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

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Date:   Thu, 7 May 2020 20:28:10 -0600
From:   Jens Axboe <axboe@...nel.dk>
To:     Al Viro <viro@...iv.linux.org.uk>
Cc:     Max Kellermann <mk@...all.com>, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org, stable@...r.kernel.org
Subject: Re: [PATCH] fs/io_uring: fix O_PATH fds in openat, openat2, statx

On 5/7/20 5:31 PM, Al Viro wrote:
> 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?

Maybe there is no issue at all, the point was obviously to not have
io_uring close itself. But we might just need an ordering of the
fput vs put_request to make that just fine. Let me experiment a bit
and see what's going on.

-- 
Jens Axboe