Re: [PATCH] selftests/sgx: fix EINIT failure dueto SGX_INVALID_SIGNATURE

[Jia Zhang <zhang.jia@linux.alibaba.com>]

On 2021/3/11 上午11:42, Jarkko Sakkinen wrote:
> On Thu, Mar 11, 2021 at 10:47:50AM +0800, Jia Zhang wrote:
>>
>>
>> On 2021/3/11 上午5:39, Jarkko Sakkinen wrote:
>>> On Wed, Mar 10, 2021 at 08:44:44PM +0800, Jia Zhang wrote:
>>>>
>>>>
>>>> On 2021/3/2 下午9:47, Jarkko Sakkinen wrote:
>>>>> On Mon, Mar 01, 2021 at 09:54:37PM -0800, Andy Lutomirski wrote:
>>>>>> On Mon, Mar 1, 2021 at 9:06 PM Tianjia Zhang
>>>>>> <tianjia.zhang@linux.alibaba.com> wrote:
>>>>>>>
>>>>>>>
>>>>>>>
>>>>>>> On 3/1/21 5:54 PM, Jarkko Sakkinen wrote:
>>>>>>>> On Mon, Mar 01, 2021 at 01:18:36PM +0800, Tianjia Zhang wrote:
>>>>>>>>> q2 is not always 384-byte length. Sometimes it only has 383-byte.
>>>>>>>>
>>>>>>>> What does determine this?
>>>>>>>>
>>>>>>>>> In this case, the valid portion of q2 is reordered reversely for
>>>>>>>>> little endian order, and the remaining portion is filled with zero.
>>>>>>>>
>>>>>>>> I'm presuming that you want to say "In this case, q2 needs to be reversed because...".
>>>>>>>>
>>>>>>>> I'm lacking these details:
>>>>>>>>
>>>>>>>> 1. Why the length of Q2 can vary?
>>>>>>>> 2. Why reversing the bytes is the correct measure to counter-measure
>>>>>>>>     this variation?
>>>>>>>>
>>>>>>>> /Jarkko
>>>>>>>>
>>>>>>>
>>>>>>> When use openssl to generate a key instead of using the built-in
>>>>>>> sign_key.pem, there is a probability that will encounter this problem.
>>>>>>>
>>>>>>> Here is a problematic key I encountered. The calculated q1 and q2 of
>>>>>>> this key are both 383 bytes, If the length is not processed, the
>>>>>>> hardware signature will fail.
>>>>>>
>>>>>> Presumably the issue is that some keys have parameters that have
>>>>>> enough leading 0 bits to be effectively shorter.  The openssl API
>>>>>> (and, sadly, a bunch  of the ASN.1 stuff) treats these parameters as
>>>>>> variable-size integers.
>>>>>
>>>>> But the test uses a static key. It used to generate a key on fly but
>>>>
>>>> IMO even though the test code, it comes from the linux kernel, meaning
>>>> that its quality has a certain guarantee and it is a good reference, so
>>>> the test code still needs to ensure its correctness.
>>>
>>> Hmm... what is working incorrectly then?
>>
>> In current implementation, it is working well, after all the static key
>> can derive the full 384-byte length of q1 and q2. As mentioned above, if
>> someone refers to the design of signing tool from selftest code, it is
>> quite possible that the actual implementation will use dynamical or
>> external signing key deriving shorter q1 and/or q2 in length.
> 
> A self-test needs is not meant to be generic to be directly used in 3rd
> party code. With the current key there is not issue => there is no issue.

Alright. So what we should do is to add a comment to explain why the
selfcode does something wrong and essentially it is intended.

Jia

> 
>>
>> Going back the technical background, I'm not a crypto expert, so I
>> choose to check the code, doc and make experiment.
>>
>> Accorindg to intel sdm vol3 37.14:
>>
>> ```
>> SIGSTRUCT includes four 3072-bit integers (MODULUS, SIGNATURE, Q1, Q2).
>> Each such integer is represented as a byte strings of length 384, with
>> the most significant byte at the position “offset + 383”, and the least
>> significant byte at position “offset”.
>>
>> ...
>>
>> The 3072-bit integers Q1 and Q2 are defined by:
>> q1 = floor(Signature^2 / Modulus);
>> q2 = floor((Signature^3 - q1 * Signature * Modulus) / Modulus);
>> ```
>>
>> and the implementation of singing tool from Intel SGX SDK
>> (https://github.com/intel/linux-sgx/blob/master/sdk/sign_tool/SignTool/sign_tool.cpp#L381
>> ), the most significant byte shuld be at the position “offset +
>> q1/q2_actuall_len”, and the padding portion should be filled with zero,
>> and don't involve the order reverse, but the selftest code always does.
>> This is the root cause of SGX_INVALID_SIGNATURE.
>>
>> Just simplily enforce size_q1 and size_q2 to SE_KEY_SIZE, and generate
>> randome siging key with `openssl genrsa -3 -out signing_key.pem 3072`,
>> the SGX_INVALID_SIGNATURE error will appear up quickly.
>>
>> Jia
>>
>>>
>>> /Jarkko
>>>
>>
> 
> /Jarkko
>