix/af_unix.c    | 183 ++++++++++++++++++++++++++++++++++++++++++++------
 2 files changed, 165 insertions(+), 19 deletions(-)

diff --git a/include/net/af_unix.h b/include/net/af_unix.h
index 609746c..084086e 100644
--- a/include/net/af_unix.h
+++ b/include/net/af_unix.h
@@ -63,6 +63,7 @@ struct unix_sock {
 #define UNIX_GC_CANDIDATE	0
 #define UNIX_GC_MAYBE_CYCLE	1
 	struct socket_wq	peer_wq;
+	wait_queue_t		peer_wake;
 };
 #define unix_sk(__sk) ((struct unix_sock *)__sk)
 
diff --git a/net/unix/af_unix.c b/net/unix/af_unix.c
index 144fcbf..d64141f 100644
--- a/net/unix/af_unix.c
+++ b/net/unix/af_unix.c
@@ -315,6 +315,118 @@ found:
 	return s;
 }
 
+/* Support code for asymmetrically connected dgram sockets
+ *
+ * If a datagram socket is connected to a socket not itself connected
+ * to the first socket (eg, /dev/log), clients may only enqueue more
+ * messages if the present receive queue of the server socket is not
+ * "too large". This means there's a second writeability condition
+ * poll and sendmsg need to test. The dgram recv code will do a wake
+ * up on the peer_wait wait queue of a socket upon reception of a
+ * datagram which needs to be propagated to sleeping would-be writers
+ * since these might not have sent anything so far. This can't be
+ * accomplished via poll_wait because the lifetime of the server
+ * socket might be less than that of its clients if these break their
+ * association with it or if the server socket is closed while clients
+ * are still connected to it and there's no way to inform "a polling
+ * implementation" that it should let go of a certain wait queue
+ *
+ * In order to propagate a wake up, a wait_queue_t of the client
+ * socket is enqueued on the peer_wait queue of the server socket
+ * whose wake function does a wake_up on the ordinary client socket
+ * wait queue. This connection is established whenever a write (or
+ * poll for write) hit the flow control condition and broken when the
+ * association to the server socket is dissolved or after a wake up
+ * was relayed.
+ */
+
+static int unix_dgram_peer_wake_relay(wait_queue_t *q, unsigned mode, int flags,
+				      void *key)
+{
+	struct unix_sock *u;
+	wait_queue_head_t *u_sleep;
+
+	u = container_of(q, struct unix_sock, peer_wake);
+
+	__remove_wait_queue(&unix_sk(u->peer_wake.private)->peer_wait,
+			    q);
+	u->peer_wake.private = NULL;
+
+	/* relaying can only happen while the wq still exists */
+	u_sleep = sk_sleep(&u->sk);
+	if (u_sleep)
+		wake_up_interruptible_poll(u_sleep, key);
+
+	return 0;
+}
+
+static int unix_dgram_peer_wake_connect(struct sock *sk, struct sock *other)
+{
+	struct unix_sock *u, *u_other;
+	int rc;
+
+	u = unix_sk(sk);
+	u_other = unix_sk(other);
+	rc = 0;
+	spin_lock(&u_other->peer_wait.lock);
+
+	if (!u->peer_wake.private) {
+		u->peer_wake.private = other;
+		__add_wait_queue(&u_other->peer_wait, &u->peer_wake);
+
+		rc = 1;
+	}
+
+	spin_unlock(&u_other->peer_wait.lock);
+	return rc;
+}
+
+static void unix_dgram_peer_wake_disconnect(struct sock *sk,
+					    struct sock *other)
+{
+	struct unix_sock *u, *u_other;
+
+	u = unix_sk(sk);
+	u_other = unix_sk(other);
+	spin_lock(&u_other->peer_wait.lock);
+
+	if (u->peer_wake.private == other) {
+		__remove_wait_queue(&u_other->peer_wait, &u->peer_wake);
+		u->peer_wake.private = NULL;
+	}
+
+	spin_unlock(&u_other->peer_wait.lock);
+}
+
+static void unix_dgram_peer_wake_disconnect_wakeup(struct sock *sk,
+						   struct sock *other)
+{
+	unix_dgram_peer_wake_disconnect(sk, other);
+	wake_up_interruptible_poll(sk_sleep(sk),
+				   POLLOUT |
+				   POLLWRNORM |
+				   POLLWRBAND);
+}
+
+/* preconditions:
+ *	- unix_peer(sk) == other
+ *	- association is stable
+ */
+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;
+}
+
 static inline int unix_writable(struct sock *sk)
 {
 	return (atomic_read(&sk->sk_wmem_alloc) << 2) <= sk->sk_sndbuf;
@@ -419,6 +531,8 @@ static void unix_release_sock(struct sock *sk, int embrion)
 			skpair->sk_state_change(skpair);
 			sk_wake_async(skpair, SOCK_WAKE_WAITD, POLL_HUP);
 		}
+
+		unix_dgram_peer_wake_disconnect(sk, skpair);
 		sock_put(skpair); /* It may now die */
 		unix_peer(sk) = NULL;
 	}
@@ -658,6 +772,7 @@ static struct sock *unix_create1(struct net *net, struct socket *sock)
 	INIT_LIST_HEAD(&u->link);
 	mutex_init(&u->readlock); /* single task reading lock */
 	init_waitqueue_head(&u->peer_wait);
+	init_waitqueue_func_entry(&u->peer_wake, unix_dgram_peer_wake_relay);
 	unix_insert_socket(unix_sockets_unbound(sk), sk);
 out:
 	if (sk == NULL)
@@ -1025,6 +1140,8 @@ restart:
 	if (unix_peer(sk)) {
 		struct sock *old_peer = unix_peer(sk);
 		unix_peer(sk) = other;
+		unix_dgram_peer_wake_disconnect_wakeup(sk, old_peer);
+
 		unix_state_double_unlock(sk, other);
 
 		if (other != old_peer)
@@ -1464,6 +1581,7 @@ static int unix_dgram_sendmsg(struct kiocb *kiocb, struct socket *sock,
 	struct scm_cookie tmp_scm;
 	int max_level;
 	int data_len = 0;
+	int sk_locked;
 
 	if (NULL == siocb->scm)
 		siocb->scm = &tmp_scm;
@@ -1545,12 +1663,14 @@ restart:
 		goto out_free;
 	}
 
+	sk_locked = 0;
 	unix_state_lock(other);
+restart_locked:
 	err = -EPERM;
 	if (!unix_may_send(sk, other))
 		goto out_unlock;
 
-	if (sock_flag(other, SOCK_DEAD)) {
+	if (unlikely(sock_flag(other, SOCK_DEAD))) {
 		/*
 		 *	Check with 1003.1g - what should
 		 *	datagram error
@@ -1558,10 +1678,14 @@ restart:
 		unix_state_unlock(other);
 		sock_put(other);
 
+		if (!sk_locked)
+			unix_state_lock(sk);
+
 		err = 0;
-		unix_state_lock(sk);
 		if (unix_peer(sk) == other) {
 			unix_peer(sk) = NULL;
+			unix_dgram_peer_wake_disconnect_wakeup(sk, other);
+
 			unix_state_unlock(sk);
 
 			unix_dgram_disconnected(sk, other);
@@ -1587,21 +1711,38 @@ restart:
 			goto out_unlock;
 	}
 
-	if (unix_peer(other) != sk && unix_recvq_full(other)) {
-		if (!timeo) {
-			err = -EAGAIN;
-			goto out_unlock;
+	if (unlikely(unix_peer(other) != sk && unix_recvq_full(other))) {
+		if (timeo) {
+			timeo = unix_wait_for_peer(other, timeo);
+
+			err = sock_intr_errno(timeo);
+			if (signal_pending(current))
+				goto out_free;
+
+			goto restart;
 		}
 
-		timeo = unix_wait_for_peer(other, timeo);
+		if (!sk_locked) {
+			unix_state_unlock(other);
+			unix_state_double_lock(sk, other);
+		}
 
-		err = sock_intr_errno(timeo);
-		if (signal_pending(current))
-			goto out_free;
+		if (unix_peer(sk) != other ||
+		    unix_dgram_peer_wake_me(sk, other)) {
+			err = -EAGAIN;
+			sk_locked = 1;
+			goto out_unlock;
+		}
 
-		goto restart;
+		if (!sk_locked) {
+			sk_locked = 1;
+			goto restart_locked;
+		}
 	}
 
+	if (unlikely(sk_locked))
+		unix_state_unlock(sk);
+
 	if (sock_flag(other, SOCK_RCVTSTAMP))
 		__net_timestamp(skb);
 	maybe_add_creds(skb, sock, other);
@@ -1615,6 +1756,8 @@ restart:
 	return len;
 
 out_unlock:
+	if (sk_locked)
+		unix_state_unlock(sk);
 	unix_state_unlock(other);
 out_free:
 	kfree_skb(skb);
@@ -2459,14 +2602,16 @@ static unsigned int unix_dgram_poll(struct file *file, struct socket *sock,
 		return mask;
 
 	writable = unix_writable(sk);
-	other = unix_peer_get(sk);
-	if (other) {
-		if (unix_peer(other) != sk) {
-			sock_poll_wait(file, &unix_sk(other)->peer_wait, wait);
-			if (unix_recvq_full(other))
-				writable = 0;
-		}
-		sock_put(other);
+	if (writable) {
+		unix_state_lock(sk);
+
+		other = unix_peer(sk);
+		if (other && unix_peer(other) != sk &&
+		    unix_recvq_full(other) &&
+		    unix_dgram_peer_wake_me(sk, other))
+			writable = 0;
+
+		unix_state_unlock(sk);
 	}
 
 	if (writable)


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