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

[Sean Christopherson <sean.j.christopherson@intel.com>]

On Fri, May 24, 2019 at 10:54:34AM -0700, Andy Lutomirski wrote:
> 
> > On May 24, 2019, at 10:42 AM, Sean Christopherson <sean.j.christopherson@intel.com> wrote:
> > 
> > Hmm, I've been thinking more about pulling permissions from the source
> > page.  Conceptually I'm not sure we need to meet the same requirements as
> > non-enclave DSOs while the enclave is being built, i.e. do we really need
> > to force userspace to fully map the enclave in normal memory?
> > 
> > Consider the Graphene scenario where it's building an enclave on the fly.
> > Pulling permissions from the source VMAs means Graphene has to map the
> > code pages of the enclave with X.  This means Graphene will need EXEDMOD
> > (or EXECMEM if Graphene isn't careful).  In a non-SGX scenario this makes
> > perfect sense since there is no way to verify the end result of RW->RX.
> > 
> > But for SGX, assuming enclaves are whitelisted by their sigstruct (checked
> > at EINIT) and because page permissions affect sigstruct.MRENCLAVE, it *is*
> > possible to verify the resulting RX contents.  E.g. for the purposes of
> > LSMs, can't we use the .sigstruct file as a proxy for the enclave and
> > require FILE__EXECUTE on the .sigstruct inode to map/run the enclave?
> 
> I think it’s sound for some but not all use cases. I would imagine that a lot
> of users won’t restrict sigstruct at all — the “use this as a sigstruct”
> permission will be granted to everything and maybe even to memfd. But even
> users like that might want to force their enclaves to be hardened such that
> writable pages are never executable, in which case Graphene may need an
> exception to run.

Heh, I belatedly had the same thought.  See my follow-up about EXECMEM.

> But maybe I’m nuts.