Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
Subject: Re: [PATCH] io_uring: fix mm use with IORING_OP_{READ,WRITE}
To:     Pavel Begunkov <asml.silence@gmail.com>, io-uring@vger.kernel.org,
        linux-kernel@vger.kernel.org
References: <951bb84c8337aaac7654261a21b03506b2b8a001.1580914723.git.asml.silence@gmail.com>
 <df11c48e-f456-3b64-88d1-6012b1ac2bc8@kernel.dk>
 <d9a15d32-a20b-a20f-9ea4-3ac355c15bb2@gmail.com>
 <c6cc1e97-f306-e3f0-3f7b-9463fdbbc5a5@kernel.dk>
 <d55fd4dc-e7f1-0f06-76bb-0e29c1db4995@kernel.dk>
 <62e33e81-7857-fb5c-92be-6757623a6478@gmail.com>
From:   Jens Axboe <axboe@kernel.dk>
Message-ID: <fe6ea91c-ad4a-092f-e98c-6ba233304636@kernel.dk>
Date:   Wed, 5 Feb 2020 09:52:41 -0700
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101
 Thunderbird/68.4.1
MIME-Version: 1.0
In-Reply-To: <62e33e81-7857-fb5c-92be-6757623a6478@gmail.com>
Content-Type: text/plain; charset=utf-8
Content-Language: en-US
Content-Transfer-Encoding: 7bit
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

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