Re: SGX vs LSM (Re: [PATCH v20 00/28] Intel SGX1 support)

[Sean Christopherson]

On Thu, May 30, 2019 at 02:23:07PM -0700, Andy Lutomirski wrote:
> On Thu, May 30, 2019 at 2:16 PM Sean Christopherson
> <sean.j.christopherson@xxxxxxxxx> wrote:
> >
> > On Thu, May 30, 2019 at 12:20:45PM -0700, Andy Lutomirski wrote:
> > > On Thu, May 30, 2019 at 11:01 AM Sean Christopherson
> > > <sean.j.christopherson@xxxxxxxxx> wrote:
> > > >
> > > > On Thu, May 30, 2019 at 09:14:10AM -0700, Andy Lutomirski wrote:
> > > > > Enclave file -- that is, the file backing the vma from which the data is loaded.
> > > >
> > > > It wasn't explicitly called out in Andy's proposal(s), but the idea is
> > > > that the SGX driver would effectively inherit permissions from the source
> > > > VMA (EADD needs a source for the initial value of the encave page).
> > >
> > > I actually meant for it to *not* work like this.  I don't want the
> > > source VMA to have to be VM_EXEC.  I think the LSM should just check
> > > permissions on ->vm_file.
> >
> > But if ->vm_file is NULL, i.e. the enclave is not backed by a file,
> > then PROCESS__EXECMEM is required (or more likely, ENCLAVE__EXECMEM).
> >
> 
> If ->vm_file is NULL, then I think some privilege is needed.  I
> suppose the policy could have a new lesser permission EXECUNTRUSTED
> which is like EXECMOD but you can't modify it.  I'm not convinced this
> is particular important.

Assuming MRENCLAVE generated by Graphene or any other hosting scheme are
stable[1], then avoiding EXEC<whatever> means the user can effectively
whitelist what enclaves are runnable by Graphene, even if the kernel
doesn't implement security_enclave_create/init().

I agree that it probably isn't all that important, it's more of a "why
not" argument, i.e. what is gained by not using sigstruct as a proxy?

[1] What in the world is being attested if MRENCLAVE isn't stable?