Subject: [PATCH 1/9] arm64: Add logic to fully remove features from sanitised id registers

From: Marc Zyngier <[email-masked]>
Date: Thu, 19 Feb 2026 19:55:24 +0000

We currently make support for some features such as Pointer Auth,
SVE or S1POE a compile time decision.

However, while we hide that feature from userspace when such support
is disabled, we still leave the value provided by the HW visible to
the rest of the kernel, including KVM.

This has the potential to result in ugly state leakage, as half of
the kernel knows about the feature, and the other doesn't.

Short of completely banning such compilation options and restore
universal knowledge, introduce the possibility to fully remove such
knowledge from the sanitised id registers.

This has more or less the same effect as the idreg override that
a user can pass on the command-line, only defined at build-time.

For that purpose, we provide a new macro (FTR_CONFIG()) that defines
the behaviour of a feature, both when enabled and disabled.

At this stage, nothing is making use of this anti-feature.

Signed-off-by: Marc Zyngier <[email-masked]>
---
 arch/arm64/include/asm/cpufeature.h | 15 ++++++++++-----
 arch/arm64/kernel/cpufeature.c      | 21 ++++++++++++++++-----
 2 files changed, 26 insertions(+), 10 deletions(-)

diff --git a/arch/arm64/include/asm/cpufeature.h b/arch/arm64/include/asm/cpufeature.h
index 4de51f8d92cba..2731ea13c2c86 100644
--- a/arch/arm64/include/asm/cpufeature.h
+++ b/arch/arm64/include/asm/cpufeature.h
@@ -53,15 +53,20 @@ enum ftr_type {
 #define FTR_SIGNED	true	/* Value should be treated as signed */
 #define FTR_UNSIGNED	false	/* Value should be treated as unsigned */
 
-#define FTR_VISIBLE	true	/* Feature visible to the user space */
-#define FTR_HIDDEN	false	/* Feature is hidden from the user */
+enum ftr_visibility {
+	FTR_HIDDEN,		/* Feature hidden from the user */
+	FTR_ALL_HIDDEN,		/* Feature hidden from kernel, user and KVM */
+	FTR_VISIBLE,		/* Feature visible to all observers */
+};
+
+#define FTR_CONFIG(c, e, d)				\
+	(IS_ENABLED(c) ? FTR_ ## e : FTR_ ## d)
 
-#define FTR_VISIBLE_IF_IS_ENABLED(config)		\
-	(IS_ENABLED(config) ? FTR_VISIBLE : FTR_HIDDEN)
+#define FTR_VISIBLE_IF_IS_ENABLED(c)	FTR_CONFIG(c, VISIBLE, HIDDEN)
 
 struct arm64_ftr_bits {
 	bool		sign;	/* Value is signed ? */
-	bool		visible;
+	enum ftr_visibility visibility;
 	bool		strict;	/* CPU Sanity check: strict matching required ? */
 	enum ftr_type	type;
 	u8		shift;
diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
index c840a93b9ef95..b34a39967d111 100644
--- a/arch/arm64/kernel/cpufeature.c
+++ b/arch/arm64/kernel/cpufeature.c
@@ -192,7 +192,7 @@ void dump_cpu_features(void)
 #define __ARM64_FTR_BITS(SIGNED, VISIBLE, STRICT, TYPE, SHIFT, WIDTH, SAFE_VAL) \
 	{						\
 		.sign = SIGNED,				\
-		.visible = VISIBLE,			\
+		.visibility = VISIBLE,			\
 		.strict = STRICT,			\
 		.type = TYPE,				\
 		.shift = SHIFT,				\
@@ -1057,17 +1057,28 @@ static void init_cpu_ftr_reg(u32 sys_reg, u64 new)
 				ftrp->shift);
 		}
 
-		val = arm64_ftr_set_value(ftrp, val, ftr_new);
-
 		valid_mask |= ftr_mask;
 		if (!ftrp->strict)
 			strict_mask &= ~ftr_mask;
-		if (ftrp->visible)
+
+		switch (ftrp->visibility) {
+		case FTR_VISIBLE:
+			val = arm64_ftr_set_value(ftrp, val, ftr_new);
 			user_mask |= ftr_mask;
-		else
+			break;
+		case FTR_ALL_HIDDEN:
+			val = arm64_ftr_set_value(ftrp, val, ftrp->safe_val);
+			reg->user_val = arm64_ftr_set_value(ftrp,
+							    reg->user_val,
+							    ftrp->safe_val);
+			break;
+		case FTR_HIDDEN:
+			val = arm64_ftr_set_value(ftrp, val, ftr_new);
 			reg->user_val = arm64_ftr_set_value(ftrp,
 							    reg->user_val,
 							    ftrp->safe_val);
+			break;
+		}
 	}
 
 	val &= valid_mask;
-- 
2.47.3