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From:   Andy Lutomirski <luto@kernel.org>
Date:   Thu, 7 May 2020 17:40:11 -0700
Message-ID: <CALCETrX9w3anusPhNWkv-hs554N6MF9GfChazrG+Br16oFfCaw@mail.gmail.com>
Subject: Re: [PATCH v29 00/20] Intel SGX foundations
To:     Sean Christopherson <sean.j.christopherson@intel.com>
Cc:     "Dr. Greg" <greg@enjellic.com>,
        Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        LKML <linux-kernel@vger.kernel.org>, X86 ML <x86@kernel.org>,
        linux-sgx@vger.kernel.org,
        Andrew Morton <akpm@linux-foundation.org>,
        Dave Hansen <dave.hansen@intel.com>,
        Neil Horman <nhorman@redhat.com>, npmccallum@redhat.com,
        "Huang, Haitao" <haitao.huang@intel.com>,
        Andy Shevchenko <andriy.shevchenko@linux.intel.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        "Svahn, Kai" <kai.svahn@intel.com>, Borislav Petkov <bp@alien8.de>,
        Josh Triplett <josh@joshtriplett.org>,
        Andrew Lutomirski <luto@kernel.org>,
        "Huang, Kai" <kai.huang@intel.com>,
        David Rientjes <rientjes@google.com>,
        "Xing, Cedric" <cedric.xing@intel.com>, puiterwijk@redhat.com
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On Wed, Apr 29, 2020 at 8:30 AM Sean Christopherson
<sean.j.christopherson@intel.com> wrote:
>
> On Sun, Apr 26, 2020 at 11:57:53AM -0500, Dr. Greg wrote:
> > In closing, it is important to note that the proposed SGX driver is
> > not available as a module.  This effectively excludes any alternative
> > implementations of the driver without replacement of the kernel at
> > large.
>
> No it doesn't.  The SGX subsytem won't allocate EPC pages unless userspace
> creates an enclave, i.e. preventing unprivileged userspace from accessing
> /dev/sgx/enclave will allow loading an alternative out-of-tree SGX module.
> Yes, SGX sanitizes the EPC on boot, but that's arguably a good thing for
> out-of-tree modules.
>
> And if you want to get crafty and squash in-kernel SGX altogether, boot
> with "clearcpuid=<SGX_LC>" and/or "clearcpuid=<SGX>" to disable in-kernel
> support entirely.  SGX won't be correctly enumerated in /proc/cpuinfo
> relative to the existence of an out-of-tree module, but that seems like a
> very minor issue if you're running with a completely different SGX driver.
>
> > It also means that any platform, with SGX hardware support,
> > running a kernel with this driver, has the potential for the
> > security/privacy issues noted above.
>
> Unless I'm mistaken, /dev/sgx is root-only by default.  There are far
> scarier mechanisms available to root for hosing the system.
>
> > If key based policy management is not allowed, then the driver needs
> > to be re-architected to have modular support so that alternative
> > implementations or the absence of any driver support are at least
> > tenable.
>
> As above, using an alternative implementation is teneble, albeit a bit
> kludgy.

It is worth noting that, if someone actualy does this, and a future
kernel patch breaks it, the upstream developers are unlikely to
apologize or even feel particularly bad.  See, for example, the
current situation with VirtualBox.