linux-kernel - Re: [PATCH v4 0/5] Next revision of the L1D flush patches

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Message-ID: <202104081319.DAB1D817@keescook>
Date:   Thu, 8 Apr 2021 13:23:06 -0700
From:   Kees Cook <keescook@...omium.org>
To:     "Singh, Balbir" <sblbir@...zon.com>, tglx@...utronix.de
Cc:     "mingo@...hat.com" <mingo@...hat.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "peterz@...radead.org" <peterz@...radead.org>,
        "torvalds@...ux-foundation.org" <torvalds@...ux-foundation.org>,
        "jpoimboe@...hat.com" <jpoimboe@...hat.com>,
        "x86@...nel.org" <x86@...nel.org>,
        "tony.luck@...el.com" <tony.luck@...el.com>,
        "dave.hansen@...el.com" <dave.hansen@...el.com>,
        "thomas.lendacky@....com" <thomas.lendacky@....com>,
        "benh@...nel.crashing.org" <benh@...nel.crashing.org>,
        linux-hardening@...r.kernel.org
Subject: Re: [PATCH v4 0/5] Next revision of the L1D flush patches

*thread necromancy*
https://lore.kernel.org/lkml/20210108121056.21940-1-sblbir@amazon.com/

On Mon, Jan 25, 2021 at 09:27:38AM +0000, Singh, Balbir wrote:
> On Fri, 2021-01-08 at 23:10 +1100, Balbir Singh wrote:
> > Implement a mechanism that allows tasks to conditionally flush
> > their L1D cache (mitigation mechanism suggested in [2]). The previous
> > posts of these patches were sent for inclusion (see [3]) and were not
> > included due to the concern for the need for additional checks,
> > those checks were:
> > 
> > 1. Implement this mechanism only for CPUs affected by the L1TF bug
> > 2. Disable the software fallback
> > 3. Provide an override to enable this mechanism
> > 4. Be SMT aware in the implementation
> > [...]
> Ping on any review comments? Suggested refactoring?

Hi!

I'd still really like to see this -- it's a big hammer, but that's the
point for cases where some new flaw appears and we can point to the
toolbox and say "you can mitigate it with this while you wait for new
kernel/CPU."

Any further thoughts from x86 maintainers? This seems like it addressed
all of tglx's review comments.

-- 
Kees Cook