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

[Sean Christopherson]

On Fri, Jun 14, 2019 at 10:16:55AM -0700, Xing, Cedric wrote:
> > From: Christopherson, Sean J
> > Sent: Thursday, June 13, 2019 5:46 PM
> > 
> > On Thu, Jun 13, 2019 at 01:02:17PM -0400, Stephen Smalley wrote:
> > > On 6/11/19 6:02 PM, Sean Christopherson wrote:
> > > >My RFC series[1] implements #1.  My understanding is that Andy
> > > >(Lutomirski) prefers #2.  Cedric's RFC series implements #3.
> > > >
> > > >Perhaps the easiest way to make forward progress is to rule out the
> > > >options we absolutely *don't* want by focusing on the potentially
> > > >blocking issue with each option:
> > > >
> > > >   #1 - SGX UAPI funkiness
> > > >
> > > >   #2 - Auditing complexity, potential enclave lock contention
> > > >
> > > >   #3 - Pushing SGX details into LSMs and complexity of kernel
> > > > implementation
> > > >
> > > >
> > > >[1]
> > > >https://lkml.kernel.org/r/20190606021145.12604-1-sean.j.christopherso
> > > >n@xxxxxxxxx
> > >
> > > Given the complexity tradeoff, what is the clear motivating example
> > > for why
> > > #1 isn't the obvious choice? That the enclave loader has no way of
> > > knowing a priori whether the enclave will require W->X or WX?  But
> > > aren't we better off requiring enclaves to be explicitly marked as
> > > needing such so that we can make a more informed decision about
> > > whether to load them in the first place?
> > 
> > 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...
> 
> How does enclave loader provide per-page ALLOW_* flags?

Unchanged from my RFC, i.e. specified at SGX_IOC_ENCLAVE_ADD_PAGE(S).

> And a related question is why they are necessary for enclaves but
> unnecessary for regular executables or shared objects.

Because at mmap()/mprotect() time we don't have the source file of the
enclave page to check SELinux's FILE__EXECUTE or AppArmor's AA_EXEC_MMAP.

> What's the story for SGX2 if mmap()'ing non-existing pages is not allowed?

Userspace will need to invoke an ioctl() to tell SGX "this range can be
EAUG'd".

> 
> What's the story for auditing?

It happens naturally when security_enclave_load() is called.  Am I
missing something?

> After everything above has been taken care of properly, will #1 still be
> simpler than #2/#3?

The state tracking of #2/#3 doesn't scare me, it's purely the auditing.
Holding an audit message for an indeterminate amount of time is a
nightmare.

Here's a thought.  What if we simply require FILE__EXECUTE or AA_EXEC_MAP
to load any enclave page from a file?  Alternatively, we could add an SGX
specific file policity, e.g. FILE__ENCLAVELOAD and AA_MAY_LOAD_ENCLAVE.
As in my other email, SELinux's W^X restrictions can be tied to the process,
i.e. they can be checked at mmap()/mprotect() without throwing a wrench in
auditing.