linux-kernel - Re: [PATCH v4] tee: Use iov_iter to better support shared buffer registration

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Date:   Mon, 4 Dec 2023 18:02:27 +0100
From:   Arnaud POULIQUEN <arnaud.pouliquen@...s.st.com>
To:     Jens Axboe <axboe@...nel.dk>, Sumit Garg <sumit.garg@...aro.org>,
        Al Viro <viro@...iv.linux.org.uk>
CC:     Jens Wiklander <jens.wiklander@...aro.org>,
        Christoph Hellwig <hch@...radead.org>,
        <op-tee@...ts.trustedfirmware.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v4] tee: Use iov_iter to better support shared buffer
 registration

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


Thanks,
Arnaud