Re: [PATCH V2 16/32] x86/sgx: Support restricting of enclave page permissions

Jarkko Sakkinen
2022-03-14T02:58:01Z

On Mon, Mar 14, 2022 at 04:50:56AM +0200, Jarkko Sakkinen wrote:
> On Mon, Mar 14, 2022 at 04:49:37AM +0200, Jarkko Sakkinen wrote:
> > On Fri, Mar 11, 2022 at 09:53:29AM -0800, Reinette Chatre wrote:
> >
> > > I saw Haitao's note that EMODPE requires "Read access permitted by enclave".
> > > This motivates that EMODPR->PROT_NONE should not be allowed since it would
> > > not be possible to relax permissions (run EMODPE) after that. Even so, I
> > > also found in the SDM that EACCEPT has the note "Read access permitted
> > > by enclave". That seems to indicate that EMODPR->PROT_NONE is not practical
> > > from that perspective either since the enclave will not be able to
> > > EACCEPT the change. Does that match your understanding?
> >
> > Yes, PROT_NONE should not be allowed.
> >
> > This is however the real problem.
> >
> > The current kernel patch set has inconsistent API and EMODPR ioctl is
> > simply unacceptable. It also requires more concurrency management from
> > user space run-time, which would be heck a lot easier to do in the kernel.
> >
> > If you really want EMODPR as ioctl, then for consistencys sake, then EAUG
> > should be too. Like this when things go opposite directions, this patch set
> > plain and simply will not work out.
> >
> > I would pick EAUG's strategy from these two as it requires half the back
> > calls to host from an enclave. I.e. please combine mprotect() and EMODPR,
> > either in the #PF handler or as part of mprotect(), which ever suits you
> > best.
> >
> > I'll try demonstrate this with two examples.
> >
> > mmap() could go something like this() (simplified):
> > 1. Execution #UD's to SYSCALL.
> > 2. Host calls enclave's mmap() handler with mmap() parameters.
> > 3. Enclave up-calls host's mmap().
> > 4. Loops the range with EACCEPTCOPY.
> >
> > mprotect() has to be done like this:
> > 1. Execution #UD's to SYSCALL.
> > 2. Host calls enclave's mprotect() handler.
> > 3. Enclave up-calls host's mprotect().
> > 4. Enclave up-calls host's ioctl() to SGX_IOC_ENCLAVE_PERMISSIONS.
> > 3. Loops the range with EACCEPT.
> ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> 5. Loops the range with EACCEPT + EMODPE.
>
> > This is just terrible IMHO. I hope these examples bring some insight.

E.g. in Enarx we have to add a special up-call (so called enarxcall in
intermediate that we call sallyport, which provides shared buffer to
communicate with the enclave) just for reseting the range with PROT_READ.
Feel very redundant, adds ugly cruft and is completely opposite strategy to
what you've chosen to do with EAUG, which is I think correct choice as far
as API is concerned.

BR, Jarkko