linux-kernel - Re: [PATCH v17 18/23] platform/x86: Intel SGX driver

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Message-Id: <0669C300-02CB-4EA6-BF88-5C4B4DDAD4C7@amacapital.net>
Date:   Sun, 25 Nov 2018 08:22:35 -0800
From:   Andy Lutomirski <luto@...capital.net>
To:     Jarkko Sakkinen <jarkko.sakkinen@...ux.intel.com>
Cc:     Andy Lutomirski <luto@...nel.org>,
        "Dr. Greg Wettstein" <greg@...ellic.com>, X86 ML <x86@...nel.org>,
        Platform Driver <platform-driver-x86@...r.kernel.org>,
        linux-sgx@...r.kernel.org, Dave Hansen <dave.hansen@...el.com>,
        "Christopherson, Sean J" <sean.j.christopherson@...el.com>,
        nhorman@...hat.com, npmccallum@...hat.com,
        "Ayoun, Serge" <serge.ayoun@...el.com>, shay.katz-zamir@...el.com,
        haitao.huang@...ux.intel.com,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        "Svahn, Kai" <kai.svahn@...el.com>, mark.shanahan@...el.com,
        Suresh Siddha <suresh.b.siddha@...el.com>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>,
        Darren Hart <dvhart@...radead.org>,
        Andy Shevchenko <andy@...radead.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v17 18/23] platform/x86: Intel SGX driver


>> On Nov 25, 2018, at 6:53 AM, Jarkko Sakkinen <jarkko.sakkinen@...ux.intel.com> wrote:
>> 
>> On Sat, Nov 24, 2018 at 09:21:14AM -0800, Jarkko Sakkinen wrote:
>> On Thu, Nov 22, 2018 at 07:21:08AM -0800, Andy Lutomirski wrote:
>>>> At a high level, addressing these issues is straight forward.  First,
>>>> the driver needs to support authorization equivalent to that which is
>>>> implemented in the current Intel Launch Enclave, ie. control over the
>>>> SGX_FLAGS_PROVISION_KEY attribute.
>>> 
>>> I agree, hence my email :)
>> 
>> Started to scratch my head that is it really an issue that any enclave
>> can provision in the end?
>> 
>> Direct quote from your first response:
>> 
>> "In particular, the ability to run enclaves with the provisioning bit set
>> is somewhat sensitive, since it effectively allows access to a stable
>> fingerprint of the system."
>> 
>> As can be seen from the key derivation table this does not exactly hold
>> so you should refine your original argument before we can consider any
>> type of change.
>> 
>> I just don't see what it is so wrong for any enclave to be able to tell
>> that it really is an enclave.
> 
> I mean I can understand why Greg wants LE although I don't understand
> what benefit does it bring to anyone to lock in for enclave to allow
> to identify itself.
> 
> What you are proposing does not really bring any additional security if
> we consider a threat model where the kernel is an adversary but it makes
> the software stack more clanky to use.

Agreed. What I’m proposing adds additional security if the kernel is *not* compromised.

There are other ways to accomplish it that might be better in some respects.  For example, there could be /dev/sgx and /dev/sgx_rights/provision.  The former exposes the whole sgx API, except that it doesn’t allow provisioning by default. The latter does nothing by itself. To run a provisioning enclave, you open both nodes, then do something like:

ioctl(sgx, SGX_IOC_ADD_RIGHT, sgx_provisioning);

This requires extra syscalls, but it doesn’t have the combinatorial explosion problem.