linux-kernel - Re: RFC: userspace exception fixups

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Message-ID: <20181126143534.GA25719@linux.intel.com>
Date:   Mon, 26 Nov 2018 06:35:34 -0800
From:   Sean Christopherson <sean.j.christopherson@...el.com>
To:     Jarkko Sakkinen <jarkko.sakkinen@...ux.intel.com>
Cc:     Jethro Beekman <jethro@...tanix.com>,
        Andy Lutomirski <luto@...nel.org>,
        Dave Hansen <dave.hansen@...ux.intel.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" <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>
Subject: Re: RFC: userspace exception fixups

On Wed, Nov 21, 2018 at 05:17:34PM +0200, Jarkko Sakkinen wrote:
> On Wed, Nov 21, 2018 at 05:17:32AM +0000, Jethro Beekman wrote:
> > Jarkko, can you please explain you solution in detail? The CPU receives an
> > exception. This will be handled by the kernel exception handler. What
> > information does the kernel exception handler use to determine whether to
> > deliver the exception as a regular signal to the process, or whether to set
> > the special registers values for userspace and just continue executing the
> > process manually?
> 
> Now we throw SIGSEGV when PF_SGX set, right? In my solution that would
> be turned just doing iret to AEP with the extra that three registers get
> exception data (type, reason, addr). No decoding or RIP adjusting
> involved.
> 
> That would mean that you would actually have to implement AEP handler
> than just have enclu there.
> 
> I've also proposed that perhaps for SGX also #UD should be propagated
> this way because for some instructions you need outside help to emulate
> "non-enclave" environment.

And how would you determine the #UD is related to SGX?  Hardware doesn't
provide any indication that a #UD (or any other fault) is related to SGX
or occurred in an enclave.  The only fault that is special-cased in a
non-virtualized environment is #PF signaled by the EPCM, which gets the
PF_SGX bit set in the error code.

> That is all I have drafted together so far. I'll try to finish v18 this
> week with other stuff and refine further next week (unless someone gives
> obvious reason why this doesn't work, which might well be because I
> haven't went too deep with my analysis yet because of lack of time).
> 
> /Jarkko