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

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Date:   Sat, 8 Jan 2022 17:51:43 +0200
From:   Jarkko Sakkinen <jarkko@...nel.org>
To:     Haitao Huang <haitao.huang@...ux.intel.com>
Cc:     Reinette Chatre <reinette.chatre@...el.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 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.

/Jarkko