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

[Greg Kroah-Hartman]

On Sun, Oct 06, 2019 at 10:32:02AM -0700, Eric Biggers wrote:
> 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;

I _think_ the backport is correct, and I know someone has verified that
the 4.4.y backport works properly and I don't see much difference here
from that version.

But I will defer to Todd and Martijn here, as they know this code _WAY_
better than I do.  The codebase has changed a lot from 4.9.y to 4.14.y
so it makes it hard to do equal comparisons simply.

Todd and Martijn, thoughts?

thanks,

greg k-h