To make use of KASAN's tag based mode on x86, Linear Address Masking
(LAM) needs to be enabled. To do that the 28th bit in CR4 has to be set.

Set the bit in early memory initialization.

When launching secondary CPUs the LAM bit gets lost. To avoid this add
it in a mask in head_64.S. The bitmask permits some bits of CR4 to pass
from the primary CPU to the secondary CPUs without being cleared.

Signed-off-by: Maciej Wieczor-Retman <maciej.wieczor-retman@intel.com>
---
 arch/x86/kernel/head_64.S | 3 +++
 arch/x86/mm/init.c        | 3 +++
 2 files changed, 6 insertions(+)

diff --git a/arch/x86/kernel/head_64.S b/arch/x86/kernel/head_64.S
index 3e9b3a3bd039..18ca77daa481 100644
--- a/arch/x86/kernel/head_64.S
+++ b/arch/x86/kernel/head_64.S
@@ -209,6 +209,9 @@ SYM_INNER_LABEL(common_startup_64, SYM_L_LOCAL)
 	 *  there will be no global TLB entries after the execution."
 	 */
 	movl	$(X86_CR4_PAE | X86_CR4_LA57), %edx
+#ifdef CONFIG_ADDRESS_MASKING
+	orl	$X86_CR4_LAM_SUP, %edx
+#endif
 #ifdef CONFIG_X86_MCE
 	/*
 	 * Preserve CR4.MCE if the kernel will enable #MC support.
diff --git a/arch/x86/mm/init.c b/arch/x86/mm/init.c
index bb57e93b4caf..756bd96c3b8b 100644
--- a/arch/x86/mm/init.c
+++ b/arch/x86/mm/init.c
@@ -763,6 +763,9 @@ void __init init_mem_mapping(void)
 	probe_page_size_mask();
 	setup_pcid();
 
+	if (boot_cpu_has(X86_FEATURE_LAM) && IS_ENABLED(CONFIG_KASAN_SW_TAGS))
+		cr4_set_bits_and_update_boot(X86_CR4_LAM_SUP);
+
 #ifdef CONFIG_X86_64
 	end = max_pfn << PAGE_SHIFT;
 #else
-- 
2.50.1

On Mon, Aug 25, 2025 at 10:24:36PM +0200, Maciej Wieczor-Retman wrote:
> diff --git a/arch/x86/mm/init.c b/arch/x86/mm/init.c
> index bb57e93b4caf..756bd96c3b8b 100644
> --- a/arch/x86/mm/init.c
> +++ b/arch/x86/mm/init.c
> @@ -763,6 +763,9 @@ void __init init_mem_mapping(void)
>  	probe_page_size_mask();
>  	setup_pcid();
>  
> +	if (boot_cpu_has(X86_FEATURE_LAM) && IS_ENABLED(CONFIG_KASAN_SW_TAGS))

cpu_feature_enabled()

> +		cr4_set_bits_and_update_boot(X86_CR4_LAM_SUP);
> +
>  #ifdef CONFIG_X86_64
>  	end = max_pfn << PAGE_SHIFT;
>  #else
> -- 

Also, for all your patches' subjects and text:

Pls read

https://www.kernel.org/doc/html/latest/process/maintainer-tip.html#patch-subject
https://www.kernel.org/doc/html/latest/process/maintainer-tip.html#changelog

and fixup.

Thx.

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette

On 2025-09-07 at 00:24:20 +0200, Borislav Petkov wrote:
>On Mon, Aug 25, 2025 at 10:24:36PM +0200, Maciej Wieczor-Retman wrote:
>> diff --git a/arch/x86/mm/init.c b/arch/x86/mm/init.c
>> index bb57e93b4caf..756bd96c3b8b 100644
>> --- a/arch/x86/mm/init.c
>> +++ b/arch/x86/mm/init.c
>> @@ -763,6 +763,9 @@ void __init init_mem_mapping(void)
>>  	probe_page_size_mask();
>>  	setup_pcid();
>>  
>> +	if (boot_cpu_has(X86_FEATURE_LAM) && IS_ENABLED(CONFIG_KASAN_SW_TAGS))
>
>cpu_feature_enabled()
>

Thanks, I'll correct it.

>> +		cr4_set_bits_and_update_boot(X86_CR4_LAM_SUP);
>> +
>>  #ifdef CONFIG_X86_64
>>  	end = max_pfn << PAGE_SHIFT;
>>  #else
>> -- 
>
>Also, for all your patches' subjects and text:
>
>Pls read
>
>https://www.kernel.org/doc/html/latest/process/maintainer-tip.html#patch-subject
>https://www.kernel.org/doc/html/latest/process/maintainer-tip.html#changelog
>
>and fixup.

Thanks, I'll recheck all the patches with that in mind.

>
>Thx.
>
>-- 
>Regards/Gruss,
>    Boris.
>
>https://people.kernel.org/tglx/notes-about-netiquette

-- 
Kind regards
Maciej Wieczór-Retman