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

[Reinette Chatre <reinette.chatre@intel.com>]

Hi Jarkko,

Apologies, after thinking about this more I believe that I misunderstood
your proposal in my original answer.

On 3/9/2022 8:59 AM, 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.

Why not keep the flexibility of supporting all permission restrictions?

It is already possible to call SGX_IOC_ENCLAVE_RESTRICT_PERMISSIONS with PROT_NONE, no?

> 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.
> 
> 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@intel.com/
>   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.
> 
>>> 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.
> 

As I understand it this is currently possible with
SGX_IOC_ENCLAVE_RESTRICT_PERMISSIONS.

Reinette