linux-kernel - Re: RFC: userspace exception fixups

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Message-ID: <20181120225518.GE8391@linux.intel.com>
Date:   Wed, 21 Nov 2018 00:55:18 +0200
From:   Jarkko Sakkinen <jarkko.sakkinen@...ux.intel.com>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     Dave Hansen <dave.hansen@...ux.intel.com>,
        "Christopherson, Sean J" <sean.j.christopherson@...el.com>,
        Jethro Beekman <jethro@...tanix.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,
        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>
Subject: Re: RFC: userspace exception fixups

On Tue, Nov 20, 2018 at 07:19:37AM -0800, Andy Lutomirski wrote:
> What is "#GP with EPCM"?  We certainly don't want to react to #UD in

A typo. Meant #PF with PF_SGX set i.e. EPCM conflict.

> 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