Hi Jarkko, On 3/9/2022 3:35 PM, 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. > Understood. I realized my mistake only after sending the email and attempted to correct it in the following. Sorry for the noise. Reinette
