linux-kernel - Re: RFC: userspace exception fixups

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Date:   Fri, 2 Nov 2018 10:33:50 -0700
From:   Sean Christopherson <sean.j.christopherson@...el.com>
To:     Dave Hansen <dave.hansen@...el.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>,
        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 Fri, Nov 02, 2018 at 10:13:23AM -0700, Dave Hansen wrote:
> On 11/2/18 10:06 AM, Sean Christopherson wrote:
> > On Fri, Nov 02, 2018 at 09:56:44AM -0700, Dave Hansen wrote:
> >> On 11/2/18 9:30 AM, Sean Christopherson wrote:
> >>> What if rather than having userspace register an address for fixup, the
> >>> kernel instead unconditionally does fixup on the ENCLU opcode?
> >>
> >> The problem is knowing what to do for the fixup.  If we have a simple
> >> action to take that's universal, like backing up %RIP, or setting some
> >> other register state, it's not bad.
> > 
> > Isn't the EENTER/RESUME behavior universal?  Or am I missing something?
> 
> Could someone write down all the ways we get in and out of the enclave?
> 
> I think we always get in from userspace calling EENTER or ERESUME.  We
> can't ever enter directly from the kernel, like via an IRET from what I
> understand.

Correct, the only way to get into the enclave is EENTER or ERESUME.
My understanding is that even SMIs bounce through the AEX target
before transitioning to SMM.
 
> We get *out* from exceptions, hardware interrupts, or enclave-explicit
> EEXITs.  Did I miss any?  Remind me where the hardware lands the control
> flow in each of those exit cases.

And VMExits.  There are basically two cases: EEXIT and everything else.
EEXIT is a glorified indirect jump, e.g. %RBX holds the target %RIP.
Everything else is an Asynchronous Enclave Exit (AEX).  On an AEX, %RIP
is set to a value specified by EENTER/ERESUME, %RBP and %RSP are
restored to pre-enclave values and all other registers are loaded with
synthetic state.  The actual interrupt/exception/VMExit then triggers,
e.g. the %RIP on the stack for an exception is always the AEX target,
not the %RIP inside the enclave that actually faulted.