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

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Date:   Tue, 28 Dec 2021 16:52:04 +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 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?

/Jarkko