linux-kernel - Re: [RFC] x86: Avoid CR3 load on compatibility mode with PTI

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Message-ID: <20180115195252.GH7804@1wt.eu>
Date:   Mon, 15 Jan 2018 20:52:52 +0100
From:   Willy Tarreau <w@....eu>
To:     Dave Hansen <dave.hansen@...ux.intel.com>
Cc:     Nadav Amit <namit@...are.com>, linux-kernel@...r.kernel.org,
        luto@...nel.org, tglx@...utronix.de, mingo@...hat.com,
        hpa@...or.com, x86@...nel.org, nadav.amit@...il.com
Subject: Re: [RFC] x86: Avoid CR3 load on compatibility mode with PTI

On Mon, Jan 15, 2018 at 11:49:19AM -0800, Dave Hansen wrote:
> If we start disabling PTI willy nilly at points _away_ from the
> capability checks (like for 32-bit binaries, say), then it gets really
> hard to decide if we are doing the right things.
> 
> Also, what's the end goal here?  Run old 32-bit binaries better?  You
> want to weaken the security of the whole implementation to do that?
> Sounds like a bad tradeoff to me.

In fact I understand it differently, which is that by running 32-bit,
he can recover the original performance without sacrifying security.
It's not that bad actually when you think about it since the vast
majority of performance-sensitive software doesn't need to access
even one GB of data.

Willy