This reverts commit 3267cb6d3a174ff83d6287dcd5b0047bbd912452.

LASS mitigates the Spectre based on LAM (SLAM) [1] and the previous
commit made LAM depend on LASS, so we no longer need to disable LAM at
compile time, so revert the commit that disables LAM.

Adjust USER_PTR_MAX if LAM enabled, allowing tag bits to be set for
userspace pointers. The value for the constant is defined in a way to
avoid overflow compiler warning on 32-bit config.

[1] https://download.vusec.net/papers/slam_sp24.pdf

Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: Pawan Gupta <pawan.kumar.gupta@linux.intel.com>
---
 arch/x86/Kconfig             | 1 -
 arch/x86/kernel/cpu/common.c | 5 +----
 2 files changed, 1 insertion(+), 5 deletions(-)

diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index 71019b3b54ea..2b48e916b754 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -2181,7 +2181,6 @@ config RANDOMIZE_MEMORY_PHYSICAL_PADDING
 config ADDRESS_MASKING
 	bool "Linear Address Masking support"
 	depends on X86_64
-	depends on COMPILE_TEST || !CPU_MITIGATIONS # wait for LASS
 	help
 	  Linear Address Masking (LAM) modifies the checking that is applied
 	  to 64-bit linear addresses, allowing software to use of the
diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
index 382b687ce7e2..7ae757498a6f 100644
--- a/arch/x86/kernel/cpu/common.c
+++ b/arch/x86/kernel/cpu/common.c
@@ -2553,11 +2553,8 @@ void __init arch_cpu_finalize_init(void)
 	if (IS_ENABLED(CONFIG_X86_64)) {
 		unsigned long USER_PTR_MAX = TASK_SIZE_MAX;
 
-		/*
-		 * Enable this when LAM is gated on LASS support
 		if (cpu_feature_enabled(X86_FEATURE_LAM))
-			USER_PTR_MAX = (1ul << 63) - PAGE_SIZE;
-		 */
+			USER_PTR_MAX = (-1UL >> 1) & PAGE_MASK;
 		runtime_const_init(ptr, USER_PTR_MAX);
 
 		/*
-- 
2.47.2

On 7/7/2025 1:03 AM, Kirill A. Shutemov wrote:
> This reverts commit 3267cb6d3a174ff83d6287dcd5b0047bbd912452.
> 
> LASS mitigates the Spectre based on LAM (SLAM) [1] and the previous
> commit made LAM depend on LASS, so we no longer need to disable LAM at
> compile time, so revert the commit that disables LAM.
> 

Reviewed-by: Sohil Mehta <sohil.mehta@intel.com>

You may have missed my comments in the previous revision.
https://lore.kernel.org/all/af709ffa-eb11-4de5-9ae8-a179cb99750c@intel.com/

Mainly, x86 maintainers prefer imperative tone and references such as
"previous commit" can be confusing sometimes.


> Adjust USER_PTR_MAX if LAM enabled, allowing tag bits to be set for
> userspace pointers. The value for the constant is defined in a way to
> avoid overflow compiler warning on 32-bit config.
> 
> [1] https://download.vusec.net/papers/slam_sp24.pdf
> 
> Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
> Cc: Pawan Gupta <pawan.kumar.gupta@linux.intel.com>
> ---
>  arch/x86/Kconfig             | 1 -
>  arch/x86/kernel/cpu/common.c | 5 +----
>  2 files changed, 1 insertion(+), 5 deletions(-)
>

On Tue, Jul 08, 2025 at 10:31:05PM -0700, Sohil Mehta wrote:
> On 7/7/2025 1:03 AM, Kirill A. Shutemov wrote:
> > This reverts commit 3267cb6d3a174ff83d6287dcd5b0047bbd912452.
> > 
> > LASS mitigates the Spectre based on LAM (SLAM) [1] and the previous
> > commit made LAM depend on LASS, so we no longer need to disable LAM at
> > compile time, so revert the commit that disables LAM.
> > 
> 
> Reviewed-by: Sohil Mehta <sohil.mehta@intel.com>
> 
> You may have missed my comments in the previous revision.
> https://lore.kernel.org/all/af709ffa-eb11-4de5-9ae8-a179cb99750c@intel.com/
> 
> Mainly, x86 maintainers prefer imperative tone and references such as
> "previous commit" can be confusing sometimes.

Indeed, missed. My bad.

I've merged last two patches and updated the commit message:

https://git.kernel.org/pub/scm/linux/kernel/git/kas/linux.git/commit/?h=x86/lass

I hope it is still okay to use your Reviewed-by tag.

-- 
  Kiryl Shutsemau / Kirill A. Shutemov

On 7/9/2025 4:00 AM, Kirill A. Shutemov wrote:

> I've merged last two patches and updated the commit message:
> 
> https://git.kernel.org/pub/scm/linux/kernel/git/kas/linux.git/commit/?h=x86/lass
> 
> I hope it is still okay to use your Reviewed-by tag.
> 

Yes, that should be fine.