[PATCH] arm64: cpufeature: make sure cpu_mitigations_off then kpti off

[PATCH] arm64: cpufeature: make sure cpu_mitigations_off then kpti off

[Liu Song]

From: Liu Song <liusong@linux.alibaba.com>

If kaslr is enabled, kpti cannot be turned off even if "mitigations=off",
adjust the code order to ensure that kpti is off when "mitigations=off".

Signed-off-by: Liu Song <liusong@linux.alibaba.com>
---
 arch/arm64/kernel/cpufeature.c | 10 +++++-----
 1 file changed, 5 insertions(+), 5 deletions(-)

diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
index 42ea2bd..27d4850 100644
--- a/arch/arm64/kernel/cpufeature.c
+++ b/arch/arm64/kernel/cpufeature.c
@@ -1616,6 +1616,11 @@ static bool unmap_kernel_at_el0(const struct arm64_cpu_capabilities *entry,
 		__kpti_forced = -1;
 	}
 
+	if (cpu_mitigations_off() && !__kpti_forced) {
+		str = "mitigations=off";
+		__kpti_forced = -1;
+	}
+
 	/* Useful for KASLR robustness */
 	if (kaslr_requires_kpti()) {
 		if (!__kpti_forced) {
@@ -1624,11 +1629,6 @@ static bool unmap_kernel_at_el0(const struct arm64_cpu_capabilities *entry,
 		}
 	}
 
-	if (cpu_mitigations_off() && !__kpti_forced) {
-		str = "mitigations=off";
-		__kpti_forced = -1;
-	}
-
 	if (!IS_ENABLED(CONFIG_UNMAP_KERNEL_AT_EL0)) {
 		pr_info_once("kernel page table isolation disabled by kernel configuration\n");
 		return false;
-- 
1.8.3.1

Re: [PATCH] arm64: cpufeature: make sure cpu_mitigations_off then kpti off

[Will Deacon]

On Thu, Jun 16, 2022 at 08:35:38PM +0800, Liu Song wrote:
> From: Liu Song <liusong@linux.alibaba.com>
> 
> If kaslr is enabled, kpti cannot be turned off even if "mitigations=off",
> adjust the code order to ensure that kpti is off when "mitigations=off".
> 
> Signed-off-by: Liu Song <liusong@linux.alibaba.com>
> ---
>  arch/arm64/kernel/cpufeature.c | 10 +++++-----
>  1 file changed, 5 insertions(+), 5 deletions(-)
> 
> diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
> index 42ea2bd..27d4850 100644
> --- a/arch/arm64/kernel/cpufeature.c
> +++ b/arch/arm64/kernel/cpufeature.c
> @@ -1616,6 +1616,11 @@ static bool unmap_kernel_at_el0(const struct arm64_cpu_capabilities *entry,
>  		__kpti_forced = -1;
>  	}
>  
> +	if (cpu_mitigations_off() && !__kpti_forced) {
> +		str = "mitigations=off";
> +		__kpti_forced = -1;
> +	}
> +
>  	/* Useful for KASLR robustness */
>  	if (kaslr_requires_kpti()) {
>  		if (!__kpti_forced) {
> @@ -1624,11 +1629,6 @@ static bool unmap_kernel_at_el0(const struct arm64_cpu_capabilities *entry,
>  		}
>  	}
>  
> -	if (cpu_mitigations_off() && !__kpti_forced) {
> -		str = "mitigations=off";
> -		__kpti_forced = -1;
> -	}

The current behaviour is intentional: KASLR is trivial to bypass when kpti
is disabled.

Will

Re: [PATCH] arm64: cpufeature: make sure cpu_mitigations_off then kpti off

[liusong]

>On Thu, Jun 16, 2022 at 08:35:38PM +0800, Liu Song wrote:
>> From: Liu Song <liusong@linux.alibaba.com>
>>
>> If kaslr is enabled, kpti cannot be turned off even if "mitigations=off",
>> adjust the code order to ensure that kpti is off when "mitigations=off".
>>
>> Signed-off-by: Liu Song <liusong@linux.alibaba.com>
>> ---
>>  arch/arm64/kernel/cpufeature.c | 10 +++++-----
>>  1 file changed, 5 insertions(+), 5 deletions(-)
>>
>> diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
>> index 42ea2bd..27d4850 100644
>> --- a/arch/arm64/kernel/cpufeature.c
>> +++ b/arch/arm64/kernel/cpufeature.c
>> @@ -1616,6 +1616,11 @@ static bool unmap_kernel_at_el0(const struct arm64_cpu_capabilities *entry,
>>    __kpti_forced = -1;
>>   }
>> 
>> + if (cpu_mitigations_off() && !__kpti_forced) {
>> +  str = "mitigations=off";
>> +  __kpti_forced = -1;
>> + }
>> +
>>   /* Useful for KASLR robustness */
>>   if (kaslr_requires_kpti()) {
>>    if (!__kpti_forced) {
>> @@ -1624,11 +1629,6 @@ static bool unmap_kernel_at_el0(const struct arm64_cpu_capabilities *entry,
>>    }
>>   }
>> 
>> - if (cpu_mitigations_off() && !__kpti_forced) {
>> -  str = "mitigations=off";
>> -  __kpti_forced = -1;
>> - }
>
>The current behaviour is intentional: KASLR is trivial to bypass when kpti
>is disabled.
>
>Will

Hi

According to the description of mitigaions=off in kernel-parameters.txt,
it is equivalent to kpti=0 under ARM64, so the description here will mislead
users. I have proposed a patch to fix this.

Thanks