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

[Jarkko Sakkinen]

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