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

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Message-ID: <Yd9pMq4lUy56B+50@iki.fi>
Date:   Thu, 13 Jan 2022 01:50:10 +0200
From:   Jarkko Sakkinen <jarkko@...nel.org>
To:     Reinette Chatre <reinette.chatre@...el.com>
Cc:     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 Tue, Jan 11, 2022 at 09:13:27AM -0800, Reinette Chatre wrote:
> Hi Jarkko,
> 
> On 1/10/2022 5:53 PM, Jarkko Sakkinen wrote:
> > On Mon, Jan 10, 2022 at 04:05:21PM -0600, Haitao Huang wrote:
> >> On Sat, 08 Jan 2022 10:22:30 -0600, Jarkko Sakkinen <jarkko@...nel.org>
> >> wrote:
> >>
> >>> On Sat, Jan 08, 2022 at 05:51:46PM +0200, Jarkko Sakkinen wrote:
> >>>> On Sat, Jan 08, 2022 at 05:45:44PM +0200, Jarkko Sakkinen wrote:
> >>>>> On Fri, Jan 07, 2022 at 10:14:29AM -0600, Haitao Huang wrote:
> >>>>>>>>> OK, so the question is: do we need both or would a
> >>>> mechanism just
> >>>>>>>> to extend
> >>>>>>>>> permissions be sufficient?
> >>>>>>>>
> >>>>>>>> I do believe that we need both in order to support pages
> >>>> having only
> >>>>>>>> the permissions required to support their intended use
> >>>> during the
> >>>>>>>> time the
> >>>>>>>> particular access is required. While technically it is
> >>>> possible to grant
> >>>>>>>> pages all permissions they may need during their lifetime it
> >>>> is safer to
> >>>>>>>> remove permissions when no longer required.
> >>>>>>>
> >>>>>>> So if we imagine a run-time: how EMODPR would be useful, and
> >>>> how using it
> >>>>>>> would make things safer?
> >>>>>>>
> >>>>>> In scenarios of JIT compilers, once code is generated into RW pages,
> >>>>>> modifying both PTE and EPCM permissions to RX would be a good
> >>>> defensive
> >>>>>> measure. In that case, EMODPR is useful.
> >>>>>
> >>>>> What is the exact threat we are talking about?
> >>>>
> >>>> To add: it should be *significantly* critical thread, given that not
> >>>> supporting only EAUG would leave us only one complex call pattern with
> >>>> EACCEPT involvement.
> >>>>
> >>>> I'd even go to suggest to leave EMODPR out of the patch set, and
> >>>> introduce
> >>>> it when there is PoC code for any of the existing run-time that
> >>>> demonstrates the demand for it. Right now this way too speculative.
> >>>>
> >>>> Supporting EMODPE is IMHO by factors more critical.
> >>>
> >>> At least it does not protected against enclave code because an enclave
> >>> can
> >>> always choose not to EACCEPT any of the EMODPR requests. I'm not only
> >>> confused here about the actual threat but also the potential adversary
> >>> and
> >>> target.
> >>>
> >> I'm not sure I follow your thoughts here. The sequence should be for enclave
> >> to request  EMODPR in the first place through runtime to kernel, then to
> >> verify with EACCEPT that the OS indeed has done EMODPR.
> >> If enclave does not verify with EACCEPT, then its own code has
> >> vulnerability. But this does not justify OS not providing the mechanism to
> >> request EMODPR.
> > 
> > The question is really simple: what is the threat scenario? In order to use
> > the word "vulnerability", you would need one.
> > 
> > Given the complexity of the whole dance with EMODPR it is mandatory to have
> > one, in order to ack it to the mainline.
> > 
> 
> Which complexity related to EMODPR are you concerned about? In a later message
> you mention "This leaves only EAUG and EMODT requiring the EACCEPT handshake"
> so it seems that you are perhaps concerned about the flow involving EACCEPT?
> The OS does not require nor depend on EACCEPT being called as part of these flows
> so a faulty or misbehaving user space omitting an EACCEPT call would not impact
> these flows in the OS, but would of course impact the enclave.

I'd say *any* complexity because I see no benefit of supporting it. E.g.
EMODPR/EACCEPT/EMODPE sequence I mentioned to Haitao concerns me. How is
EMODPR going to help with any sort of workload?

/Jarkko