linux-kernel - Re: RFC: userspace exception fixups

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Message-ID: <0a78772e-4f42-254b-d91a-f0f80a24ff90@fortanix.com>
Date:   Wed, 21 Nov 2018 05:17:32 +0000
From:   Jethro Beekman <jethro@...tanix.com>
To:     Jarkko Sakkinen <jarkko.sakkinen@...ux.intel.com>,
        Andy Lutomirski <luto@...nel.org>
CC:     Dave Hansen <dave.hansen@...ux.intel.com>,
        "Christopherson, Sean J" <sean.j.christopherson@...el.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 2018-11-21 04:25, Jarkko Sakkinen wrote:
> On Tue, Nov 20, 2018 at 07:19:37AM -0800, Andy Lutomirski wrote:
>> general by mucking with some regs and retrying -- that will infinite
>> loop and confuse everyone.  I'm not even 100% convinced that decoding
>> the insn stream is useful -- AEP can point to something that isn't
>> ENCLU.
> 
> In my return-to-AEP approach to whole point was not to do any decoding
> but instead have something else always in the AEP handler than just
> ENCLU.
> 
> No instruction decoding. No RIP manipulation.
> 
>> IOW the kernel needs to know *when* to apply this special behavior.
>> Sadly there is no bit in the exception frame that says "came from
>> SGX".

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?

--
Jethro Beekman | Fortanix


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