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Subject: Re: Re: [PATCH v12 00/18] Enable FSGSBASE instructions
To:     Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>,
        Thomas Gleixner <tglx@linutronix.de>
Cc:     Andi Kleen <ak@linux.intel.com>, Sasha Levin <sashal@kernel.org>,
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        hpa@zytor.com, dave.hansen@intel.com, tony.luck@intel.com,
        ravi.v.shankar@intel.com, chang.seok.bae@intel.com
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From:   Don Porter <porter@cs.unc.edu>
Message-ID: <7eb45e02-03bf-0af0-c915-794bf49d66d7@cs.unc.edu>
Date:   Fri, 22 May 2020 16:14:20 -0400
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On 5/19/20 12:48 PM, Jarkko Sakkinen wrote:
> On Tue, May 19, 2020 at 01:03:25AM +0200, Thomas Gleixner wrote:
>> Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com> writes:
>>> On Mon, 2020-05-18 at 08:34 -0700, Andi Kleen wrote:
>>>>> Yes, for SGX this is functional feature because enclave entry points,
>>>>> thread control structures (aka TCS's), reset FSBASE and GSBASE registers
>>>>> to fixed (albeit user defined) values. And syscall's can be done only
>>>>> outside of enclave.
>>>>>
>>>>> This is a required feature for fancier runtimes (such as Graphene).
>>>>
>>>> Can you please explain a bit more? What do they need GS for?
>>>
>>> Apparently, uses only wrfsbase:
>>>
>>> https://raw.githubusercontent.com/oscarlab/graphene/master/Pal/src/host/Linux-SGX/db_misc.c
>>>
>>> I'm not too familiar with the codebase yet but by reading some research
>>> papers in the past the idea is to multiplex one TCS for multiple virtual
>>> threads inside the enclave.
>>>
>>> E.g. TCS could represent a vcpu for a libos type of container and on
>>> entry would pick on a thread and set fsbase accordingly for a thread
>>> control block.
>>
>> That justifies to write books which recommend to load a kernel module
>> which creates a full unpriviledged root hole. I bet none of these papers
>> ever mentioned that.
> 
> Fully agree that oot lkm for this is a worst idea ever.
> 
> That's why I want to help with this.
> 
> /Jarkko
> 

 >

Hi all, and apologies for the resend,

I wanted to clarify that we never intended the Graphene kernel module 
you mention for production use, as well as to comment in support of this 
patch.

Setting the fs register in userspace is an essential feature for running 
legacy code in SGX.  We have been following LKML discussions on this 
instruction for years, and hoping this feature would be supported by 
Linux, so that we can retire this module.  To our knowledge, every SGX 
library OS has a similar module, waiting for this or a similar patch to 
be merged into Linux.  This indicates a growing user base that needs 
this instruction.

Just for some history, Graphene was originally a research 
proof-of-concept that started in my lab, and has since received 
substantial contributions as an open source project from companies 
including Intel.  This code base is explicitly not intended or ready for 
production use at this point, as it is still missing essential features.

We wrote the kernel module as a way to get something working quickly, so 
that we could focus on studying more difficult aspects of porting code 
to SGX.  We had always assumed that the Linux community would eventually 
offer a correct and safe mechanism to enable this instruction, but we 
generally err on the side of publishing code we write for research 
studies as open source in the interest of supporting reproducibility and 
further science.

Nonetheless, Graphene is moving towards adoption in production systems, 
and we are actively working to make the code base secure and robust. 
This issue has been on our to-do list before a production release.  It 
would certainly make our lives easier to deprecate our module and just 
use a robust, in-kernel implementation.

All the best,
Don Porter
Graphene Maintainer
https://grapheneproject.io/