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Date:   Tue, 5 Dec 2023 14:45:35 +0100
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Subject: Re: [PATCH v4] tee: Use iov_iter to better support shared buffer
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To:     Sumit Garg <sumit.garg@linaro.org>
CC:     Jens Axboe <axboe@kernel.dk>, Al Viro <viro@zeniv.linux.org.uk>,
        Jens Wiklander <jens.wiklander@linaro.org>,
        Christoph Hellwig <hch@infradead.org>,
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From:   Arnaud POULIQUEN <arnaud.pouliquen@foss.st.com>
Organization: STMicroelectronics
In-Reply-To: <CAFA6WYMi52WTWho5y=967fm8utqtdq9fuCjVJFA9G0MaHtNYgg@mail.gmail.com>
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Hi Sumit,

On 12/5/23 13:07, Sumit Garg wrote:
> Hi Arnaud,
> 
> On Mon, 4 Dec 2023 at 22:32, Arnaud POULIQUEN
> <arnaud.pouliquen@foss.st.com> wrote:
>>
>> Hi,
>>
>> On 12/4/23 17:40, Jens Axboe wrote:
>>> On 12/4/23 9:36 AM, Jens Axboe wrote:
>>>> On 12/4/23 5:42 AM, Sumit Garg wrote:
>>>>> IMO, access_ok() should be the first thing that import_ubuf() or
>>>>> import_single_range() should do, something as follows:
>>>>>
>>>>> diff --git a/lib/iov_iter.c b/lib/iov_iter.c
>>>>> index 8ff6824a1005..4aee0371824c 100644
>>>>> --- a/lib/iov_iter.c
>>>>> +++ b/lib/iov_iter.c
>>>>> @@ -1384,10 +1384,10 @@ EXPORT_SYMBOL(import_single_range);
>>>>>
>>>>>  int import_ubuf(int rw, void __user *buf, size_t len, struct iov_iter *i)
>>>>>  {
>>>>> -       if (len > MAX_RW_COUNT)
>>>>> -               len = MAX_RW_COUNT;
>>>>>         if (unlikely(!access_ok(buf, len)))
>>>>>                 return -EFAULT;
>>>>> +       if (len > MAX_RW_COUNT)
>>>>> +               len = MAX_RW_COUNT;
>>>>>
>>>>>         iov_iter_ubuf(i, rw, buf, len);
>>>>>         return 0;
>>>>>
>>>>> Jens A., Al Viro,
>>>>>
>>>>> Was there any particular reason which I am unaware of to perform
>>>>> access_ok() check on modified input length?
>>>>
>>>> This change makes sense to me, and seems consistent with what is done
>>>> elsewhere too.
>>>
>>> For some reason I missed import_single_range(), which does it the same
>>> way as import_ubuf() currently does - cap the range before the
>>> access_ok() check. The vec variants sum as they go, but access_ok()
>>> before the range.
>>>
>>> I think part of the issue here is that the single range imports return 0
>>> for success and -ERROR otherwise. This means that the caller does not
>>> know if the full range was imported or not. OTOH, we always cap any data
>>> transfer at MAX_RW_COUNT, so may make more sense to fix up the caller
>>> here.
>>>
>>
>> Should we limit to MAX_RW_COUNT or return an error?
>> Seems to me that limiting could generate side effect later that could be not
>> simple to debug.
>>
>>
>>>>>  int import_ubuf(int rw, void __user *buf, size_t len, struct iov_iter *i)
>>>>>  {
>>>>> -       if (len > MAX_RW_COUNT)
>>>>> +               return -EFAULT;
>>>>>         if (unlikely(!access_ok(buf, len)))
>>>>>                 return -EFAULT;
>>>>>
>>>>>         iov_iter_ubuf(i, rw, buf, len);
>>>>>         return 0;
>>
>> or perhaps just remove the test as __access_ok() already tests that the
>> size < TASK_SIZE
>>
>> https://elixir.bootlin.com/linux/v6.7-rc4/source/include/asm-generic/access_ok.h#L31
>>
> 
> It looks like there are predefined constraints for using import_ubuf()
> which doesn't properly match our needs. So let's directly use:
> iov_iter_ubuf() instead.

Yes, this seems a safer alternative. I will send a new version based on it.

Thanks,
Arnaud

> 
> -Sumit
> 
>>
>> Thanks,
>> Arnaud
>>