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 01:42:13PM -0700, Xing, Cedric wrote:
> > From: linux-sgx-owner@vger.kernel.org [mailto:linux-sgx-
> > owner@vger.kernel.org] On Behalf Of Sean Christopherson
> > Sent: Friday, May 24, 2019 12:14 PM
> > 
> > My point is that enclaves have different properties than shared objects.
> > 
> > Normal LSM behavior with regard to executing files is to label files
> > with e.g. FILE__EXECUTE.  Because an enclave must be built to the exact
> > specifications of .sigstruct, requring FILE__EXECUTE on the .sigstruct
> > is effectively the same as requiring FILE__EXECUTE on the enclave itself.
> > 
> > Addressing your scenario of loading an executable page in EPC, doing so
> > would require one of the following:
> > 
> >   - Ability to install a .sigstruct with FILE__EXECUTE
> > 
> >   - PROCESS__EXECMEM
> > 
> >   - FILE__EXECMOD and SGX2 support
> 
> Now I got your point. It sounds a great idea to me!
> 
> But instead of using .sigstruct file, I'd still recommend using file mapping
> (i.e. SIGSTRUCT needs to reside in executable memory). But then there'll be a

Why?  Even in the Graphene case the final .sigstruct can be known ahead of
time.  Userspace can always use memfd() if it's generating SIGSTRUCT on
the fly.

> hole - a process having FILE__EXECMOD on any file could use that file as a
> SIGSTRUCT. Probably we'll need a new type in SELinux to label
> enclave/sigstruct files.
>
> > I don't see a fundamental difference between having RWX in an enclave
> > and RWX in normal memory, either way the process can execute arbitrary
> > code, i.e. PROCESS__EXECMEM is appropriate.  Yes, an enclave will #UD on
> > certain instructions, but that's easily sidestepped by having a
> > trampoline in the host (marked RX) and piping arbitrary code into the
> > enclave.  Or using EEXIT to do a bit of ROP.
> 
> I'm with you.
> 
> With your proposal only FILE__EXECMOD is needed on /dev/sgx/enclave to launch
> Graphene enclaves or the like.

It wouldn't even need FILE__EXECMOD, assuming Graphene does all of its
libc rewriting before building the enclave, i.e. doesn't EADD RWX pages.

> > > > > No changes are required to LSMs, SGX1 has a single LSM touchpoint
> > > > > in
> > > > its
> > > > > mmap(), and I *think* the only required userspace change is to
> > > > > mmap() PROT_NONE when allocating the enclave's virtual address
> > range.
> > >
> > > I'm not sure I understand the motivation behind this proposal to
> > > decouple initial EPC permissions from source pages.
> > 
> > Pulling permissions from source pages means userspace needs to fully map
> > the in normal memory, including marking pages executable.  That exposes
> > the loader to having executable pages in its address space that it has
> > no intention of executing (outside of the enclave).  And for Graphene,
> > it means having to actively avoid PROCESS__EXECMEM, e.g. by using a
> > dummy backing file to build the enclave instead of anon memory.
> 
> Agreed.
> 
> > 
> > > I don't think it a big deal to fully mmap() enclave files, which have
> > > to be parsed by user mode anyway to determine various things including
> > > but not limited to the size of heap(s), size and number of
> > > TCSs/stacks/TLS areas, and the overall enclave size. So with PHDRs
> > > parsed, it's trivial to mmap() each segment with permissions from its
> > PHDR.
> > >
> > > > > As for Graphene, it doesn't need extra permissions to run its
> > > > > enclaves, it just needs a way to install .sigstruct, which is a
> > > > > generic permissions problem and not SGX specific.
> > > > >
> > > > >
> > > > > For SGX2 maybe:
> > > > >
> > > > >   - No additional requirements to map an EAUG'd page as RW page.
> > Not
> > > > >     aligned with standard MAP_SHARED behavior, but we really don't
> > want
> > > > >     to require FILE__WRITE, and thus allow writes to .sigstruct.
> > > > >
> > > > >   - Require FILE__EXECMOD on the .sigstruct to map previously
> > writable
> > > > >     page as executable (which indirectly includes all EAUG'd
> > pages).
> > > > >     Wiring this up will be a little funky, but we again we don't
> > want
> > > > >     to require FILE__WRITE on .sigstruct.
> > > > >
> > >
> > > I'm lost. Why is EAUG tied to permissions on .sigstruct?
> > 
> > Because for the purposes of LSM checks, .sigstruct is the enclave's
> > backing file, and mapping a previously writable enclave page as
> > exectuable is roughly equivalent to mapping a CoW'd page as exectuable.
> 
> I think I've got your idea. You are trying to use permissions on .sigstruct
> to determine whether EAUG will be available to that specific enclave. Am I
> right?

Yep.

> I'd tie EAUG to the permissions of /dev/sgx/enclave instead. But why? There
> are couple of reasons. For one, a SIGSTRUCT identifies the behavior of the
> enclave, hence the SGX features needed by that enclave. So if an enclave
> requires EAUG, the .sigstruct has to allow EAUG or the enclave wouldn't work.
> That means the system admin wouldn't have a choice but to match up what's
> needed by the enclave. For two, whether to allow, say loading code
> dynamically into an enclave, depends on whether the host process can tolerate
> the inherent risk. And that decision is seldom made on individual enclaves
> but to the host process as a whole. And /dev/sgx/enclave serves that purpose.

I think I'd be ok either way?  What I really care about is having line of
sight to a sane way to support for SGX2, and both seem sane.  I.e. we can
hash this detail out when SGX2 goes in.