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

[Jarkko Sakkinen <jarkko@kernel.org>]

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.

> 
> 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