ANDROID: binder: remove waitqueue when thread exits.

BugLink: https://bugs.launchpad.net/bugs/1848598

commit f5cb779b upstream.

binder_poll() passes the thread->wait waitqueue that
can be slept on for work. When a thread that uses
epoll explicitly exits using BINDER_THREAD_EXIT,
the waitqueue is freed, but it is never removed
from the corresponding epoll data structure. When
the process subsequently exits, the epoll cleanup
code tries to access the waitlist, which results in
a use-after-free.

Prevent this by using POLLFREE when the thread exits.
Signed-off-by: Martijn Coenen <maco@android.com>
Reported-by: syzbot <syzkaller@googlegroups.com>
Cc: stable <stable@vger.kernel.org> # 4.14
[backport BINDER_LOOPER_STATE_POLL logic as well]
Signed-off-by: Mattias Nissler <mnissler@chromium.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Connor Kuehl <connor.kuehl@canonical.com>
Signed-off-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>

drivers/android/binder.c

@@ -334,7 +334,8 @@ enum {
 	BINDER_LOOPER_STATE_EXITED      = 0x04,
 	BINDER_LOOPER_STATE_INVALID     = 0x08,
 	BINDER_LOOPER_STATE_WAITING     = 0x10,
-	BINDER_LOOPER_STATE_NEED_RETURN = 0x20
+	BINDER_LOOPER_STATE_NEED_RETURN = 0x20,
+	BINDER_LOOPER_STATE_POLL	= 0x40,
 };
 
 struct binder_thread {
@@ -2610,6 +2611,18 @@ static int binder_free_thread(struct binder_proc *proc,
 		} else
 			BUG();
 	}
+
+	/*
+	 * If this thread used poll, make sure we remove the waitqueue
+	 * from any epoll data structures holding it with POLLFREE.
+	 * waitqueue_active() is safe to use here because we're holding
+	 * the global lock.
+	 */
+	if ((thread->looper & BINDER_LOOPER_STATE_POLL) &&
+	    waitqueue_active(&thread->wait)) {
+		wake_up_poll(&thread->wait, POLLHUP | POLLFREE);
+	}
+
 	if (send_reply)
 		binder_send_failed_reply(send_reply, BR_DEAD_REPLY);
 	binder_release_work(&thread->todo);
@@ -2633,6 +2646,8 @@ static unsigned int binder_poll(struct file *filp,
 		return POLLERR;
 	}
 
+	thread->looper |= BINDER_LOOPER_STATE_POLL;
+
 	wait_for_proc_work = thread->transaction_stack == NULL &&
 		list_empty(&thread->todo) && thread->return_error == BR_OK;