KVM: arm64: Extend masking facility to arbitrary registers

We currently only use the masking (RES0/RES1) facility for VNCR
registers, as they are memory-based and thus easy to sanitise.

But we could apply the same thing to other registers if we:

- split the sanitisation from __VNCR_START__
- apply the sanitisation when reading from a HW register

This involves a new "marker" in the vcpu_sysreg enum, which
defines the point at which the sanitisation applies (the VNCR
registers being of course after this marker).

Whle we are at it, rename kvm_vcpu_sanitise_vncr_reg() to
kvm_vcpu_apply_reg_masks(), which is vaguely more explicit,
and harden set_sysreg_masks() against setting masks for
random registers...

Signed-off-by: Marc Zyngier <maz@kernel.org>
Reviewed-by: Joey Gouly <joey.gouly@arm.com>
Link: https://lore.kernel.org/r/20241023145345.1613824-10-maz@kernel.org
Signed-off-by: Oliver Upton <oliver.upton@linux.dev>

Author: Marc Zyngier

arch/arm64/include/asm/kvm_host.h

@@ -374,7 +374,7 @@ struct kvm_arch {
 
 	u64 ctr_el0;
 
-	/* Masks for VNCR-baked sysregs */
+	/* Masks for VNCR-backed and general EL2 sysregs */
 	struct kvm_sysreg_masks	*sysreg_masks;
 
 	/*
@@ -408,6 +408,9 @@ struct kvm_vcpu_fault_info {
 	r = __VNCR_START__ + ((VNCR_ ## r) / 8),	\
 	__after_##r = __MAX__(__before_##r - 1, r)
 
+#define MARKER(m)				\
+	m, __after_##m = m - 1
+
 enum vcpu_sysreg {
 	__INVALID_SYSREG__,   /* 0 is reserved as an invalid value */
 	MPIDR_EL1,	/* MultiProcessor Affinity Register */
@@ -494,7 +497,11 @@ enum vcpu_sysreg {
 	CNTHV_CTL_EL2,
 	CNTHV_CVAL_EL2,
 
-	__VNCR_START__,	/* Any VNCR-capable reg goes after this point */
+	/* Anything from this can be RES0/RES1 sanitised */
+	MARKER(__SANITISED_REG_START__),
+
+	/* Any VNCR-capable reg goes after this point */
+	MARKER(__VNCR_START__),
 
 	VNCR(SCTLR_EL1),/* System Control Register */
 	VNCR(ACTLR_EL1),/* Auxiliary Control Register */
@@ -554,7 +561,7 @@ struct kvm_sysreg_masks {
 	struct {
 		u64	res0;
 		u64	res1;
-	} mask[NR_SYS_REGS - __VNCR_START__];
+	} mask[NR_SYS_REGS - __SANITISED_REG_START__];
 };
 
 struct kvm_cpu_context {
@@ -1002,13 +1009,13 @@ static inline u64 *___ctxt_sys_reg(const struct kvm_cpu_context *ctxt, int r)
 
 #define ctxt_sys_reg(c,r)	(*__ctxt_sys_reg(c,r))
 
-u64 kvm_vcpu_sanitise_vncr_reg(const struct kvm_vcpu *, enum vcpu_sysreg);
+u64 kvm_vcpu_apply_reg_masks(const struct kvm_vcpu *, enum vcpu_sysreg, u64);
 #define __vcpu_sys_reg(v,r)						\
 	(*({								\
 		const struct kvm_cpu_context *ctxt = &(v)->arch.ctxt;	\
 		u64 *__r = __ctxt_sys_reg(ctxt, (r));			\
-		if (vcpu_has_nv((v)) && (r) >= __VNCR_START__)		\
-			*__r = kvm_vcpu_sanitise_vncr_reg((v), (r));	\
+		if (vcpu_has_nv((v)) && (r) >= __SANITISED_REG_START__)	\
+			*__r = kvm_vcpu_apply_reg_masks((v), (r), *__r);\
 		__r;							\
 	}))
 

arch/arm64/kvm/nested.c

@@ -933,15 +933,15 @@ static void limit_nv_id_regs(struct kvm *kvm)
 	kvm_set_vm_id_reg(kvm, SYS_ID_AA64DFR0_EL1, val);
 }
 
-u64 kvm_vcpu_sanitise_vncr_reg(const struct kvm_vcpu *vcpu, enum vcpu_sysreg sr)
+u64 kvm_vcpu_apply_reg_masks(const struct kvm_vcpu *vcpu,
+			     enum vcpu_sysreg sr, u64 v)
 {
-	u64 v = ctxt_sys_reg(&vcpu->arch.ctxt, sr);
 	struct kvm_sysreg_masks *masks;
 
 	masks = vcpu->kvm->arch.sysreg_masks;
 
 	if (masks) {
-		sr -= __VNCR_START__;
+		sr -= __SANITISED_REG_START__;
 
 		v &= ~masks->mask[sr].res0;
 		v |= masks->mask[sr].res1;
@@ -952,7 +952,11 @@ u64 kvm_vcpu_sanitise_vncr_reg(const struct kvm_vcpu *vcpu, enum vcpu_sysreg sr)
 
 static void set_sysreg_masks(struct kvm *kvm, int sr, u64 res0, u64 res1)
 {
-	int i = sr - __VNCR_START__;
+	int i = sr - __SANITISED_REG_START__;
+
+	BUILD_BUG_ON(!__builtin_constant_p(sr));
+	BUILD_BUG_ON(sr < __SANITISED_REG_START__);
+	BUILD_BUG_ON(sr >= NR_SYS_REGS);
 
 	kvm->arch.sysreg_masks->mask[i].res0 = res0;
 	kvm->arch.sysreg_masks->mask[i].res1 = res1;

arch/arm64/kvm/sys_regs.c

@@ -189,6 +189,9 @@ u64 vcpu_read_sys_reg(const struct kvm_vcpu *vcpu, int reg)
 
 		/* Get the current version of the EL1 counterpart. */
 		WARN_ON(!__vcpu_read_sys_reg_from_cpu(el1r, &val));
+		if (reg >= __SANITISED_REG_START__)
+			val = kvm_vcpu_apply_reg_masks(vcpu, reg, val);
+
 		return val;
 	}