linux-kernel - Re: [PATCH RESEND] x86/pti: Fix kernel warnings for pti= and nopti cmdline options.

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Date:   Tue, 8 Aug 2023 17:13:12 -0700
From:   Sohil Mehta <sohil.mehta@...el.com>
To:     Jo Van Bulck <jo.vanbulck@...kuleuven.be>,
        <linux-kernel@...r.kernel.org>, <dave.hansen@...ux.intel.com>,
        <luto@...nel.org>, <peterz@...radead.org>, <mingo@...hat.com>
CC:     <x86@...nel.org>, <bp@...en8.de>, <tglx@...utronix.de>,
        <hpa@...or.com>
Subject: Re: [PATCH RESEND] x86/pti: Fix kernel warnings for pti= and nopti
 cmdline options.

On 8/8/2023 12:56 PM, Jo Van Bulck wrote:

> -
> -	if (cmdline_find_option_bool(boot_command_line, "nopti") ||
> -	    cpu_mitigations_off()) {
> +	if (pti_mode == PTI_FORCE_OFF || cpu_mitigations_off()) {

Can mitigations be off through some other mechanisms such as kernel config?

Maybe split the mitigations_off check into a separate if and it's own
unique print message?

The existing code might have the same issue as well.

Also, with the separated check you can avoid the unnecessary re-setting
of pti_mode when pti_mode == PTI_FORCE_OFF is true.


>  		pti_mode = PTI_FORCE_OFF;>  		pti_print_if_insecure("disabled on command line.");
>  		return;
>  	}
>  
> -autosel:
> -	if (!boot_cpu_has_bug(X86_BUG_CPU_MELTDOWN))
> +	if (pti_mode == PTI_AUTO && !boot_cpu_has_bug(X86_BUG_CPU_MELTDOWN))
>  		return;
> -enable:
> +
> +	if (pti_mode == PTI_FORCE_ON)
> +		pti_print_if_secure("force enabled on command line.");
>  	setup_force_cpu_cap(X86_FEATURE_PTI);
>  }
>  
> +static int __init pti_parse_cmdline(char *arg)
> +{
> +	if (!strcmp(arg, "off"))
> +		pti_mode = PTI_FORCE_OFF;
> +	else if (!strcmp(arg, "on"))
> +		pti_mode = PTI_FORCE_ON;
> +	else if (!strcmp(arg, "auto"))
> +		pti_mode = PTI_AUTO;
> +	else
> +		return -EINVAL;
> +	return 0;
> +}
> +early_param("pti", pti_parse_cmdline);
> +
> +static int __init pti_parse_cmdline_nopti(char *arg)
> +{
> +	pti_mode = PTI_FORCE_OFF;
> +	return 0;
> +}
> +early_param("nopti", pti_parse_cmdline_nopti);
> +

In the rare case that both pti= and nopti is set the existing code seems
to ignore the nopti option. Would the new implementation do the same?

Sohil