linux-kernel - Re: [PATCH V2 16/32] x86/sgx: Support restricting of enclave page permissions

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Message-ID: <YkRxA0g4O29Sv14Y@iki.fi>
Date:   Wed, 30 Mar 2022 18:02:27 +0300
From:   Jarkko Sakkinen <jarkko@...nel.org>
To:     Reinette Chatre <reinette.chatre@...el.com>
Cc:     Haitao Huang <haitao.huang@...ux.intel.com>,
        "Dhanraj, Vijay" <vijay.dhanraj@...el.com>,
        "dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "bp@...en8.de" <bp@...en8.de>,
        "Lutomirski, Andy" <luto@...nel.org>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "linux-sgx@...r.kernel.org" <linux-sgx@...r.kernel.org>,
        "x86@...nel.org" <x86@...nel.org>,
        "Christopherson,, Sean" <seanjc@...gle.com>,
        "Huang, Kai" <kai.huang@...el.com>,
        "Zhang, Cathy" <cathy.zhang@...el.com>,
        "Xing, Cedric" <cedric.xing@...el.com>,
        "Huang, Haitao" <haitao.huang@...el.com>,
        "Shanahan, Mark" <mark.shanahan@...el.com>,
        "hpa@...or.com" <hpa@...or.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH V2 16/32] x86/sgx: Support restricting of enclave page
 permissions

On Wed, Mar 30, 2022 at 06:00:30PM +0300, Jarkko Sakkinen wrote:
> On Mon, Mar 28, 2022 at 04:22:35PM -0700, Reinette Chatre wrote:
> > Hi Jarkko,
> > 
> > On 3/19/2022 5:24 PM, Jarkko Sakkinen wrote:
> > > On Thu, Mar 17, 2022 at 05:11:40PM -0700, Reinette Chatre wrote:
> > >> Hi Jarkko,
> > >>
> > >> On 3/17/2022 3:51 PM, Jarkko Sakkinen wrote:
> > >>> On Thu, Mar 17, 2022 at 03:08:04PM -0700, Reinette Chatre wrote:
> > >>>> Hi Jarkko,
> > >>>>
> > >>>> On 3/16/2022 9:30 PM, Jarkko Sakkinen wrote:
> > >>>>> On Mon, Mar 14, 2022 at 08:32:28AM -0700, Reinette Chatre wrote:
> > >>>>>> Hi Jarkko,
> > >>>>>>
> > >>>>>> On 3/13/2022 8:42 PM, Jarkko Sakkinen wrote:
> > >>>>>>> On Fri, Mar 11, 2022 at 11:28:27AM -0800, Reinette Chatre wrote:
> > >>>>>>>> Supporting permission restriction in an ioctl() enables the runtime to manage
> > >>>>>>>> the enclave memory without needing to map it.
> > >>>>>>>
> > >>>>>>> Which is opposite what you do in EAUG. You can also augment pages without
> > >>>>>>> needing the map them. Sure you get that capability, but it is quite useless
> > >>>>>>> in practice.
> > >>>>>>>
> > >>>>>>>> I have considered the idea of supporting the permission restriction with
> > >>>>>>>> mprotect() but as you can see in this response I did not find it to be
> > >>>>>>>> practical.
> > >>>>>>>
> > >>>>>>> Where is it practical? What is your application? How is it practical to
> > >>>>>>> delegate the concurrency management of a split mprotect() to user space?
> > >>>>>>> How do we get rid off a useless up-call to the host?
> > >>>>>>>
> > >>>>>>
> > >>>>>> The email you responded to contained many obstacles against using mprotect()
> > >>>>>> but you chose to ignore them and snipped them all from your response. Could
> > >>>>>> you please address the issues instead of dismissing them? 
> > >>>>>
> > >>>>> I did read the whole email but did not see anything that would make a case
> > >>>>> for fully exposed EMODPR, or having asymmetrical towards how EAUG works.
> > >>>>
> > >>>> I believe that on its own each obstacle I shared with you is significant enough
> > >>>> to not follow that approach. You simply respond that I am just not making a
> > >>>> case without acknowledging any obstacle or providing a reason why the obstacles
> > >>>> are not valid.
> > >>>>
> > >>>> To help me understand your view, could you please respond to each of the
> > >>>> obstacles I list below and how it is not an issue?
> > >>>>
> > >>>>
> > >>>> 1) ABI change:
> > >>>>    mprotect() is currently supported to modify VMA permissions
> > >>>>    irrespective of EPCM permissions. Supporting EPCM permission
> > >>>>    changes with mprotect() would change this behavior.
> > >>>>    For example, currently it is possible to have RW enclave
> > >>>>    memory and support multiple tasks accessing the memory. Two
> > >>>>    tasks can map the memory RW and later one can run mprotect()
> > >>>>    to reduce the VMA permissions to read-only without impacting
> > >>>>    the access of the other task.
> > >>>>    By moving EPCM permission changes to mprotect() this usage
> > >>>>    will no longer be supported and current behavior will change.
> > >>>
> > >>> Your concurrency scenario is somewhat artificial. Obviously you need to
> > >>> synchronize somehow, and breaking something that could be done with one
> > >>> system call into two separates is not going to help with that. On the
> > >>> contrary, it will add a yet one more difficulty layer.
> > >>
> > >> This is about supporting multiple threads in a single enclave, they can
> > >> all have their own memory mappings based on the needs. This is currently
> > >> supported in mainline as part of SGX1.
> > 
> > 
> > Could you please comment on the above?
> 
> 
> I've probably spent probably over two weeks of my life addressing concerns
> to the point that I feel as I was implementing this feature (that could be
> faster way to get it done).
> 
> So I'll just wait the next version and see how it is like and give my
> feedback based on that. It's not really my problem to address every
> possible concern.

Once v3 is out, I'll check what I think is right, and what is wrong
and might send some fixups and see where that leads to. I think it
is more costructive way to move forward. Repeating same arguments
leads to nowhere.

BR, Jarkko