linux-kernel - [PATCHv6 16/16] x86: Re-enable Linear Address Masking

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Message-ID: <20250620135325.3300848-17-kirill.shutemov@linux.intel.com>
Date: Fri, 20 Jun 2025 16:53:24 +0300
From: "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
To: Andy Lutomirski <luto@...nel.org>,
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	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
Subject: [PATCHv6 16/16] x86: Re-enable Linear Address Masking

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@...ux.intel.com>
Cc: Pawan Gupta <pawan.kumar.gupta@...ux.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 edb342214328..0adf1b4722b8 100644
--- a/arch/x86/kernel/cpu/common.c
+++ b/arch/x86/kernel/cpu/common.c
@@ -2565,11 +2565,8 @@ 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