Re: [PATCH] io_uring: fix mm use with IORING_OP_{READ,WRITE}

[Jens Axboe]

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.

-- 
Jens Axboe