linux-kernel - Re: RFC: userspace exception fixups

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Date:   Fri, 2 Nov 2018 10:12:51 -0700
From:   Sean Christopherson <sean.j.christopherson@...el.com>
To:     Jethro Beekman <jethro@...tanix.com>
Cc:     Andy Lutomirski <luto@...nel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Rich Felker <dalias@...c.org>, Jann Horn <jannh@...gle.com>,
        Dave Hansen <dave.hansen@...ux.intel.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" <nhorman@...hat.com>,
        "npmccallum@...hat.com" <npmccallum@...hat.com>,
        "Ayoun, Serge" <serge.ayoun@...el.com>,
        "shay.katz-zamir@...el.com" <shay.katz-zamir@...el.com>,
        "linux-sgx@...r.kernel.org" <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" <adhemerval.zanella@...aro.org>
Subject: Re: RFC: userspace exception fixups

On Fri, Nov 02, 2018 at 04:56:36PM +0000, Jethro Beekman wrote:
> On 2018-11-02 09:52, Sean Christopherson wrote:
> >On Fri, Nov 02, 2018 at 04:37:10PM +0000, Jethro Beekman wrote:
> >>On 2018-11-02 09:30, Sean Christopherson wrote:
> >>>... The intended convention for EENTER is to have an ENCLU at the AEX target ...
> >>>
> >>>... to further enforce that the AEX target needs to be ENCLU.
> >>
> >>Some SGX runtimes may want to use a different AEX target.
> >
> >To what end?  Userspace gets no indication as to why the AEX occurred.
> >And if exceptions are getting transfered to userspace the trampoline
> >would effectively be handling only INTR, NMI, #MC and EPC #PF.
> >
> 
> Various reasons...
> 
> Userspace may have established an exception handling convention with the
> enclave (by setting TCS.NSSA > 1) and may want to call EENTER instead of
> ERESUME.

The ERESUME trampoline would only be invoked for exceptions that aren't
transferred to userspace.  On #BR, #UD, etc..., the kernel would fixup
%RIP to effectively point at @fault_handler.  Userspace can then do
whatever it wants to handle the fault, e.g. do EENTER if the fault needs
to be serviced by the enclave.

> Userspace may want fine-grained control over enclave scheduling (e.g.
> SGX-Step)

Uh, isn't SGX-Step an attack on SGX?  Preventing userspace from playing
games with enclave scheduling seems like a good thing.