linux-kernel - Re: [PATCH v4 18/36] cpu: Define attack vectors

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Message-ID: <uhfexplln3n736m4yxdrfqg666x4hs5pokxqw6ycnaktastvko@zi2saqxeoidj>
Date: Thu, 10 Apr 2025 11:12:09 -0700
From: Josh Poimboeuf <jpoimboe@...nel.org>
To: David Kaplan <david.kaplan@....com>
Cc: Thomas Gleixner <tglx@...utronix.de>, Borislav Petkov <bp@...en8.de>, 
	Peter Zijlstra <peterz@...radead.org>, Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>, 
	Ingo Molnar <mingo@...hat.com>, Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org, 
	"H . Peter Anvin" <hpa@...or.com>, linux-kernel@...r.kernel.org, 
	Brendan Jackman <jackmanb@...gle.com>, Derek Manwaring <derekmn@...zon.com>
Subject: Re: [PATCH v4 18/36] cpu: Define attack vectors

On Mon, Mar 10, 2025 at 11:40:05AM -0500, David Kaplan wrote:
> @@ -3178,8 +3179,38 @@ void __init boot_cpu_hotplug_init(void)
>  
>  #ifdef CONFIG_CPU_MITIGATIONS
>  /*
> - * These are used for a global "mitigations=" cmdline option for toggling
> - * optional CPU mitigations.
> + * All except the cross-thread attack vector are mitigated by default.
> + * Cross-thread mitigation often requires disabling SMT which is too expensive
> + * to be enabled by default.

Cross-thread is *partially* mitigated by default (everything except
disabling SMT).

> +bool cpu_mitigate_attack_vector(enum cpu_attack_vectors v)
> +{
> +	if (v < NR_CPU_ATTACK_VECTORS)
> +		return attack_vectors[v];
> +
> +	WARN_ON_ONCE(v >= NR_CPU_ATTACK_VECTORS);

This can be a WARN_ONCE(), v is already known to be invalid here.

>  static int __init mitigations_parse_cmdline(char *arg)
>  {
> -	if (!strcmp(arg, "off"))
> -		cpu_mitigations = CPU_MITIGATIONS_OFF;
> -	else if (!strcmp(arg, "auto"))
> -		cpu_mitigations = CPU_MITIGATIONS_AUTO;
> -	else if (!strcmp(arg, "auto,nosmt"))
> -		cpu_mitigations = CPU_MITIGATIONS_AUTO_NOSMT;
> -	else
> -		pr_crit("Unsupported mitigations=%s, system may still be vulnerable\n",
> -			arg);
> +	char *s, *p;
> +	int len;
> +
> +	len = mitigations_parse_global_opt(arg);
> +
> +	if (cpu_mitigations_off()) {
> +		memset(attack_vectors, 0, sizeof(attack_vectors));
> +		smt_mitigations = SMT_MITIGATIONS_OFF;
> +	} else if (cpu_mitigations_auto_nosmt())
> +		smt_mitigations = SMT_MITIGATIONS_ON;

Kernel coding style wants consistent braces for if-then-else:

	if (cpu_mitigations_off()) {
		memset(attack_vectors, 0, sizeof(attack_vectors));
		smt_mitigations = SMT_MITIGATIONS_OFF;
	} else if (cpu_mitigations_auto_nosmt()) {
		smt_mitigations = SMT_MITIGATIONS_ON;
	}

-- 
Josh