linux-kernel - Re: RFC: userspace exception fixups

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Date:   Tue, 20 Nov 2018 07:19:37 -0800
From:   Andy Lutomirski <luto@...nel.org>
To:     Jarkko Sakkinen <jarkko.sakkinen@...ux.intel.com>
Cc:     Andrew Lutomirski <luto@...nel.org>,
        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 Tue, Nov 20, 2018 at 2:11 AM Jarkko Sakkinen
<jarkko.sakkinen@...ux.intel.com> wrote:
>
> On Mon, Nov 19, 2018 at 09:00:08AM -0800, Andy Lutomirski wrote:
> > On Mon, Nov 19, 2018 at 8:02 AM Jarkko Sakkinen
> > <jarkko.sakkinen@...ux.intel.com> wrote:
> > >
> > > 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.
> >
> > Sure, but how does the kernel know when to do that and when to send a
> > signal?  I don't really like decoding the instruction stream to figure
> > it out.
>
> Hmm... why you have to decode instruction stream to find that out? Would
> just depend on exception type (#GP with EPCM, #UD).

What is "#GP with EPCM"?  We certainly don't want to react to #UD in
general by mucking with some regs and retrying -- that will infinite
loop and confuse everyone.  I'm not even 100% convinced that decoding
the insn stream is useful -- AEP can point to something that isn't
ENCLU.

IOW the kernel needs to know *when* to apply this special behavior.
Sadly there is no bit in the exception frame that says "came from
SGX".