Re: [RFC PATCH v1 2/3] LSM/x86/sgx: Implement SGX specific hooks in SELinux

[Andy Lutomirski <luto@kernel.org>]

On Mon, Jun 17, 2019 at 9:49 AM Sean Christopherson
<sean.j.christopherson@intel.com> wrote:
>
> On Sun, Jun 16, 2019 at 03:14:51PM -0700, Andy Lutomirski wrote:
> > On Fri, Jun 14, 2019 at 8:38 AM Sean Christopherson
> > <sean.j.christopherson@intel.com> wrote:
> > > > Andy and/or Cedric, can you please weigh in with a concrete (and practical)
> > > > use case that will break if we go with #1?  The auditing issues for #2/#3
> > > > are complex to say the least...
> >
> > The most significant issue I see is the following.  Consider two
> > cases. First, an SGX2 enclave that dynamically allocates memory but
> > doesn't execute code from dynamic memory.  Second, an SGX2 enclave
> > that *does* execute code from dynamic memory.  In #1, the untrusted
> > stack needs to decide whether to ALLOW_EXEC when the memory is
> > allocated, which means that it either needs to assume the worst or it
> > needs to know at allocation time whether the enclave ever intends to
> > change the permission to X.
>
> I'm just not convinced that folks running enclaves that can't communicate
> their basic functionality will care one whit about SELinux restrictions,
> i.e. will happily give EXECMOD even if it's not strictly necessary.

At least when permissions are learned, if there's no ALLOW_EXEC for
EAUG, then EXECMOD won't get learned if there's no eventual attempt to
execute the memory.

>
> > I suppose there's a middle ground.  The driver could use model #1 for
> > driver-filled pages and model #2 for dynamic pages.  I haven't tried
> > to fully work it out, but I think there would be the ALLOW_READ /
> > ALLOW_WRITE / ALLOW_EXEC flag for EADD-ed pages but, for EAUG-ed
> > pages, there would be a different policy.  This might be as simple as
> > internally having four flags instead of three:
> >
> > ALLOW_READ, ALLOW_WRITE, ALLOW_EXEC: as before
> >
> > ALLOW_EXEC_COND: set implicitly by the driver for EAUG.
> >
> > As in #1, if you try to mmap or protect a page with neither ALLOW_EXEC
> > variant, it fails (-EACCES, perhaps).  But, if you try to mmap or
> > mprotect an ALLOW_EXEC_COND page with PROT_EXEC, you ask LSM for
> > permission.  There is no fancy DIRTY tracking here, since it's
> > reasonable to just act as though *every* ALLOW_EXEC_COND page is
> > dirty.  There is no real auditing issue here, since LSM can just log
> > what permission is missing.
> >
> > Does this seem sensible?  It might give us the best of #1 and #2.
>
> It would work and is easy to implement *if* SELinux ties permissions to
> the process, as the SIGSTRUCT vma/file won't be available at
> EAUG+mprotect().  I already have a set of patches to that effect, I'll
> send 'em out in a bit.

I'm okay with that.

>
> FWIW, we still need to differentiate W->X from WX on SGX1, i.e. declaring
> ALLOW_WRITE + ALLOW_EXEC shouldn't imply WX.  This is also addressed in
> the forthcoming updated RFC.

Sounds good.