linux-kernel - Re: [RFC PATCH v3 0/4] x86: Add exception fixup for SGX ENCLU

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Date:   Tue, 11 Dec 2018 08:52:53 -0800
From:   Sean Christopherson <sean.j.christopherson@...el.com>
To:     Andy Lutomirski <luto@...capital.net>
Cc:     Josh Triplett <josh@...htriplett.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        x86@...nel.org, Jarkko Sakkinen <jarkko.sakkinen@...ux.intel.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        "H. Peter Anvin" <hpa@...or.com>, linux-kernel@...r.kernel.org,
        linux-sgx@...r.kernel.org,
        Haitao Huang <haitao.huang@...ux.intel.com>,
        Jethro Beekman <jethro@...tanix.com>,
        "Dr . Greg Wettstein" <greg@...ellic.com>
Subject: Re: [RFC PATCH v3 0/4] x86: Add exception fixup for SGX ENCLU

On Tue, Dec 11, 2018 at 07:41:27AM -0800, Andy Lutomirski wrote:
> 
> 
> > On Dec 10, 2018, at 3:24 PM, Josh Triplett <josh@...htriplett.org> wrote:
> > 
> >> On Mon, Dec 10, 2018 at 03:21:37PM -0800, Sean Christopherson wrote:
> >> At that point I realized it's a hell of a lot easier to simply provide
> >> an IOCTL via /dev/sgx that allows userspace to register a per-process
> >> ENCLU exception handler.  At a high level, the basic idea is the same
> >> as the vDSO approach: provide a hardcoded fixup handler for ENCLU and
> >> attempt to fixup select unhandled exceptions that occurred in user code.
> > 
> > So, on the one hand, this is *absolutely* much cleaner than the VDSO
> > approach. On the other hand, this is global process state and has some
> > of the same problems as a signal handler as a result.
> 
> I liked the old version better for this reason 

This isn't fundamentally different than forcing all EENTER calls through
the vDSO, which is also per-process.  Technically this is more flexible
in that regard since userspace gets to choose where their one ENCLU gets
to reside.  Userspace can have per-enclave entry flows so long as the
actual ENLU[EENTER] is common, same as vDSO.

> and for another reason:
> while this new one looks very very simple, it still has the hidden
> complexity that the magic values written to registers in the event of an
> exception are very much Linux specific.

Definitely more magical, but not necessarily more difficult to document.
It'd essentially be an extension of hardware's AEE/AEP behavior.

> OTOH, the old approach clobbered more regs than needed, but that’s a easy fix.