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@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. > >>> 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
