linux-kernel - Re: [PATCH] io_uring: fix mm use with IORING_OP

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Date:   Wed, 5 Feb 2020 19:59:13 +0300
From:   Pavel Begunkov <asml.silence@...il.com>
To:     Jens Axboe <axboe@...nel.dk>, io-uring@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] io_uring: fix mm use with IORING_OP_{READ,WRITE}

On 05/02/2020 19:52, Jens Axboe wrote:
> On 2/5/20 9:50 AM, Pavel Begunkov wrote:
>> On 05/02/2020 19:16, Jens Axboe wrote:
>>> 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.
>>>
>>>
>> On the other hand, we don't send requests async without @mm. So, if we fail them
>> whenever can't grab mm, it solves all the problems even without extra checks.
>> What do you think?
> 
> I agree, the check is/was just there as a safe guard, it's not really
> needed.
> 
Cool, I'll deal with it.

-- 
Pavel Begunkov



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