Re: [PATCH 05/25] x86/sgx: Introduce runtime protection bits

[Reinette Chatre]

Hi Jarkko,

On 1/17/2022 6:22 PM, Jarkko Sakkinen wrote:
> On Tue, Jan 18, 2022 at 03:59:29AM +0200, Jarkko Sakkinen wrote:
>> On Mon, Jan 17, 2022 at 08:13:32AM -0500, Nathaniel McCallum wrote:
>>> On Sat, Jan 15, 2022 at 6:57 AM Jarkko Sakkinen <jarkko@xxxxxxxxxx> wrote:
>>>>
>>>> On Sat, Jan 15, 2022 at 03:18:04AM +0200, Jarkko Sakkinen wrote:
>>>>> On Fri, Jan 14, 2022 at 04:41:59PM -0800, Reinette Chatre wrote:
>>>>>> Hi Jarkko,
>>>>>>
>>>>>> On 1/14/2022 4:27 PM, Jarkko Sakkinen wrote:
>>>>>>> On Fri, Jan 14, 2022 at 04:01:33PM -0800, Reinette Chatre wrote:
>>>>>>>> Hi Jarkko,
>>>>>>>>
>>>>>>>> On 1/14/2022 3:15 PM, Jarkko Sakkinen wrote:
>>>>>>>>> On Fri, Jan 14, 2022 at 03:05:21PM -0800, Reinette Chatre wrote:
>>>>>>>>>> Hi Jarkko,
>>>>>>>>>
>>>>>>>>> How enclave can check a page range that EPCM has the expected permissions?
>>>>>>>>
>>>>>>>> Only way to change EPCM permissions from outside enclave is to run ENCLS[EMODPR]
>>>>>>>> that needs to be accepted from within the enclave via ENCLU[EACCEPT]. At that
>>>>>>>> time the enclave provides the expected permissions and that will fail
>>>>>>>> if there is a mismatch with the EPCM permissions (SGX_PAGE_ATTRIBUTES_MISMATCH).
>>>>>>>
>>>>>>> This is a very valid point but that does make the introspection possible
>>>>>>> only at the time of EACCEPT.
>>>>>>>
>>>>>>> It does not give tools for enclave to make sure that EMODPR-ETRACK dance
>>>>>>> was ever exercised.
>>>>>>
>>>>>> Could you please elaborate? EACCEPT is available to the enclave as a tool
>>>>>> and it would fail if ETRACK was not completed (error SGX_NOT_TRACKED).
>>>>>>
>>>>>> Here is the relevant snippet from the SDM from the section where it
>>>>>> describes EACCEPT:
>>>>>>
>>>>>> IF (Tracking not correct)
>>>>>>     THEN
>>>>>>         RFLAGS.ZF := 1;
>>>>>>         RAX := SGX_NOT_TRACKED;
>>>>>>         GOTO DONE;
>>>>>> FI;
>>>>>>
>>>>>> Reinette
>>>>>
>>>>> Yes, if enclave calls EACCEPT it does the necessary introspection and makes
>>>>> sure that ETRACK is completed. I have trouble understanding how enclave
>>>>> makes sure that EACCEPT was called.
>>>>
>>>> I'm not concerned of anything going wrong once EMODPR has been started.
>>>>
>>>> The problem nails down to that the whole EMODPR process is spawned by
>>>> the entity that is not trusted so maybe that should further broke down
>>>> to three roles:
>>>>
>>>> 1. Build process B
>>>> 2. Runner process R.
>>>> 3. Enclave E.
>>>>
>>>> And to the costraint that we trust B *more* than R. Once B has done all the
>>>> needed EMODPR calls it would send the file descriptor to R. Even if R would
>>>> have full access to /dev/sgx_enclave, it would not matter, since B has done
>>>> EMODPR-EACCEPT dance with E.
>>>>
>>>> So what you can achieve with EMODPR is not protection against mistrusted
>>>> *OS*. There's absolutely no chance you could use it for that purpose
>>>> because mistrusted OS controls the whole process.
>>>>
>>>> EMODPR is to help to protect enclave against mistrusted *process*, i.e.
>>>> in the above scenario R.
>>>
>>> There are two general cases that I can see. Both are valid.
>>>
>>> 1. The OS moves from a trusted to an untrusted state. This could be
>>> the multi-process system you've described. But it could also be that
>>> the kernel becomes compromised after the enclave is fully initialized.
>>>
>>> 2. The OS is untrustworthy from the start.
>>>
>>> The second case is the stronger one and if you can solve it, the first
>>> one is solved implicitly. And our end goal is that if the OS does
>>> anything malicious we will crash in a controlled way.
>>>
>>> A defensive enclave will always want to have the least number of
>>> privileges for the maximum protection. Therefore, the enclave will
>>> want the OS to call EMODPR. If that were it, the host could just lie.
>>> But the enclave also verifies that the EMODPR operation was, in fact,
>>> executed by doing EACCEPT. When the enclave calls EACCEPT, if the
>>> kernel hasn't restricted permissions then we get a controlled crash.
>>> Therefore, we have solved the second case.
>>
>> So you're referring to this part of the SDM pseude code in the SDM:
>>
>> (* Check the destination EPC page for concurrency *)
>> IF ( EPC page in use )
>>     THEN #GP(0); FI;
>>
>> I wonder does "EPC page in use" unconditionally trigger when EACCEPT
>> is invoked for a page for which all of these conditions hold:
>>
>> - .PR := 0 (no EMODPR in progress)
>> - .MODIFIED := 0 (no EMODT in progress)
>> - .PENDING := 0 (no EMODPR in progress)
>>
>> I don't know the exact scope and scale of "EPC page in use".
>>
>> Then, yes, EACCEPT could be at least used to validate that one of the
>> three operations above was requested. However, enclave thread cannot say
>> which one was it, so it is guesswork.
> 
> OK, I got it, and this last paragraph is not true. SECINFO given EACCEPT
> will lock in rest of the details and make the operation deterministic.

Indeed - so the SDM pseudo code that is relevant here can be found under
the "(* Verify that accept request matches current EPC page settings *)"
comment where the enclave can verify that all EPCM values are as they should
and would fail with SGX_PAGE_ATTRIBUTES_MISMATCH if there is anything
amiss.

> 
> The only question mark then is the condition when no requests are active.

Could you please elaborate what you mean with this question? If no request
is active then I understand that to mean that no request has started. 

Reinette