mptcp: fix UaF in listener shutdown

As reported by Christoph, the mptcp listener shutdown path is prone
to an UaF issue.

    BUG: KASAN: use-after-free in _raw_spin_lock_bh+0x73/0xe0
    Write of size 4 at addr ffff88810cb23098 by task syz-executor731/1266

    CPU: 1 PID: 1266 Comm: syz-executor731 Not tainted 6.2.0-rc59af4eaa31c1f6c00c8f1e448ed99a45c66340dd5 #6
    Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.13.0-0-gf21b5a4aeb02-prebuilt.qemu.org 04/01/2014
    Call Trace:
     <TASK>
     dump_stack_lvl+0x6e/0x91
     print_report+0x16a/0x46f
     kasan_report+0xad/0x130
     kasan_check_range+0x14a/0x1a0
     _raw_spin_lock_bh+0x73/0xe0
     subflow_error_report+0x6d/0x110
     sk_error_report+0x3b/0x190
     tcp_disconnect+0x138c/0x1aa0
     inet_child_forget+0x6f/0x2e0
     inet_csk_listen_stop+0x209/0x1060
     __mptcp_close_ssk+0x52d/0x610
     mptcp_destroy_common+0x165/0x640
     mptcp_destroy+0x13/0x80
     __mptcp_destroy_sock+0xe7/0x270
     __mptcp_close+0x70e/0x9b0
     mptcp_close+0x2b/0x150
     inet_release+0xe9/0x1f0
     __sock_release+0xd2/0x280
     sock_close+0x15/0x20
     __fput+0x252/0xa20
     task_work_run+0x169/0x250
     exit_to_user_mode_prepare+0x113/0x120
     syscall_exit_to_user_mode+0x1d/0x40
     do_syscall_64+0x48/0x90
     entry_SYSCALL_64_after_hwframe+0x72/0xdc

The msk grace period can legitly expire in between the last
reference count dropped in mptcp_subflow_queue_clean() and
the later eventual access in inet_csk_listen_stop()

After the previous patch we don't need anymore special-casing
msk listener socket cleanup: the mptcp worker will process each
of the unaccepted msk sockets.

Just drop the now unnecessary code.

Fixes: 6aeed90 ("mptcp: fix race on unaccepted mptcp sockets")
Reported-and-tested-by: Christoph Paasch <cpaasch@apple.com>
Closes: #346
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

net/mptcp/protocol.c

@@ -2357,7 +2357,6 @@ static void __mptcp_close_ssk(struct sock *sk, struct sock *ssk,
 		/* otherwise tcp will dispose of the ssk and subflow ctx */
 		if (ssk->sk_state == TCP_LISTEN) {
 			tcp_set_state(ssk, TCP_CLOSE);
-			mptcp_subflow_queue_clean(sk, ssk);
 			inet_csk_listen_stop(ssk);
 			mptcp_event_pm_listener(ssk, MPTCP_EVENT_LISTENER_CLOSED);
 		}

net/mptcp/protocol.h

@@ -629,7 +629,6 @@ void mptcp_close_ssk(struct sock *sk, struct sock *ssk,
 		     struct mptcp_subflow_context *subflow);
 void __mptcp_subflow_send_ack(struct sock *ssk);
 void mptcp_subflow_reset(struct sock *ssk);
-void mptcp_subflow_queue_clean(struct sock *sk, struct sock *ssk);
 void mptcp_sock_graft(struct sock *sk, struct socket *parent);
 struct socket *__mptcp_nmpc_socket(const struct mptcp_sock *msk);
 bool __mptcp_close(struct sock *sk, long timeout);

net/mptcp/subflow.c

@@ -1801,78 +1801,6 @@ static void subflow_state_change(struct sock *sk)
 	}
 }
 
-void mptcp_subflow_queue_clean(struct sock *listener_sk, struct sock *listener_ssk)
-{
-	struct request_sock_queue *queue = &inet_csk(listener_ssk)->icsk_accept_queue;
-	struct mptcp_sock *msk, *next, *head = NULL;
-	struct request_sock *req;
-
-	/* build a list of all unaccepted mptcp sockets */
-	spin_lock_bh(&queue->rskq_lock);
-	for (req = queue->rskq_accept_head; req; req = req->dl_next) {
-		struct mptcp_subflow_context *subflow;
-		struct sock *ssk = req->sk;
-		struct mptcp_sock *msk;
-
-		if (!sk_is_mptcp(ssk))
-			continue;
-
-		subflow = mptcp_subflow_ctx(ssk);
-		if (!subflow || !subflow->conn)
-			continue;
-
-		/* skip if already in list */
-		msk = mptcp_sk(subflow->conn);
-		if (msk->dl_next || msk == head)
-			continue;
-
-		msk->dl_next = head;
-		head = msk;
-	}
-	spin_unlock_bh(&queue->rskq_lock);
-	if (!head)
-		return;
-
-	/* can't acquire the msk socket lock under the subflow one,
-	 * or will cause ABBA deadlock
-	 */
-	release_sock(listener_ssk);
-
-	for (msk = head; msk; msk = next) {
-		struct sock *sk = (struct sock *)msk;
-		bool do_cancel_work;
-
-		lock_sock_nested(sk, SINGLE_DEPTH_NESTING);
-		next = msk->dl_next;
-		msk->first = NULL;
-		msk->dl_next = NULL;
-
-		do_cancel_work = __mptcp_close(sk, 0);
-		release_sock(sk);
-		if (do_cancel_work) {
-			/* lockdep will report a false positive ABBA deadlock
-			 * between cancel_work_sync and the listener socket.
-			 * The involved locks belong to different sockets WRT
-			 * the existing AB chain.
-			 * Using a per socket key is problematic as key
-			 * deregistration requires process context and must be
-			 * performed at socket disposal time, in atomic
-			 * context.
-			 * Just tell lockdep to consider the listener socket
-			 * released here.
-			 */
-			mutex_release(&listener_sk->sk_lock.dep_map, _RET_IP_);
-			mptcp_cancel_work(sk);
-			mutex_acquire(&listener_sk->sk_lock.dep_map,
-				      SINGLE_DEPTH_NESTING, 0, _RET_IP_);
-		}
-		sock_put(sk);
-	}
-
-	/* we are still under the listener msk socket lock */
-	lock_sock_nested(listener_ssk, SINGLE_DEPTH_NESTING);
-}
-
 static int subflow_ulp_init(struct sock *sk)
 {
 	struct inet_connection_sock *icsk = inet_csk(sk);