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

Sohil Mehta
2025-10-07T06:54:08Z

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@intel.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