linux-kernel - Re: RFC: userspace exception fixups

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Message-ID: <20181119160204.GD13298@linux.intel.com>
Date:   Mon, 19 Nov 2018 18:02:04 +0200
From:   Jarkko Sakkinen <jarkko.sakkinen@...ux.intel.com>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     Dave Hansen <dave.hansen@...ux.intel.com>,
        "Christopherson, Sean J" <sean.j.christopherson@...el.com>,
        Jethro Beekman <jethro@...tanix.com>,
        Florian Weimer <fweimer@...hat.com>,
        Linux API <linux-api@...r.kernel.org>,
        Jann Horn <jannh@...gle.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        X86 ML <x86@...nel.org>,
        linux-arch <linux-arch@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Rich Felker <dalias@...c.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>
Subject: Re: RFC: userspace exception fixups

On Mon, Nov 19, 2018 at 07:29:36AM -0800, Andy Lutomirski wrote:
> 1. The kernel needs some way to know *when* to apply this fixup.
> Decoding the instruction stream and doing it to all exceptions that
> hit an ENCLU instruction seems like a poor design.

I'm not sure why you would ever need to do any type of fixup as the idea
is to just return to AEP i.e. from chosen exceptions (EPCM, #UD) the AEP
would work the same way as for exceptions that the kernel can deal with
except filling the exception information to registers.

> 2. It starts exposing what looks like a more generic exception
> handling mechanism to userspace, except that it's nonsensical for
> anything other than ENCLU.

Well, I see the user space and namely the run-time the host for the
enclave i.e. middle-man to provide services for emulating instructions
etc.

/Jarkko