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

[Sean Christopherson]

On Thu, May 23, 2019 at 07:17:52AM -0700, Sean Christopherson wrote:
> On Thu, May 23, 2019 at 01:26:28PM +0300, Jarkko Sakkinen wrote:
> > On Wed, May 22, 2019 at 07:35:17PM -0700, Sean Christopherson wrote:
> > > But actually, there's no need to disallow mmap() after ECREATE since the
> > > LSM checks also apply to mmap(), e.g. FILE__EXECUTE would be needed to
> > > mmap() any enclave pages PROT_EXEC.  I guess my past self thought mmap()
> > > bypassed LSM checks?  The real problem is that mmap()'ng an existing
> > > enclave would require FILE__WRITE and FILE__EXECUTE, which puts us back
> > > at square one.
> > 
> > I'm lost with the constraints we want to set.
> 
> As is today, SELinux policies would require enclave loaders to have
> FILE__WRITE and FILE__EXECUTE permissions on /dev/sgx/enclave.  Presumably
> other LSMs have similar requirements.  Requiring all processes to have
> FILE__{WRITE,EXECUTE} permissions means the permissions don't add much
> value, e.g. they can't be used to distinguish between an enclave that is
> being loaded from an unmodified file and an enclave that is being
> generated on the fly, e.g. Graphene.
> 
> Looking back at Andy's mail, he was talking about requiring FILE__EXECUTE
> to run an enclave, so perhaps it's only FILE__WRITE that we're trying to
> special case.

Argh, as I was working through Andy's latest proposal I realized that I
was subconciously making FILE__READ imply FILE__EXECUTE.

The idea behind inheriting permissions from the source VMA is to exempt
"standard" enclaves from needing FILE__WRITE.  But if we don't add an
exemption for FILE__EXECUTE as well, then all enclaves need FILE__EXECUTE,
which means FILE__EXECUTE can't be used to identify the case where
userspace is mapping an inherited PROT_WRITE page as PROT_EXEC.  And if
the SGX magic exempts FILE__EXECUTE, then FILE__READ implies FILE__EXECUTE.

Yuck.