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

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Message-ID: <YeK2hFcy72tYL61S@iki.fi>
Date:   Sat, 15 Jan 2022 13:56:52 +0200
From:   Jarkko Sakkinen <jarkko@...nel.org>
To:     Reinette Chatre <reinette.chatre@...el.com>
Cc:     Nathaniel McCallum <nathaniel@...fian.com>,
        Haitao Huang <haitao.huang@...ux.intel.com>,
        Andy Lutomirski <luto@...nel.org>, dave.hansen@...ux.intel.com,
        tglx@...utronix.de, bp@...en8.de, mingo@...hat.com,
        linux-sgx@...r.kernel.org, x86@...nel.org, seanjc@...gle.com,
        kai.huang@...el.com, cathy.zhang@...el.com, cedric.xing@...el.com,
        haitao.huang@...el.com, mark.shanahan@...el.com, hpa@...or.com,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 05/25] x86/sgx: Introduce runtime protection bits

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.

/Jarkko