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

Jarkko Sakkinen
2022-03-14T02:49:59Z

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.

BR, Jarkko