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

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Date:   Fri, 07 Jan 2022 10:14:29 -0600
From:   "Haitao Huang" <haitao.huang@...ux.intel.com>
To:     "Reinette Chatre" <reinette.chatre@...el.com>,
        "Jarkko Sakkinen" <jarkko@...nel.org>
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 Fri, 07 Jan 2022 06:16:21 -0600, Jarkko Sakkinen <jarkko@...nel.org>  
wrote:

> 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?
>
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.

Haitao