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Subject: Re: RFC: userspace exception fixups
To:     Andy Lutomirski <luto@amacapital.net>,
        "Christopherson, Sean J" <sean.j.christopherson@intel.com>
Cc:     Andrew Lutomirski <luto@kernel.org>, Jann Horn <jannh@google.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Rich Felker <dalias@libc.org>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Jethro Beekman <jethro@fortanix.com>,
        Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>,
        Florian Weimer <fweimer@redhat.com>,
        Linux API <linux-api@vger.kernel.org>, X86 ML <x86@kernel.org>,
        linux-arch <linux-arch@vger.kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Peter Zijlstra <peterz@infradead.org>, nhorman@redhat.com,
        npmccallum@redhat.com, "Ayoun, Serge" <serge.ayoun@intel.com>,
        shay.katz-zamir@intel.com, linux-sgx@vger.kernel.org,
        Andy Shevchenko <andriy.shevchenko@linux.intel.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        Carlos O'Donell <carlos@redhat.com>,
        adhemerval.zanella@linaro.org
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From:   Dave Hansen <dave.hansen@intel.com>
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Message-ID: <7027c3dc-addb-1b96-027e-a57fccf1f812@intel.com>
Date:   Thu, 8 Nov 2018 12:10:30 -0800
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On 11/8/18 12:05 PM, Andy Lutomirski wrote:
> Hmm.  The idea being that the SDK preserves RBP but not RSP.  That's
> not the most terrible thing in the world.  But could the SDK live with
> something more like my suggestion where the vDSO supplies a normal
> function that takes a struct containing registers that are visible to
> the enclave?  This would make it extremely awkward for the enclave to
> use the untrusted stack per se, but it would make it quite easy (I
> think) for the untrusted part of the SDK to allocate some extra memory
> and just tell the enclave that *that* memory is the stack.

I really think the enclave should keep its grubby mitts off the
untrusted stack.  There are lots of ways to get memory, even with
stack-like semantics, that don't involve mucking with the stack itself.

I have not heard a good, hard argument for why there is an absolute
*need* to store things on the actual untrusted stack.

We could quite easily have the untrusted code just promise to allocate a
stack-sized virtual area (even derived from the stack rlimit size) and
pass that into the enclave for parameter use.