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Subject: Re: [PATCH] io_uring: fix mm use with IORING_OP_{READ,WRITE}
From:   Jens Axboe <axboe@kernel.dk>
To:     Pavel Begunkov <asml.silence@gmail.com>, io-uring@vger.kernel.org,
        linux-kernel@vger.kernel.org
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Date:   Wed, 5 Feb 2020 09:16:55 -0700
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On 2/5/20 9:05 AM, Jens Axboe wrote:
> On 2/5/20 9:02 AM, Pavel Begunkov wrote:
>> On 05/02/2020 18:54, Jens Axboe wrote:
>>> On 2/5/20 8:46 AM, Pavel Begunkov wrote:
>>>> IORING_OP_{READ,WRITE} need mm to access user buffers, hence
>>>> req->has_user check should go for them as well. Move the corresponding
>>>> imports past the check.
>>>
>>> I'd need to double check, but I think the has_user check should just go.
>>> The import checks for access anyway, so we'll -EFAULT there if we
>>> somehow messed up and didn't acquire the right mm.
>>>
>> It'd be even better. I have plans to remove it, but I was thinking from a
>> different angle.
> 
> Let me just confirm it in practice, but it should be fine. Then we can just
> kill it.

OK now I remember - in terms of mm it's fine, we'll do the right thing.
But the iov_iter_init() has this gem:

	/* It will get better.  Eventually... */
	if (uaccess_kernel()) {
		i->type = ITER_KVEC | direction;
		i->kvec = (struct kvec *)iov;
	} else {
		i->type = ITER_IOVEC | direction;
		i->iov = iov;
	}

which means that if we haven't set USER_DS, then iov_iter_init() will
magically set the type to ITER_KVEC which then crashes when the iterator
tries to copy.

Which is pretty lame. How about a patch that just checks for
uaccess_kernel() and -EFAULTs if true for the non-fixed variants where
we don't init the iter ourselves? Then we can still kill req->has_user
and not have to fill it in.


-- 
Jens Axboe