Parse the pti= and nopti cmdline options using early_param to fix 'Unknown
kernel command line parameters "nopti", will be passed to user space'
warnings in the kernel log when nopti or pti= are passed to the kernel
cmdline on x86 platforms. Additionally allow the kernel to warn for
malformed pti= options.

Link: https://lore.kernel.org/all/b9bbb279-fa8f-0784-900f-114ce186cbb3@intel.com/
Signed-off-by: Jo Van Bulck <jo.vanbulck@cs.kuleuven.be>
---
 arch/x86/mm/pti.c | 55 ++++++++++++++++++++++++-----------------------
 1 file changed, 28 insertions(+), 27 deletions(-)

diff --git a/arch/x86/mm/pti.c b/arch/x86/mm/pti.c
index 78414c6d1..7575e224d 100644
--- a/arch/x86/mm/pti.c
+++ b/arch/x86/mm/pti.c
@@ -69,6 +69,7 @@ static void __init pti_print_if_secure(const char *reason)
 		pr_info("%s\n", reason);
 }
 
+/* Assume mode is auto unless overridden via cmdline below. */
 static enum pti_mode {
 	PTI_AUTO = 0,
 	PTI_FORCE_OFF,
@@ -77,50 +78,50 @@ static enum pti_mode {
 
 void __init pti_check_boottime_disable(void)
 {
-	char arg[5];
-	int ret;
-
-	/* Assume mode is auto unless overridden. */
-	pti_mode = PTI_AUTO;
-
 	if (hypervisor_is_type(X86_HYPER_XEN_PV)) {
 		pti_mode = PTI_FORCE_OFF;
 		pti_print_if_insecure("disabled on XEN PV.");
 		return;
 	}
 
-	ret = cmdline_find_option(boot_command_line, "pti", arg, sizeof(arg));
-	if (ret > 0)  {
-		if (ret == 3 && !strncmp(arg, "off", 3)) {
-			pti_mode = PTI_FORCE_OFF;
-			pti_print_if_insecure("disabled on command line.");
-			return;
-		}
-		if (ret == 2 && !strncmp(arg, "on", 2)) {
-			pti_mode = PTI_FORCE_ON;
-			pti_print_if_secure("force enabled on command line.");
-			goto enable;
-		}
-		if (ret == 4 && !strncmp(arg, "auto", 4)) {
-			pti_mode = PTI_AUTO;
-			goto autosel;
-		}
-	}
-
-	if (cmdline_find_option_bool(boot_command_line, "nopti") ||
-	    cpu_mitigations_off()) {
+	if (cpu_mitigations_off())
 		pti_mode = PTI_FORCE_OFF;
+	if (pti_mode == PTI_FORCE_OFF) {
 		pti_print_if_insecure("disabled on command line.");
 		return;
 	}
+	if (pti_mode == PTI_FORCE_ON) {
+		pti_print_if_secure("force enabled on command line.");
+		goto enable;
+	}
 
-autosel:
 	if (!boot_cpu_has_bug(X86_BUG_CPU_MELTDOWN))
 		return;
 enable:
 	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);
+
 pgd_t __pti_set_user_pgtbl(pgd_t *pgdp, pgd_t pgd)
 {
 	/*
-- 
2.25.1

On 8/12/2023 8:54 AM, Jo Van Bulck wrote:
>  arch/x86/mm/pti.c | 55 ++++++++++++++++++++++++-----------------------
>  1 file changed, 28 insertions(+), 27 deletions(-)
> 

This version is very similar to the original patch and much simpler.
Sorry about the unnecessary churn.

Apart from the minor nits below,
Reviewed-by: Sohil Mehta <sohil.mehta@intel.com>


> +	if (cpu_mitigations_off())
>  		pti_mode = PTI_FORCE_OFF;
> +	if (pti_mode == PTI_FORCE_OFF) {
>  		pti_print_if_insecure("disabled on command line.");
>  		return;
>  	}

A new line here would be useful.

> +	if (pti_mode == PTI_FORCE_ON) {
> +		pti_print_if_secure("force enabled on command line.");
> +		goto enable;
> +	}
>  
> -autosel:
>  	if (!boot_cpu_has_bug(X86_BUG_CPU_MELTDOWN))
>  		return;
>  enable:
>  	setup_force_cpu_cap(X86_FEATURE_PTI);
>  }
>  

Was there an issue with the flow you had in the original patch? It was
avoiding the goto label and flow was a bit more linear.

> if (pti_mode == PTI_AUTO && !boot_cpu_has_bug(X86_BUG_CPU_MELTDOWN))
>  		return;
> 
> if (pti_mode == PTI_FORCE_ON)
> 	pti_print_if_secure("force enabled on command line.");
> 
> setup_force_cpu_cap(X86_FEATURE_PTI);

Sohil

On 14.08.23 14:12, Sohil Mehta wrote:
> On 8/12/2023 8:54 AM, Jo Van Bulck wrote:
>>   arch/x86/mm/pti.c | 55 ++++++++++++++++++++++++-----------------------
>>   1 file changed, 28 insertions(+), 27 deletions(-)
>>
> 
> This version is very similar to the original patch and much simpler.
> Sorry about the unnecessary churn.
> 
> Apart from the minor nits below,
> Reviewed-by: Sohil Mehta <sohil.mehta@intel.com>

No problem, thanks for the help!

> 
>> +	if (cpu_mitigations_off())
>>   		pti_mode = PTI_FORCE_OFF;
>> +	if (pti_mode == PTI_FORCE_OFF) {
>>   		pti_print_if_insecure("disabled on command line.");
>>   		return;
>>   	}
> 
> A new line here would be useful.

Added in next revision.

> Was there an issue with the flow you had in the original patch? It was
> avoiding the goto label and flow was a bit more linear.

No, the original flow also works and I agree that an explicit PTI_AUTO 
check may indeed be preferable. Reverting this in the next patch iteration.

Best,
Jo