Re: [PATCH 4.9 30/47] ANDROID: binder: remove waitqueue when thread exits.

[Eric Biggers]

On Sun, Oct 06, 2019 at 07:21:17PM +0200, Greg Kroah-Hartman wrote:
> From: Martijn Coenen <maco@xxxxxxxxxxx>
> 
> commit f5cb779ba16334b45ba8946d6bfa6d9834d1527f 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@xxxxxxxxxxx>
> Reported-by: syzbot <syzkaller@xxxxxxxxxxxxxxxx>
> Cc: stable <stable@xxxxxxxxxxxxxxx> # 4.14
> [backport BINDER_LOOPER_STATE_POLL logic as well]
> Signed-off-by: Mattias Nissler <mnissler@xxxxxxxxxxxx>
> Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
> ---
>  drivers/android/binder.c |   17 ++++++++++++++++-
>  1 file changed, 16 insertions(+), 1 deletion(-)
> 
> --- a/drivers/android/binder.c
> +++ b/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 {
> @@ -2628,6 +2629,18 @@ static int binder_free_thread(struct bin
>  		} 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 inner 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);
> @@ -2651,6 +2664,8 @@ static unsigned int binder_poll(struct f
>  		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;
>  

Are you sure this backport is correct, given that in 4.9, binder_poll()
sometimes uses proc->wait instead of thread->wait?:

        wait_for_proc_work = thread->transaction_stack == NULL &&
                list_empty(&thread->todo) && thread->return_error == BR_OK;

        binder_unlock(__func__);

        if (wait_for_proc_work) {
                if (binder_has_proc_work(proc, thread))
                        return POLLIN;
                poll_wait(filp, &proc->wait, wait);
                if (binder_has_proc_work(proc, thread))
                        return POLLIN;
        } else {
                if (binder_has_thread_work(thread))
                        return POLLIN;
                poll_wait(filp, &thread->wait, wait);
                if (binder_has_thread_work(thread))
                        return POLLIN;
        }
        return 0;