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

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Message-ID: <YdgvFTIRboHwTgRT@iki.fi>
Date:   Fri, 7 Jan 2022 14:16:21 +0200
From:   Jarkko Sakkinen <jarkko@...nel.org>
To:     Reinette Chatre <reinette.chatre@...el.com>
Cc:     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 Thu, Jan 06, 2022 at 09:46:06AM -0800, Reinette Chatre wrote:
> Hi Jarkko,
> 
> On 12/28/2021 6:52 AM, Jarkko Sakkinen wrote:
> > On Mon, Dec 13, 2021 at 02:10:17PM -0800, Reinette Chatre wrote:
> >> Hi Jarkko,
> >>
> >> On 12/10/2021 11:42 PM, Jarkko Sakkinen wrote:
> >>> On Mon, 2021-12-06 at 13:20 -0800, Reinette Chatre wrote:
> >>>>> This is a valid question. Since EMODPE exists why not just make things for
> >>>>> EMODPE, and ignore EMODPR altogether?
> >>>>>
> >>>>
> >>>> I believe that we should support the best practice of principle of least
> >>>> privilege - once a page no longer needs a particular permission there
> >>>> should be a way to remove it (the unneeded permission).
> >>>
> >>> What if EMODPR was not used at all, since EMODPE is there anyway?
> >>
> >> EMODPR and EMODPE are not equivalent.
> >>
> >> EMODPE can only be used to "extend"/relax permissions while EMODPR can only
> >> be used to restrict permissions.
> >>
> >> Notice in the EMODPE instruction reference of the SDM:
> >>
> >> (* Update EPCM permissions *)
> >> EPCM(DS:RCX).R := EPCM(DS:RCX).R | SCRATCH_SECINFO.FLAGS.R;
> >> EPCM(DS:RCX).W := EPCM(DS:RCX).W | SCRATCH_SECINFO.FLAGS.W;
> >> EPCM(DS:RCX).X := EPCM(DS:RCX).X | SCRATCH_SECINFO.FLAGS.X;
> >>
> >> So, when using EMODPE it is only possible to add permissions, not remove
> >> permissions.
> >>
> >> If a user wants to remove permissions from an EPCM page it is only possible
> >> when using EMODPR. Notice in its instruction reference found in the SDM how
> >> it in turn can only be used to restrict permissions:
> >>
> >> (* Update EPCM permissions *)
> >> EPCM(DS:RCX).R := EPCM(DS:RCX).R & SCRATCH_SECINFO.FLAGS.R;
> >> EPCM(DS:RCX).W := EPCM(DS:RCX).W & SCRATCH_SECINFO.FLAGS.W;
> >> EPCM(DS:RCX).X := EPCM(DS:RCX).X & SCRATCH_SECINFO.FLAGS.X;
> > 
> > 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?

/Jarkko