Re: [PATCH RFC 2/3] io_uring: add IOURING_REGISTER_RESTRICTIONS opcode

[Stefano Garzarella <sgarzare@redhat.com>]

On Fri, Jul 10, 2020 at 11:52:48AM -0600, Jens Axboe wrote:
> On 7/10/20 8:19 AM, Stefano Garzarella wrote:
> > The new io_uring_register(2) IOURING_REGISTER_RESTRICTIONS opcode
> > permanently installs a feature whitelist on an io_ring_ctx.
> > The io_ring_ctx can then be passed to untrusted code with the
> > knowledge that only operations present in the whitelist can be
> > executed.
> > 
> > The whitelist approach ensures that new features added to io_uring
> > do not accidentally become available when an existing application
> > is launched on a newer kernel version.
> 
> Keeping with the trend of the times, you should probably use 'allowlist'
> here instead of 'whitelist'.

Sure, it is better!

> > 
> > Currently is it possible to restrict sqe opcodes and register
> > opcodes. It is also possible to allow only fixed files.
> > 
> > IOURING_REGISTER_RESTRICTIONS can only be made once. Afterwards
> > it is not possible to change restrictions anymore.
> > This prevents untrusted code from removing restrictions.
> 
> A few comments below.
> 
> > @@ -337,6 +344,7 @@ struct io_ring_ctx {
> >  	struct llist_head		file_put_llist;
> >  
> >  	struct work_struct		exit_work;
> > +	struct io_restriction		restrictions;
> >  };
> >  
> >  /*
> 
> Since very few will use this feature, was going to suggest that we make
> it dynamically allocated. But it's just 32 bytes, currently, so probably
> not worth the effort...
> 

Yeah, I'm not sure it will grow in the future, so I'm tempted to leave it
as it is, but I can easily change it if you prefer.

> > @@ -5491,6 +5499,11 @@ static int io_req_set_file(struct io_submit_state *state, struct io_kiocb *req,
> >  	if (unlikely(!fixed && io_async_submit(req->ctx)))
> >  		return -EBADF;
> >  
> > +	if (unlikely(!fixed && req->ctx->restrictions.enabled &&
> > +		     test_bit(IORING_RESTRICTION_FIXED_FILES_ONLY,
> > +			      req->ctx->restrictions.restriction_op)))
> > +		return -EACCES;
> > +
> >  	return io_file_get(state, req, fd, &req->file, fixed);
> >  }
> 
> This one hurts, though. I don't want any extra overhead from the
> feature, and you're digging deep in ctx here to figure out of we need to
> check.
> 
> Generally, all the checking needs to be out-of-line, and it needs to
> base the decision on whether to check something or not on a cache hot
> piece of data. So I'd suggest to turn all of these into some flag.
> ctx->flags generally mirrors setup flags, so probably just add a:
> 
> 	unsigned int restrictions : 1;
> 
> after eventfd_async : 1 in io_ring_ctx. That's free, plenty of room
> there and that cacheline is already pulled in for reading.
> 

Thanks for the clear explanation!

I left a TODO comment near the 'enabled' field to look for something better,
and what you're suggesting is what I was looking for :-)

I'll change it!

Thanks,
Stefano