Re: [RFC PATCH v2.1 14/30] x86/sgx: Support restricting of enclave page permissions

[Reinette Chatre]

Hi Jarkko,

On 3/9/2022 3:42 PM, Jarkko Sakkinen wrote:
> On Thu, Mar 10, 2022 at 01:35:53AM +0200, Jarkko Sakkinen wrote:
>> On Wed, Mar 09, 2022 at 08:59:42AM -0800, Reinette Chatre wrote:
>>> Hi Jarkko,
>>>
>>> On 3/9/2022 1:35 AM, Jarkko Sakkinen wrote:
>>>> On Wed, Mar 09, 2022 at 10:52:22AM +0200, Jarkko Sakkinen wrote:
>>>>> On Fri, Mar 04, 2022 at 11:35:08AM +0200, Jarkko Sakkinen wrote:
>>>>>> +#define SGX_IOC_ENCLAVE_RESTRICT_PERMISSIONS \
>>>>>> +	_IOWR(SGX_MAGIC, 0x05, struct sgx_enclave_restrict_perm)
>>>>>
>>>>> What if this was replaced with just SGX_IOC_ENCLAVE_RESET_PAGES, which
>>>>> would simply do EMODPR with PROT_NONE? The main ingredient of EMODPR is to
>>>>> flush out the TLB's, and move a page to pending state, which cannot be done
>>>>> from inside the enclave.
>>>
>>> I see the main ingredient as running EMODPR to restrict the EPCM permissions. If
>>> the user wants to use SGX_IOC_ENCLAVE_RESTRICT_PERMISSIONS just to flush TLB it is
>>> already possible since attempting to use EMODPR to relax permissions does not
>>> change any permissions (although it still sets EPCM.PR) but yet will still
>>> flush the TLB.
>>
>> It's not just to flush the TLB. It also resets permissions to zero from
>> which it is easy to set the exact permissions with EMODPE.
>>
>>> Even so, you have a very good point that removing SGX_IOC_ENCLAVE_RELAX_PERMISSIONS
>>> removes the ability for users to flush the TLB after an EMODPE. If there are
>>> thus PTEs present at the time the user runs EMODPE the pages would not be
>>> accessible with the new permissions.
>>>
>>> Repurposing SGX_IOC_ENCLAVE_RESTRICT_PERMISSIONS with PROT_NONE to accomplish
>>> this is not efficient because:
>>> - For the OS to flush the TLB the enclave pages need not be in the EPC but
>>>   in order to run EMODPR the enclave page needs to be in the EPC. In an 
>>>   oversubscribed environment running EMODPR unnecessarily can thus introduce
>>>   a significant delay. Please see the performance comparison I did in
>>>   https://lore.kernel.org/linux-sgx/77e81306-6b03-4b09-2df2-48e09e2e79d5@xxxxxxxxx/
>>>   The test shows that running EMODPR unnecessarily can be orders of magnitude slower.
>>> - Running EMODPR on an enclave page sets the EPCM.PR bin in the enclave page
>>>   that needs to be cleared with an EACCEPT from within the enclave.
>>>   If the user just wants to reset the TLB after running EMODPE then it should
>>>   not be necessary to run EACCEPT again to reset EPCM.PR.
>>>
>>> Resetting the TLB is exactly what SGX_IOC_ENCLAVE_RELAX_PERMISSIONS did in an 
>>> efficient way - it is quick (no need to load pages into EPC) and it does not
>>> require EACCEPT to clear EPCM.PR. 
>>>
>>> It looks like we need SGX_IOC_ENCLAVE_RELAX_PERMISSIONS back. We could
>>> rename it to SGX_IOC_ENCLAVE_RESET_PAGES if you prefer.
>>
>> Please do not add it. We do not have any use for it. It's not only used
>> to flush TLB's so it would not do any good. I just use it with fixed
>> PROT_NONE permissions.
>>
>>>>> It's there because of microarchitecture constraints, and less so to work as
>>>>> a reasonable permission control mechanism (actually it does terrible job on
>>>>> that side and only confuses).
>>>>>
>>>>> Once you have this magic TLB reset button in place you can just do one
>>>>> EACCEPT and EMODPE inside the enclave and you're done.
>>>>>
>>>>> This is also kind of atomic in the sense that EACCEPT free's a page with no
>>>>> rights so no misuse can happend before EMODPE has tuned EPCM.
>>>>
>>>> I wonder if this type of pattern could be made work out for Graphene:
>>>>
>>>> 1. SGX_IOC_ENCLAVE_RESET_PAGES
>>>> 2. EACCEPT + EMODPE
>>>>
>>>> This kind of delivers EMODP that everyone has been looking for.
>>>
>>> EACCEPT will result in page table entries created for the enclave page. EMODPE
>>> will be able to relax the permissions but TLB flush would be required to
>>> access the page with the new permissions. SGX_IOC_ENCLAVE_RELAX_PERMISSIONS
>>> (renamed to SGX_IOC_ENCLAVE_RESET_PAGES?) that does just a TLB flush is
>>> required to be after EMODPE.
>>
>> For EMODPE TLB flush is not required. I even verified this from Mark
>> Shanahan. And since access rights are zero, the page cannot be
>> deferenced by threads before EMODPE.
> 
> I'm fine of course keeping SGX_IOC_ENCLAVE_RESTRICT_PERMISSIONS as it
> is too. It's at least future-proof that way.

Will do.

Reinette