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

[Jarkko Sakkinen]

On Wed, Mar 09, 2022 at 04:10:27PM -0800, Reinette Chatre wrote:
> 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.

Please do not! It's really important this is looked from every angle
before it hits the mainline :-)

BR, Jarkko