linux-kernel - Re: RFC: userspace exception fixups

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Message-ID: <20181108150405.GB14072@linux.intel.com>
Date:   Thu, 8 Nov 2018 17:04:05 +0200
From:   Jarkko Sakkinen <jarkko.sakkinen@...ux.intel.com>
To:     Dave Hansen <dave.hansen@...el.com>
Cc:     Sean Christopherson <sean.j.christopherson@...el.com>,
        Andy Lutomirski <luto@...nel.org>,
        Jann Horn <jannh@...gle.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Rich Felker <dalias@...c.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Jethro Beekman <jethro@...tanix.com>,
        Florian Weimer <fweimer@...hat.com>,
        Linux API <linux-api@...r.kernel.org>, X86 ML <x86@...nel.org>,
        linux-arch <linux-arch@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.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>,
        Carlos O'Donell <carlos@...hat.com>,
        adhemerval.zanella@...aro.org
Subject: Re: RFC: userspace exception fixups

On Wed, Nov 07, 2018 at 12:56:58PM -0800, Dave Hansen wrote:
> On 11/7/18 11:01 AM, Sean Christopherson wrote:
> > Going off comments in similar code related to UMIP, we'd need to figure
> > out how to handle protection keys.
> 
> There are two options:
> 1. Don't depend on the userspace mapping.  Do get_user_pages() to find
>    the instruction in the kernel direct map, and use that.
> 2. Do a WRPKRU that allows read access, do the read, then put PKRU back.
>    This is a pain because of preemption and all that jazz.
> 
> Right now, we just let the prefetch instruction detection fail if you
> mark it unreadable with pkeys.  Tough cookies, basically.  But, that's
> just the kernel being nice, but you need it for functionality, so it's
> tougher.

I would go with one because it is the stable way to do it and we are
100% sure to not conflict with pk's.

/Jarkko