l2tp: fix races with tunnel socket close

The tunnel socket tunnel->sock (struct sock) is accessed when
preparing a new ppp session on a tunnel at pppol2tp_session_init. If
the socket is closed by a thread while another is creating a new
session, the threads race. In pppol2tp_connect, the tunnel object may
be created if the pppol2tp socket is associated with the special
session_id 0 and the tunnel socket is looked up using the provided
fd. When handling this, pppol2tp_connect cannot sock_hold the tunnel
socket to prevent it being destroyed during pppol2tp_connect since
this may itself may race with the socket being destroyed. Doing
sockfd_lookup in pppol2tp_connect isn't sufficient to prevent
tunnel->sock going away either because a given tunnel socket fd may be
reused between calls to pppol2tp_connect. Instead, have
l2tp_tunnel_create sock_hold the tunnel socket before it does
sockfd_put. This ensures that the tunnel's socket is always extant
while the tunnel object exists. Hold a ref on the socket until the
tunnel is destroyed and ensure that all tunnel destroy paths go
through a common function (l2tp_tunnel_delete) since this will do the
final sock_put to release the tunnel socket.

Since the tunnel's socket is now guaranteed to exist if the tunnel
exists, we no longer need to use sockfd_lookup via l2tp_sock_to_tunnel
to derive the tunnel from the socket since this is always
sk_user_data.

Also, sessions no longer sock_hold the tunnel socket since sessions
already hold a tunnel ref and the tunnel sock will not be freed until
the tunnel is freed. Removing these sock_holds in
l2tp_session_register avoids a possible sock leak in the
pppol2tp_connect error path if l2tp_session_register succeeds but
attaching a ppp channel fails. The pppol2tp_connect error path could
have been fixed instead and have the sock ref dropped when the session
is freed, but doing a sock_put of the tunnel socket when the session
is freed would require a new session_free callback. It is simpler to
just remove the sock_hold of the tunnel socket in
l2tp_session_register, now that the tunnel socket lifetime is
guaranteed.

Finally, some init code in l2tp_tunnel_create is reordered to ensure
that the new tunnel object's refcount is set and the tunnel socket ref
is taken before the tunnel socket destructor callbacks are set.

kasan: CONFIG_KASAN_INLINE enabled
kasan: GPF could be caused by NULL-ptr deref or user memory access
general protection fault: 0000 [#1] SMP KASAN
Modules linked in:
CPU: 0 PID: 4360 Comm: syzbot_19c09769 Not tainted 4.16.0-rc2+ #34
Hardware name: innotek GmbH VirtualBox/VirtualBox, BIOS VirtualBox 12/01/2006
RIP: 0010:pppol2tp_session_init+0x1d6/0x500
RSP: 0018:ffff88001377fb40 EFLAGS: 00010212
RAX: dffffc0000000000 RBX: ffff88001636a940 RCX: ffffffff84836c1d
RDX: 0000000000000045 RSI: 0000000055976744 RDI: 0000000000000228
RBP: ffff88001377fb60 R08: ffffffff84836bc8 R09: 0000000000000002
R10: ffff88001377fab8 R11: 0000000000000001 R12: 0000000000000000
R13: ffff88001636aac8 R14: ffff8800160f81c0 R15: 1ffff100026eff76
FS:  00007ffb3ea66700(0000) GS:ffff88001a400000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000000020e77000 CR3: 0000000016261000 CR4: 00000000000006f0
Call Trace:
 pppol2tp_connect+0xd18/0x13c0
 ? pppol2tp_session_create+0x170/0x170
 ? __might_fault+0x115/0x1d0
 ? lock_downgrade+0x860/0x860
 ? __might_fault+0xe5/0x1d0
 ? security_socket_connect+0x8e/0xc0
 SYSC_connect+0x1b6/0x310
 ? SYSC_bind+0x280/0x280
 ? __do_page_fault+0x5d1/0xca0
 ? up_read+0x1f/0x40
 ? __do_page_fault+0x3c8/0xca0
 SyS_connect+0x29/0x30
 ? SyS_accept+0x40/0x40
 do_syscall_64+0x1e0/0x730
 ? trace_hardirqs_off_thunk+0x1a/0x1c
 entry_SYSCALL_64_after_hwframe+0x42/0xb7
RIP: 0033:0x7ffb3e376259
RSP: 002b:00007ffeda4f6508 EFLAGS: 00000202 ORIG_RAX: 000000000000002a
RAX: ffffffffffffffda RBX: 0000000020e77012 RCX: 00007ffb3e376259
RDX: 000000000000002e RSI: 0000000020e77000 RDI: 0000000000000004
RBP: 00007ffeda4f6540 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000202 R12: 0000000000400b60
R13: 00007ffeda4f6660 R14: 0000000000000000 R15: 0000000000000000
Code: 80 3d b0 ff 06 02 00 0f 84 07 02 00 00 e8 13 d6 db fc 49 8d bc 24 28 02 00 00 48 b8 00 00 00 00 00 fc ff df 48 89 f
a 48 c1 ea 03 <80> 3c 02 00 0f 85 ed 02 00 00 4d 8b a4 24 28 02 00 00 e8 13 16

Fixes: 80d84ef3 ("l2tp: prevent l2tp_tunnel_delete racing with userspace close")
Signed-off-by: James Chapman <jchapman@katalix.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/l2tp/l2tp_core.c

@@ -136,51 +136,6 @@ l2tp_session_id_hash_2(struct l2tp_net *pn, u32 session_id)
 
 }
 
-/* Lookup the tunnel socket, possibly involving the fs code if the socket is
- * owned by userspace.  A struct sock returned from this function must be
- * released using l2tp_tunnel_sock_put once you're done with it.
- */
-static struct sock *l2tp_tunnel_sock_lookup(struct l2tp_tunnel *tunnel)
-{
-	int err = 0;
-	struct socket *sock = NULL;
-	struct sock *sk = NULL;
-
-	if (!tunnel)
-		goto out;
-
-	if (tunnel->fd >= 0) {
-		/* Socket is owned by userspace, who might be in the process
-		 * of closing it.  Look the socket up using the fd to ensure
-		 * consistency.
-		 */
-		sock = sockfd_lookup(tunnel->fd, &err);
-		if (sock)
-			sk = sock->sk;
-	} else {
-		/* Socket is owned by kernelspace */
-		sk = tunnel->sock;
-		sock_hold(sk);
-	}
-
-out:
-	return sk;
-}
-
-/* Drop a reference to a tunnel socket obtained via. l2tp_tunnel_sock_put */
-static void l2tp_tunnel_sock_put(struct sock *sk)
-{
-	struct l2tp_tunnel *tunnel = l2tp_sock_to_tunnel(sk);
-	if (tunnel) {
-		if (tunnel->fd >= 0) {
-			/* Socket is owned by userspace */
-			sockfd_put(sk->sk_socket);
-		}
-		sock_put(sk);
-	}
-	sock_put(sk);
-}
-
 /* Session hash list.
  * The session_id SHOULD be random according to RFC2661, but several
  * L2TP implementations (Cisco and Microsoft) use incrementing
@@ -193,6 +148,13 @@ l2tp_session_id_hash(struct l2tp_tunnel *tunnel, u32 session_id)
 	return &tunnel->session_hlist[hash_32(session_id, L2TP_HASH_BITS)];
 }
 
+void l2tp_tunnel_free(struct l2tp_tunnel *tunnel)
+{
+	sock_put(tunnel->sock);
+	/* the tunnel is freed in the socket destructor */
+}
+EXPORT_SYMBOL(l2tp_tunnel_free);
+
 /* Lookup a tunnel. A new reference is held on the returned tunnel. */
 struct l2tp_tunnel *l2tp_tunnel_get(const struct net *net, u32 tunnel_id)
 {
@@ -345,13 +307,11 @@ int l2tp_session_register(struct l2tp_session *session,
 			}
 
 		l2tp_tunnel_inc_refcount(tunnel);
-		sock_hold(tunnel->sock);
 		hlist_add_head_rcu(&session->global_hlist, g_head);
 
 		spin_unlock_bh(&pn->l2tp_session_hlist_lock);
 	} else {
 		l2tp_tunnel_inc_refcount(tunnel);
-		sock_hold(tunnel->sock);
 	}
 
 	hlist_add_head(&session->hlist, head);
@@ -969,7 +929,7 @@ int l2tp_udp_encap_recv(struct sock *sk, struct sk_buff *skb)
 {
 	struct l2tp_tunnel *tunnel;
 
-	tunnel = l2tp_sock_to_tunnel(sk);
+	tunnel = l2tp_tunnel(sk);
 	if (tunnel == NULL)
 		goto pass_up;
 
@@ -977,13 +937,10 @@ int l2tp_udp_encap_recv(struct sock *sk, struct sk_buff *skb)
 		 tunnel->name, skb->len);
 
 	if (l2tp_udp_recv_core(tunnel, skb, tunnel->recv_payload_hook))
-		goto pass_up_put;
+		goto pass_up;
 
-	sock_put(sk);
 	return 0;
 
-pass_up_put:
-	sock_put(sk);
 pass_up:
 	return 1;
 }
@@ -1214,7 +1171,6 @@ static void l2tp_tunnel_destruct(struct sock *sk)
 
 	l2tp_info(tunnel, L2TP_MSG_CONTROL, "%s: closing...\n", tunnel->name);
 
-
 	/* Disable udp encapsulation */
 	switch (tunnel->encap) {
 	case L2TP_ENCAPTYPE_UDP:
@@ -1237,12 +1193,11 @@ static void l2tp_tunnel_destruct(struct sock *sk)
 	list_del_rcu(&tunnel->list);
 	spin_unlock_bh(&pn->l2tp_tunnel_list_lock);
 
-	tunnel->sock = NULL;
-	l2tp_tunnel_dec_refcount(tunnel);
-
 	/* Call the original destructor */
 	if (sk->sk_destruct)
 		(*sk->sk_destruct)(sk);
+
+	kfree_rcu(tunnel, rcu);
 end:
 	return;
 }
@@ -1303,30 +1258,22 @@ EXPORT_SYMBOL_GPL(l2tp_tunnel_closeall);
 /* Tunnel socket destroy hook for UDP encapsulation */
 static void l2tp_udp_encap_destroy(struct sock *sk)
 {
-	struct l2tp_tunnel *tunnel = l2tp_sock_to_tunnel(sk);
-	if (tunnel) {
-		l2tp_tunnel_closeall(tunnel);
-		sock_put(sk);
-	}
+	struct l2tp_tunnel *tunnel = l2tp_tunnel(sk);
+
+	if (tunnel)
+		l2tp_tunnel_delete(tunnel);
 }
 
 /* Workqueue tunnel deletion function */
 static void l2tp_tunnel_del_work(struct work_struct *work)
 {
-	struct l2tp_tunnel *tunnel = NULL;
-	struct socket *sock = NULL;
-	struct sock *sk = NULL;
-
-	tunnel = container_of(work, struct l2tp_tunnel, del_work);
+	struct l2tp_tunnel *tunnel = container_of(work, struct l2tp_tunnel,
+						  del_work);
+	struct sock *sk = tunnel->sock;
+	struct socket *sock = sk->sk_socket;
 
 	l2tp_tunnel_closeall(tunnel);
 
-	sk = l2tp_tunnel_sock_lookup(tunnel);
-	if (!sk)
-		goto out;
-
-	sock = sk->sk_socket;
-
 	/* If the tunnel socket was created within the kernel, use
 	 * the sk API to release it here.
 	 */
@@ -1337,8 +1284,10 @@ static void l2tp_tunnel_del_work(struct work_struct *work)
 		}
 	}
 
-	l2tp_tunnel_sock_put(sk);
-out:
+	/* drop initial ref */
+	l2tp_tunnel_dec_refcount(tunnel);
+
+	/* drop workqueue ref */
 	l2tp_tunnel_dec_refcount(tunnel);
 }
 
@@ -1591,13 +1540,22 @@ int l2tp_tunnel_create(struct net *net, int fd, int version, u32 tunnel_id, u32
 		sk->sk_user_data = tunnel;
 	}
 
+	/* Bump the reference count. The tunnel context is deleted
+	 * only when this drops to zero. A reference is also held on
+	 * the tunnel socket to ensure that it is not released while
+	 * the tunnel is extant. Must be done before sk_destruct is
+	 * set.
+	 */
+	refcount_set(&tunnel->ref_count, 1);
+	sock_hold(sk);
+	tunnel->sock = sk;
+	tunnel->fd = fd;
+
 	/* Hook on the tunnel socket destructor so that we can cleanup
 	 * if the tunnel socket goes away.
 	 */
 	tunnel->old_sk_destruct = sk->sk_destruct;
 	sk->sk_destruct = &l2tp_tunnel_destruct;
-	tunnel->sock = sk;
-	tunnel->fd = fd;
 	lockdep_set_class_and_name(&sk->sk_lock.slock, &l2tp_socket_class, "l2tp_sock");
 
 	sk->sk_allocation = GFP_ATOMIC;
@@ -1607,11 +1565,6 @@ int l2tp_tunnel_create(struct net *net, int fd, int version, u32 tunnel_id, u32
 
 	/* Add tunnel to our list */
 	INIT_LIST_HEAD(&tunnel->list);
-
-	/* Bump the reference count. The tunnel context is deleted
-	 * only when this drops to zero. Must be done before list insertion
-	 */
-	refcount_set(&tunnel->ref_count, 1);
 	spin_lock_bh(&pn->l2tp_tunnel_list_lock);
 	list_add_rcu(&tunnel->list, &pn->l2tp_tunnel_list);
 	spin_unlock_bh(&pn->l2tp_tunnel_list_lock);
@@ -1652,8 +1605,6 @@ void l2tp_session_free(struct l2tp_session *session)
 
 	if (tunnel) {
 		BUG_ON(tunnel->magic != L2TP_TUNNEL_MAGIC);
-		sock_put(tunnel->sock);
-		session->tunnel = NULL;
 		l2tp_tunnel_dec_refcount(tunnel);
 	}
 

net/l2tp/l2tp_core.h

@@ -214,27 +214,8 @@ static inline void *l2tp_session_priv(struct l2tp_session *session)
 	return &session->priv[0];
 }
 
-static inline struct l2tp_tunnel *l2tp_sock_to_tunnel(struct sock *sk)
-{
-	struct l2tp_tunnel *tunnel;
-
-	if (sk == NULL)
-		return NULL;
-
-	sock_hold(sk);
-	tunnel = (struct l2tp_tunnel *)(sk->sk_user_data);
-	if (tunnel == NULL) {
-		sock_put(sk);
-		goto out;
-	}
-
-	BUG_ON(tunnel->magic != L2TP_TUNNEL_MAGIC);
-
-out:
-	return tunnel;
-}
-
 struct l2tp_tunnel *l2tp_tunnel_get(const struct net *net, u32 tunnel_id);
+void l2tp_tunnel_free(struct l2tp_tunnel *tunnel);
 
 struct l2tp_session *l2tp_session_get(const struct net *net,
 				      struct l2tp_tunnel *tunnel,
@@ -283,7 +264,7 @@ static inline void l2tp_tunnel_inc_refcount(struct l2tp_tunnel *tunnel)
 static inline void l2tp_tunnel_dec_refcount(struct l2tp_tunnel *tunnel)
 {
 	if (refcount_dec_and_test(&tunnel->ref_count))
-		kfree_rcu(tunnel, rcu);
+		l2tp_tunnel_free(tunnel);
 }
 
 /* Session reference counts. Incremented when code obtains a reference

net/l2tp/l2tp_ip.c

@@ -234,17 +234,13 @@ static void l2tp_ip_close(struct sock *sk, long timeout)
 static void l2tp_ip_destroy_sock(struct sock *sk)
 {
 	struct sk_buff *skb;
-	struct l2tp_tunnel *tunnel = l2tp_sock_to_tunnel(sk);
+	struct l2tp_tunnel *tunnel = sk->sk_user_data;
 
 	while ((skb = __skb_dequeue_tail(&sk->sk_write_queue)) != NULL)
 		kfree_skb(skb);
 
-	if (tunnel) {
-		l2tp_tunnel_closeall(tunnel);
-		sock_put(sk);
-	}
-
-	sk_refcnt_debug_dec(sk);
+	if (tunnel)
+		l2tp_tunnel_delete(tunnel);
 }
 
 static int l2tp_ip_bind(struct sock *sk, struct sockaddr *uaddr, int addr_len)

net/l2tp/l2tp_ip6.c

@@ -248,16 +248,14 @@ static void l2tp_ip6_close(struct sock *sk, long timeout)
 
 static void l2tp_ip6_destroy_sock(struct sock *sk)
 {
-	struct l2tp_tunnel *tunnel = l2tp_sock_to_tunnel(sk);
+	struct l2tp_tunnel *tunnel = sk->sk_user_data;
 
 	lock_sock(sk);
 	ip6_flush_pending_frames(sk);
 	release_sock(sk);
 
-	if (tunnel) {
-		l2tp_tunnel_closeall(tunnel);
-		sock_put(sk);
-	}
+	if (tunnel)
+		l2tp_tunnel_delete(tunnel);
 
 	inet6_destroy_sock(sk);
 }