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

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Message-ID: <YeIgzLzPu0U8s0sh@iki.fi>
Date:   Sat, 15 Jan 2022 03:18:04 +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 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.

/Jarkko