Re: [PATCH] fs/io_uring: fix O_PATH fds in openat, openat2, statx

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

On Thu, May 07, 2020 at 05:03:17PM -0600, Jens Axboe wrote:
> On 5/7/20 4:44 PM, Al Viro wrote:
> > On Thu, May 07, 2020 at 04:25:24PM -0600, Jens Axboe wrote:
> > 
> >>  static int io_close(struct io_kiocb *req, bool force_nonblock)
> >>  {
> >> +	struct files_struct *files = current->files;
> >>  	int ret;
> >>  
> >>  	req->close.put_file = NULL;
> >> -	ret = __close_fd_get_file(req->close.fd, &req->close.put_file);
> >> +	spin_lock(&files->file_lock);
> >> +	if (req->file->f_op == &io_uring_fops ||
> >> +	    req->close.fd == req->ctx->ring_fd) {
> >> +		spin_unlock(&files->file_lock);
> >> +		return -EBADF;
> >> +	}
> >> +
> >> +	ret = __close_fd_get_file_locked(files, req->close.fd,
> >> +						&req->close.put_file);
> > 
> > Pointless.  By that point req->file might have nothing in common with
> > anything in any descriptor table.
> 
> How about the below then? Stop using req->file, defer the lookup until
> we're in the handler instead. Not sure the 'fd' check makes sense
> at this point, but at least we should be consistent in terms of
> once we lookup the file and check the f_op.

Actually, what _is_ the reason for that check?  Note, BTW, that if the
file in question happens to be an AF_UNIX socket, closing it will
close all references held in SCM_RIGHTS datagrams sitting in its queue,
which might very well include io_uring files.

IOW, if tries to avoid something really unpleasant, it's not enough.
And if it doesn't, then what is it for?