linux-kernel - Re: [PATCH RESEND] x86/speculation: Fix user-mode spectre-v2 protection with KERNEL

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Date:   Mon, 20 Feb 2023 10:56:38 -0800
From:   KP Singh <kpsingh@...nel.org>
To:     Borislav Petkov <bp@...en8.de>
Cc:     Josh Poimboeuf <jpoimboe@...nel.org>, linux-kernel@...r.kernel.org,
        pjt@...gle.com, evn@...gle.com, tglx@...utronix.de,
        mingo@...hat.com, dave.hansen@...ux.intel.com, x86@...nel.org,
        hpa@...or.com, peterz@...radead.org,
        pawan.kumar.gupta@...ux.intel.com, kim.phillips@....com,
        alexandre.chartre@...cle.com, daniel.sneddon@...ux.intel.com,
        José Oliveira <joseloliveira11@...il.com>,
        Rodrigo Branco <rodrigo@...nelhacking.com>,
        Alexandra Sandulescu <aesa@...gle.com>,
        Jim Mattson <jmattson@...gle.com>, stable@...r.kernel.org
Subject: Re: [PATCH RESEND] x86/speculation: Fix user-mode spectre-v2
 protection with KERNEL_IBRS

On Mon, Feb 20, 2023 at 10:52 AM Borislav Petkov <bp@...en8.de> wrote:
>
> On Mon, Feb 20, 2023 at 10:44:21AM -0800, KP Singh wrote:
> > No it cannot with IBRS which is really just KERNEL_IBRS enabled, we
>
> See my other reply. The intent is there to be able to do it. What needs
> to be figured out now is *why* we said no STIBP with IBRS? Was it an
> omission or was there some intent behind it.
>

Sure, it looks like an omission to me, we wrote a POC on Skylake that
was able to do cross-thread training with the current set of
mitigations.

STIBP with IBRS is still correct if spectre_v2=ibrs had really meant
IBRS everywhere, but just means KERNEL_IBRS, which means only kernel
is protected, userspace is still unprotected.

> --
> Regards/Gruss,
>     Boris.
>
> https://people.kernel.org/tglx/notes-about-netiquette