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

Jarkko Sakkinen
2022-03-04T01:33:14Z

On Fri, Mar 04, 2022 at 03:25:38AM +0200, Jarkko Sakkinen wrote:
> On Fri, Mar 04, 2022 at 03:16:35AM +0200, 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.
> >
> > For EAUG'd pages it should be sufficient to let mmap(), mprotect() and
> > SGX opcodes to control the permissions. Thus effectively disable
> > pre-initialization policy by setting vm_max_prot_bit and
> > vm_run_prot_bits to RWX.
> >
> > Cc: Reinette Chatre <reinette.chatre@intel.com>
> > Cc: Nathaniel McCallum <nathaniel@profian.com>
> > Signed-off-by: Jarkko Sakkinen <jarkko@kernel.org>
> > ---
> > arch/x86/kernel/cpu/sgx/encl.c | 8 +-------
> > 1 file changed, 1 insertion(+), 7 deletions(-)
> >
> > diff --git a/arch/x86/kernel/cpu/sgx/encl.c b/arch/x86/kernel/cpu/sgx/encl.c
> > index 5fe7189eac9d..17feb6fa5578 100644
> > --- a/arch/x86/kernel/cpu/sgx/encl.c
> > +++ b/arch/x86/kernel/cpu/sgx/encl.c
> > @@ -200,13 +200,7 @@ static vm_fault_t sgx_encl_eaug_page(struct vm_area_struct *vma,
> > encl_page->desc = addr;
> > encl_page->encl = encl;
> >
> > - /*
> > - * Adding a regular page that is architecturally allowed to only
> > - * be created with RW permissions.
> > - * TBD: Interface with user space policy to support max permissions
> > - * of RWX.
> > - */
> > - prot = PROT_READ | PROT_WRITE;
> > + prot = PROT_READ | PROT_WRITE | PROT_EXEC;
> > encl_page->vm_run_prot_bits = calc_vm_prot_bits(prot, 0);
> > encl_page->vm_max_prot_bits = encl_page->vm_run_prot_bits;
> >
> > --
> > 2.35.1
> >
>
> This does not break any existing ABI and at least makes the current
> patch set usable.

Also it would be a sane limitation to deny EMODPR and EMODT completely for
EADD'd pages.

Then, you can discard vm_run_prot_bits. It's not needed for anything
anymore.

This should make implementation considerably less obfuscated.

BR, Jarkko