Date: Wed, 10 Jan 2018 15:31:08 +0000
From: "Dr. David Alan Gilbert" <dgilbert@redhat.com>
To: "Woodhouse, David" <dwmw@amazon.co.uk>
Cc: Paul Turner <pjt@google.com>,
        Tom Lendacky <thomas.lendacky@amd.com>,
        Andi Kleen <ak@linux.intel.com>,
        LKML <linux-kernel@vger.kernel.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Greg Kroah-Hartman <gregkh@linux-foundation.org>,
        Tim Chen <tim.c.chen@linux.intel.com>,
        Dave Hansen <dave.hansen@intel.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Kees Cook <keescook@google.com>,
        Rik van Riel <riel@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Andy Lutomirski <luto@amacapital.net>,
        Jiri Kosina <jikos@kernel.org>,
        One Thousand Gnomes <gnomes@lxorguk.ukuu.org.uk>
Subject: Re: [PATCH v6 00/10] Retpoline: Avoid speculative indirect calls in
 kernel
Message-ID: <20180110153107.GB2451@work-vm>
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* Woodhouse, David (dwmw@amazon.co.uk) wrote:
> On Mon, 2018-01-08 at 02:42 -0800, Paul Turner wrote:
> > 
> > While the cases above involve the crafting and use of poisoned
> > entries.? Recall also that one of the initial conditions was that we
> > should avoid RSB underflow as some CPUs may try to use other indirect
> > predictors when this occurs.
> 
> I think we should start by deliberately ignoring the CPUs which use the
> other indirect predictors on RSB underflow. Those CPUs don't perform
> *quite* so badly with IBRS anyway.
> 
> Let's get the minimum amount of RSB handling in to cope with the pre-
> SKL CPUs, and then see if we really do want to extend it to make SKL
> 100% secure in retpoline mode or not.

How do you make decisions on which CPU you're running on?
I'm worried about the case of a VM that starts off on an older host
and then gets live migrated to a new Skylake.
For Intel CPUs we've historically been safe to live migrate
to any newer host based on having all the features that the old one had;
with the guest still seeing the flags etc for the old CPU.

Dave
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK