linux-kernel - [PATCH v10 15/15] x86/cpu: Enable LASS by default during CPU initialization

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Message-ID: <20251007065119.148605-16-sohil.mehta@intel.com>
Date: Mon,  6 Oct 2025 23:51:19 -0700
From: Sohil Mehta <sohil.mehta@...el.com>
To: x86@...nel.org,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	Borislav Petkov <bp@...en8.de>
Cc: Jonathan Corbet <corbet@....net>,
	"H . Peter Anvin" <hpa@...or.com>,
	Andy Lutomirski <luto@...nel.org>,
	Josh Poimboeuf <jpoimboe@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Ard Biesheuvel <ardb@...nel.org>,
	"Kirill A . Shutemov" <kas@...nel.org>,
	Sohil Mehta <sohil.mehta@...el.com>,
	Xin Li <xin@...or.com>,
	David Woodhouse <dwmw@...zon.co.uk>,
	Sean Christopherson <seanjc@...gle.com>,
	Rick Edgecombe <rick.p.edgecombe@...el.com>,
	Vegard Nossum <vegard.nossum@...cle.com>,
	Andrew Cooper <andrew.cooper3@...rix.com>,
	David Laight <david.laight.linux@...il.com>,
	Randy Dunlap <rdunlap@...radead.org>,
	Geert Uytterhoeven <geert@...ux-m68k.org>,
	Kees Cook <kees@...nel.org>,
	Tony Luck <tony.luck@...el.com>,
	Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
	linux-doc@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	linux-efi@...r.kernel.org
Subject: [PATCH v10 15/15] x86/cpu: Enable LASS by default during CPU initialization

Linear Address Space Separation (LASS) mitigates a class of side-channel
attacks that rely on speculative access across the user/kernel boundary.
Enable it by default if the platform supports it.

While at it, remove the comment above the SMAP/SMEP/UMIP/LASS setup
instead of updating it, as the whole sequence is quite self-explanatory.

Signed-off-by: Sohil Mehta <sohil.mehta@...el.com>
---
v10
 - No change.
---
 arch/x86/kernel/cpu/common.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
index 57d5824465b0..7f0f1b56cbe7 100644
--- a/arch/x86/kernel/cpu/common.c
+++ b/arch/x86/kernel/cpu/common.c
@@ -401,6 +401,12 @@ static __always_inline void setup_umip(struct cpuinfo_x86 *c)
 	cr4_clear_bits(X86_CR4_UMIP);
 }
 
+static __always_inline void setup_lass(struct cpuinfo_x86 *c)
+{
+	if (cpu_feature_enabled(X86_FEATURE_LASS))
+		cr4_set_bits(X86_CR4_LASS);
+}
+
 /* These bits should not change their value after CPU init is finished. */
 static const unsigned long cr4_pinned_mask = X86_CR4_SMEP | X86_CR4_SMAP | X86_CR4_UMIP |
 					     X86_CR4_FSGSBASE | X86_CR4_CET | X86_CR4_FRED |
@@ -2019,10 +2025,10 @@ static void identify_cpu(struct cpuinfo_x86 *c)
 	/* Disable the PN if appropriate */
 	squash_the_stupid_serial_number(c);
 
-	/* Set up SMEP/SMAP/UMIP */
 	setup_smep(c);
 	setup_smap(c);
 	setup_umip(c);
+	setup_lass(c);
 
 	/* Enable FSGSBASE instructions if available. */
 	if (cpu_has(c, X86_FEATURE_FSGSBASE)) {
-- 
2.43.0