Re: [PATCH v3] x86/sgx: Do not limit EAUG'd pages by pre-initialization policy

[Jarkko Sakkinen <jarkko@kernel.org>]

On Fri, Mar 04, 2022 at 11:09:36AM -0800, Reinette Chatre wrote:
> Hi Jarkko,
> 
> On 3/3/2022 7:39 PM, Jarkko Sakkinen wrote:
> > Pre-initialization policy is meant for EADD'd pages because they are
> > part of the enclave identity. It's a good practice to not let touch the
> > permissions after initialization, and does provide guarantees to e.g.
> > LSM's about the enclave.
> 
> I disagree. There are scenarios where it is indeed good practice to
> modify the permissions after initialization.  For example, pages that
> may be used for relocatable code can start with RWX permissions but once
> the pages have been populated with the code they should be able to restrict
> permissions to RX only. It is not good practice to require RWX permission
> over their entire lifetime. Ideally pages should only have the lowest
> permissions possible. 

The only permissions kernel has real control is PTE permissions when
the enclave has been initialized.

You are introducing an artificial limitation with vm_run_prot_bits
that makes e.g. EMODPE more costly for no good reason, and in-kernel
variable has nothing to do with the permissions. They are located
in EPCM.

> Supporting the modification of permissions after initialization enables
> the security conscious enclave owner to support the security
> principle of least privilege.

1. Kernel has the control of PTE permissions.
2. Enclave has the control of EPCM permissions.

vm_run_prot_bits does not help making anything more secure.

BR, Jarkko