linux-kernel - Re: [PATCH v14 09/19] x86/mm: x86/sgx: Signal SEGV_SGXERR for #PFs w/ PF

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Date:   Wed, 26 Sep 2018 14:45:17 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     Andy Lutomirski <luto@...capital.net>
Cc:     "Christopherson, Sean J" <sean.j.christopherson@...el.com>,
        Andrew Lutomirski <luto@...nel.org>,
        Jarkko Sakkinen <jarkko.sakkinen@...ux.intel.com>,
        X86 ML <x86@...nel.org>,
        Platform Driver <platform-driver-x86@...r.kernel.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>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v14 09/19] x86/mm: x86/sgx: Signal SEGV_SGXERR for #PFs w/
 PF_SGX

On 09/26/2018 02:15 PM, Andy Lutomirski wrote:
> Could we perhaps have a little vDSO entry (or syscall, I suppose) that
> runs an enclave an returns an error code, and rig up the #PF handler
> to check if the error happened in the vDSO entry and fix it up rather
> than sending a signal?

Yeah, signals suck.

So, instead of doing the enclave entry instruction (EENTER is it?), the
app would do the vDSO call.  It would have some calling convention, like
"set %rax to 0 before entering".  Then, we just teach the page fault
handler about the %RIP in the vDSO that can fault and how to move one
instruction later, munge %RIP to a value that tells about the error,
then return from the fault.  It would basically be like the kernel
exception tables, but for userspace.  Right?

How would a syscall work, though?  I assume we can't just enter the
enclave from ring0.