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
