Re: [PATCHv8 17/17] x86: Re-enable Linear Address Masking

[Sohil Mehta <sohil.mehta@intel.com>]

On 7/1/2025 2:58 AM, Kirill A. Shutemov wrote:
> This reverts commit 3267cb6d3a174ff83d6287dcd5b0047bbd912452.
> 

This patch isn't truly a revert. This line can be skipped since the
additional changes going in are more than the reverted single line.

> 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.

Also, wording such as previous commit should be avoided since it can be
misleading. For example, in this series "the previous commit" is
enabling LASS. The commit before that adds the actual dependency between
LAM and LASS.

Other than that, the code changes look good to me.

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

> 
> 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 97a228f917a9..6f2ae9e702bc 100644
> --- a/arch/x86/kernel/cpu/common.c
> +++ b/arch/x86/kernel/cpu/common.c
> @@ -2558,11 +2558,8 @@ 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);
>  
>  		/*