Re: [PATCH 03/25] x86/sgx: Support VMA permissions exceeding enclave permissions

[Reinette Chatre]

Hi Jarkko,

On 12/4/2021 2:27 PM, Jarkko Sakkinen wrote:
> On Sun, Dec 05, 2021 at 12:25:59AM +0200, Jarkko Sakkinen wrote:
> > On Wed, Dec 01, 2021 at 11:23:01AM -0800, Reinette Chatre wrote:
> > > === Summary ===
> > >
> > > An SGX VMA can only be created if its permissions are the same or
> > > weaker than the Enclave Page Cache Map (EPCM) permissions. After VMA
> > > creation this rule continues to be enforced by the page fault handler.
> > >
> > > With SGX2 the EPCM permissions of a page can change after VMA
> > > creation resulting in the VMA exceeding the EPCM permissions and the
> > > page fault handler incorrectly blocking access.
> > >
> > > Enable the VMA's pages to remain accessible while ensuring that
> > > the page table entries are installed to match the EPCM permissions
> > > without exceeding the VMA perms issions.
> >
> > I don't understand what the short summary means in English, and the
> > commit message is way too bloated to make any conclusions. It really
> > needs a rewrite.
> >
> > These were the questions I could not find answer for:
> >
> > 1. Why it would be by any means safe to remove a permission check?

The permission check is redundant for SGX1 and incorrect for SGX2.

In the current SGX1 implementation the permission check in 
sgx_encl_load_page() is redundant because an SGX VMA can only be created 
if its permissions are the same or weaker than the EPCM permissions.

In SGX2 a user is able to change EPCM permissions during runtime (while 
VMA has the memory mapped). A RW VMA may thus originally have mapped an 
enclave page with RW EPCM permissions but since then the enclave page 
may have its permissions changed to read-only. The VMA should still be 
able to read those enclave pages but the check in sgx_encl_load_page() 
will prevent that.

> > 2. Why not re-issuing mmap()'s is unfeasible? I.e. close existing
> >     VMA's and mmap() new ones.

User is not prevented from closing existing VMAs and creating new ones.

> 3. Isn't this an API/ABI break?

Could you please elaborate where you see the API/ABI break? The rule 
that new VMAs cannot exceed EPCM permissions is untouched.

Reinette