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. BR, Jarkko
