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From:   Andy Lutomirski <luto@kernel.org>
Date:   Mon, 19 Nov 2018 07:29:36 -0800
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Subject: Re: RFC: userspace exception fixups
To:     Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>
Cc:     Andrew Lutomirski <luto@kernel.org>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        "Christopherson, Sean J" <sean.j.christopherson@intel.com>,
        Jethro Beekman <jethro@fortanix.com>,
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        Linux API <linux-api@vger.kernel.org>,
        Jann Horn <jannh@google.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        X86 ML <x86@kernel.org>,
        linux-arch <linux-arch@vger.kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Rich Felker <dalias@libc.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>
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On Sat, Nov 17, 2018 at 11:16 PM Jarkko Sakkinen
<jarkko.sakkinen@linux.intel.com> wrote:
>
> On Thu, Nov 01, 2018 at 10:53:40AM -0700, Andy Lutomirski wrote:
> > Hi all-
> >
> > The people working on SGX enablement are grappling with a somewhat
> > annoying issue: the x86 EENTER instruction is used from user code and
> > can, as part of its normal-ish operation, raise an exception.  It is
> > also highly likely to be used from a library, and signal handling in
> > libraries is unpleasant at best.
> >
> > There's been some discussion of adding a vDSO entry point to wrap
> > EENTER and do something sensible with the exceptions, but I'm
> > wondering if a more general mechanism would be helpful.
>
> I haven't really followed all of this discussion because I've been busy
> working on the patch set but for me all of these approaches look awfully
> complicated.
>
> I'll throw my own suggestion and apologize if this has been already
> suggested and discarded: return-to-AEP.
>
> My idea is to do just a small extension to SGX AEX handling. At the
> moment hardware will RAX, RBX and RCX with ERESUME parameters. We can
> fill extend this by filling other three spare registers with exception
> information.

I have two issues with this approach:

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.

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.