Re: [PATCH 07/30] aio: add delayed cancel support

[Al Viro <viro@ZenIV.linux.org.uk>]

On Thu, Mar 29, 2018 at 10:53:05AM +0200, Christoph Hellwig wrote:
> On Wed, Mar 28, 2018 at 05:35:26PM +0100, Al Viro wrote:
> > >  	ret = vfs_fsync(req->file, req->datasync);
> > > -	fput(req->file);
> > > -	aio_complete(container_of(req, struct aio_kiocb, fsync), ret, 0);
> > > +	if (aio_complete(iocb, ret, 0, 0))
> > > +		fput(file);
> > 
> > IDGI.
> > 	1) can aio_complete() ever return false here?
> 
> It won't.  But sometimes checking the return value and sometimes not
> seems like a bad pattern.
> 
> > 	2) do we ever have aio_kiocb that would not have an associated
> > struct file * that needs to be dropped on successful aio_complete()?  AFAICS,
> > rw, fsync and poll variants all have one, and I'm not sure what kind of
> > async IO *could* be done without an opened file.
> 
> All have a file assoiated at least right now.  As mentioned last time
> finding a struct to pass that file would be rather annoying, so we'd either
> have to pass it explicitly, or do something nasty like duplicating the
> pointer in the aio_kiocb in addition to struct kiocb.  Which might not
> be that bad after all, as it would only bloat the aio_kiocb and not
> struct kiocb used on stack all over.

OK.  Let's leave that alone for now.  Re deferred cancels - AFAICS, we *must*
remove the sucker from ctx->active_reqs before dropping ->ctx_lock.

As it is, you are creating a io_cancel()/io_cancel() race leading to double
fput().  It's not that hard to fix; I can do that myself while applying your
series (as described in previous posting - kiocb_cancel_locked() returning
NULL or ERR_PTR() in non-deferred case and pointer to aio_kiocb removed from
->active_reqs in deferred one) or you could fix it in some other way and
update your branch.

As it is, the race is user-exploitable and not that hard to trigger - AIO_POLL,
then have two threads try and cancel it at the same time.