linux-kernel - Re: [PATCH v3 3/3] x86/retpoline: Convert generic specific retpolines to use RETPOLINE

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Date:   Wed, 10 Jan 2018 14:46:34 +0000
From:   "Woodhouse, David" <dwmw@...zon.co.uk>
To:     Tom Lendacky <thomas.lendacky@....com>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "andi@...stfloor.org" <andi@...stfloor.org>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "peterz@...radead.org" <peterz@...radead.org>,
        "tim.c.chen@...ux.intel.com" <tim.c.chen@...ux.intel.com>,
        "torvalds@...ux-foundation.org" <torvalds@...ux-foundation.org>,
        "ak@...ux.intel.com" <ak@...ux.intel.com>,
        "x86@...nel.org" <x86@...nel.org>,
        "pjt@...gle.com" <pjt@...gle.com>,
        "luto@...nel.org" <luto@...nel.org>,
        "dave.hansen@...el.com" <dave.hansen@...el.com>,
        "jikos@...nel.org" <jikos@...nel.org>,
        "gregkh@...ux-foundation.org" <gregkh@...ux-foundation.org>
Subject: Re: [PATCH v3 3/3] x86/retpoline: Convert generic specific
 retpolines to use RETPOLINE_GENERIC

On Wed, 2018-01-10 at 08:23 -0600, Tom Lendacky wrote:
> On 1/10/2018 7:15 AM, Woodhouse, David wrote:
> > On Tue, 2018-01-09 at 18:28 -0800, Andi Kleen wrote:
> >> From: Andi Kleen <ak@...ux.intel.com>
> >>
> >> X86_FEATURE_RETPOLINE has been renamed to X86_FEATURE_RETPOLINE_GENERIC.
> >> Convert the sequences using it.
> > 
> > AMD documentation says they need the RSB fill too, so these should stay
> > under X86_FEATURE_RETPOLINE I think.
> 
> AMD processors don't fall back to the indirect predictor on an underflow
> so this isn't needed on AMD if it is purely for underflow prevention.

It isn't purely for underflow protection. This is needed (see the AMD
doc) for protecting against RSB entries that actually point to
userspace addresses. It's only not needed if we have SMEP.
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