Re: [PATCH 4.9 13/20] x86/speculation/mmio: Add mitigation for Processor MMIO Stale Data

From: Pavel Machek <pavel@denx.de>
To: Greg Kroah-Hartman <gregkh@linuxfoundation.org>, rdunlap@infradead.org
Cc: linux-kernel@vger.kernel.org, stable@vger.kernel.org,
	Pawan Gupta <pawan.kumar.gupta@linux.intel.com>,
	Borislav Petkov <bp@suse.de>,
	Thomas Gleixner <tglx@linutronix.de>,
	Thadeu Lima de Souza Cascardo <cascardo@canonical.com>
Subject: Re: [PATCH 4.9 13/20] x86/speculation/mmio: Add mitigation for Processor MMIO Stale Data
Date: Wed, 22 Jun 2022 13:48:44 +0200	[thread overview]
Message-ID: <20220622114844.GA6854@duo.ucw.cz> (raw)
In-Reply-To: <20220614183725.181834522@linuxfoundation.org>

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Hi!

> +static int __init mmio_stale_data_parse_cmdline(char *str)
> +{
> +	if (!boot_cpu_has_bug(X86_BUG_MMIO_STALE_DATA))
> +		return 0;
> +
> +	if (!str)
> +		return -EINVAL;
> +
> +	if (!strcmp(str, "off")) {
> +		mmio_mitigation = MMIO_MITIGATION_OFF;
> +	} else if (!strcmp(str, "full")) {
> +		mmio_mitigation = MMIO_MITIGATION_VERW;
> +	} else if (!strcmp(str, "full,nosmt")) {
> +		mmio_mitigation = MMIO_MITIGATION_VERW;
> +		mmio_nosmt = true;
> +	}
> +
> +	return 0;
> +}

This is wrong, AFAICT. Returning 0 will pollute init's environment;
Randy was cleaning those lately and we are even seeing them in
-stable. See for example b793a01000122d2bd133ba451a76cc135b5e162c.

The early return 0 should disappear, too; we should validate the
option even on non-buggy machines.

Best regards,
								Pavel

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