Re: [PATCH 05/25] x86/sgx: Introduce runtime protection bits

[Jarkko Sakkinen]

On Mon, Jan 17, 2022 at 08:13:32AM -0500, Nathaniel McCallum wrote:
> On Sat, Jan 15, 2022 at 6:57 AM Jarkko Sakkinen <jarkko@xxxxxxxxxx> wrote:
> >
> > On Sat, Jan 15, 2022 at 03:18:04AM +0200, Jarkko Sakkinen wrote:
> > > On Fri, Jan 14, 2022 at 04:41:59PM -0800, Reinette Chatre wrote:
> > > > Hi Jarkko,
> > > >
> > > > On 1/14/2022 4:27 PM, Jarkko Sakkinen wrote:
> > > > > On Fri, Jan 14, 2022 at 04:01:33PM -0800, Reinette Chatre wrote:
> > > > >> Hi Jarkko,
> > > > >>
> > > > >> On 1/14/2022 3:15 PM, Jarkko Sakkinen wrote:
> > > > >>> On Fri, Jan 14, 2022 at 03:05:21PM -0800, Reinette Chatre wrote:
> > > > >>>> Hi Jarkko,
> > > > >>>
> > > > >>> How enclave can check a page range that EPCM has the expected permissions?
> > > > >>
> > > > >> Only way to change EPCM permissions from outside enclave is to run ENCLS[EMODPR]
> > > > >> that needs to be accepted from within the enclave via ENCLU[EACCEPT]. At that
> > > > >> time the enclave provides the expected permissions and that will fail
> > > > >> if there is a mismatch with the EPCM permissions (SGX_PAGE_ATTRIBUTES_MISMATCH).
> > > > >
> > > > > This is a very valid point but that does make the introspection possible
> > > > > only at the time of EACCEPT.
> > > > >
> > > > > It does not give tools for enclave to make sure that EMODPR-ETRACK dance
> > > > > was ever exercised.
> > > >
> > > > Could you please elaborate? EACCEPT is available to the enclave as a tool
> > > > and it would fail if ETRACK was not completed (error SGX_NOT_TRACKED).
> > > >
> > > > Here is the relevant snippet from the SDM from the section where it
> > > > describes EACCEPT:
> > > >
> > > > IF (Tracking not correct)
> > > >     THEN
> > > >         RFLAGS.ZF := 1;
> > > >         RAX := SGX_NOT_TRACKED;
> > > >         GOTO DONE;
> > > > FI;
> > > >
> > > > Reinette
> > >
> > > Yes, if enclave calls EACCEPT it does the necessary introspection and makes
> > > sure that ETRACK is completed. I have trouble understanding how enclave
> > > makes sure that EACCEPT was called.
> >
> > I'm not concerned of anything going wrong once EMODPR has been started.
> >
> > The problem nails down to that the whole EMODPR process is spawned by
> > the entity that is not trusted so maybe that should further broke down
> > to three roles:
> >
> > 1. Build process B
> > 2. Runner process R.
> > 3. Enclave E.
> >
> > And to the costraint that we trust B *more* than R. Once B has done all the
> > needed EMODPR calls it would send the file descriptor to R. Even if R would
> > have full access to /dev/sgx_enclave, it would not matter, since B has done
> > EMODPR-EACCEPT dance with E.
> >
> > So what you can achieve with EMODPR is not protection against mistrusted
> > *OS*. There's absolutely no chance you could use it for that purpose
> > because mistrusted OS controls the whole process.
> >
> > EMODPR is to help to protect enclave against mistrusted *process*, i.e.
> > in the above scenario R.
> 
> There are two general cases that I can see. Both are valid.
> 
> 1. The OS moves from a trusted to an untrusted state. This could be
> the multi-process system you've described. But it could also be that
> the kernel becomes compromised after the enclave is fully initialized.
> 
> 2. The OS is untrustworthy from the start.
> 
> The second case is the stronger one and if you can solve it, the first
> one is solved implicitly. And our end goal is that if the OS does
> anything malicious we will crash in a controlled way.
> 
> A defensive enclave will always want to have the least number of
> privileges for the maximum protection. Therefore, the enclave will
> want the OS to call EMODPR. If that were it, the host could just lie.
> But the enclave also verifies that the EMODPR operation was, in fact,
> executed by doing EACCEPT. When the enclave calls EACCEPT, if the
> kernel hasn't restricted permissions then we get a controlled crash.
> Therefore, we have solved the second case.

So you're referring to this part of the SDM pseude code in the SDM:

(* Check the destination EPC page for concurrency *)
IF ( EPC page in use )
    THEN #GP(0); FI;

I wonder does "EPC page in use" unconditionally trigger when EACCEPT
is invoked for a page for which all of these conditions hold:

- .PR := 0 (no EMODPR in progress)
- .MODIFIED := 0 (no EMODT in progress)
- .PENDING := 0 (no EMODPR in progress)

I don't know the exact scope and scale of "EPC page in use".

Then, yes, EACCEPT could be at least used to validate that one of the
three operations above was requested. However, enclave thread cannot say
which one was it, so it is guesswork.

I guess that still is enough to quantitatively argue that EMODPR is an
operation that does improve the confidentiality properities of an
enclave, even if it has it flaws.

/Jarkko