linux-kernel - Re: RFC: userspace exception fixups

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Date:   Thu, 8 Nov 2018 12:10:30 -0800
From:   Dave Hansen <dave.hansen@...el.com>
To:     Andy Lutomirski <luto@...capital.net>,
        "Christopherson, Sean J" <sean.j.christopherson@...el.com>
Cc:     Andrew Lutomirski <luto@...nel.org>, Jann Horn <jannh@...gle.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Rich Felker <dalias@...c.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Jethro Beekman <jethro@...tanix.com>,
        Jarkko Sakkinen <jarkko.sakkinen@...ux.intel.com>,
        Florian Weimer <fweimer@...hat.com>,
        Linux API <linux-api@...r.kernel.org>, X86 ML <x86@...nel.org>,
        linux-arch <linux-arch@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>, nhorman@...hat.com,
        npmccallum@...hat.com, "Ayoun, Serge" <serge.ayoun@...el.com>,
        shay.katz-zamir@...el.com, linux-sgx@...r.kernel.org,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Carlos O'Donell <carlos@...hat.com>,
        adhemerval.zanella@...aro.org
Subject: Re: RFC: userspace exception fixups

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.