linux-kernel - Re: [PATCH 1/3] x86/cpu: Actually turn off mitigations by default for SPECULATION

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Date: Mon, 15 Apr 2024 13:13:36 +0200
From: Geert Uytterhoeven <geert@...ux-m68k.org>
To: Sean Christopherson <seanjc@...gle.com>
Cc: Jonathan Corbet <corbet@....net>, Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>, 
	Borislav Petkov <bp@...en8.de>, Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org, 
	Peter Zijlstra <peterz@...radead.org>, Josh Poimboeuf <jpoimboe@...nel.org>, linux-doc@...r.kernel.org, 
	linux-kernel@...r.kernel.org, Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>, 
	Daniel Sneddon <daniel.sneddon@...ux.intel.com>, Catalin Marinas <catalin.marinas@....com>, 
	Will Deacon <will@...nel.org>, Linux ARM <linux-arm-kernel@...ts.infradead.org>, 
	Linux-Renesas <linux-renesas-soc@...r.kernel.org>
Subject: Re: [PATCH 1/3] x86/cpu: Actually turn off mitigations by default for SPECULATION_MITIGATIONS=n

Hi Sean,

On Tue, Apr 9, 2024 at 7:51 PM Sean Christopherson <seanjc@...gle.com> wrote:
> Initialize cpu_mitigations to CPU_MITIGATIONS_OFF if the kernel is built
> with CONFIG_SPECULATION_MITIGATIONS=n, as the help text quite clearly
> states that disabling SPECULATION_MITIGATIONS is supposed to turn off all
> mitigations by default.
>
>   │ If you say N, all mitigations will be disabled. You really
>   │ should know what you are doing to say so.
>
> As is, the kernel still defaults to CPU_MITIGATIONS_AUTO, which results in
> some mitigations being enabled in spite of SPECULATION_MITIGATIONS=n.
>
> Fixes: f43b9876e857 ("x86/retbleed: Add fine grained Kconfig knobs")
> Cc: stable@...r.kernel.org
> Signed-off-by: Sean Christopherson <seanjc@...gle.com>

Thanks for your patch, which is now commit f337a6a21e2fd67e
("x86/cpu: Actually turn off mitigations by default
for SPECULATION_MITIGATIONS=n") in v6.9-rc4.

This causes the following suspicious messages on R-Car H3:

        CPU features: kernel page table isolation forced OFF by mitigations=off
        spectre-v4 mitigation disabled by command-line option
        spectre-v2 mitigation disabled by command line option
        spectre-v2 mitigation disabled by command line option

and R-Car V4H:

        CPU features: kernel page table isolation forced OFF by mitigations=off
        spectre-v4 mitigation disabled by command-line option
        spectre-bhb mitigation disabled by command line option
        spectre-bhb mitigation disabled by command line option

Interestingly, no mitigations are disabled on the command-line.

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-m68korg

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds