KVM: arm64: Use arm64_ftr_bits to sanitise ID register writes

Rather than reinventing the wheel in KVM to do ID register sanitisation
we can rely on the work already done in the core kernel. Implement a
generalized sanitisation of ID registers based on the combination of the
arm64_ftr_bits definitions from the core kernel and (optionally) a set
of KVM-specific overrides.

This all amounts to absolutely nothing for now, but will be used in
subsequent changes to realize user-configurable ID registers.
Signed-off-by: Jing Zhang <jingzhangos@google.com>
Link: https://lore.kernel.org/r/20230609190054.1542113-8-oliver.upton@linux.dev
[Oliver: split off from monster patch, rewrote commit description,
 reworked RAZ handling, return EINVAL to userspace]
Signed-off-by: Oliver Upton <oliver.upton@linux.dev>

arch/arm64/include/asm/cpufeature.h

@@ -915,6 +915,7 @@ static inline unsigned int get_vmid_bits(u64 mmfr1)
 	return 8;
 }
 
+s64 arm64_ftr_safe_value(const struct arm64_ftr_bits *ftrp, s64 new, s64 cur);
 struct arm64_ftr_reg *get_arm64_ftr_reg(u32 sys_id);
 
 extern struct arm64_ftr_override id_aa64mmfr1_override;

arch/arm64/kernel/cpufeature.c

@@ -798,7 +798,7 @@ static u64 arm64_ftr_set_value(const struct arm64_ftr_bits *ftrp, s64 reg,
 	return reg;
 }
 
-static s64 arm64_ftr_safe_value(const struct arm64_ftr_bits *ftrp, s64 new,
+s64 arm64_ftr_safe_value(const struct arm64_ftr_bits *ftrp, s64 new,
 				s64 cur)
 {
 	s64 ret = 0;

arch/arm64/kvm/sys_regs.c

@@ -1201,6 +1201,91 @@ static u8 vcpu_pmuver(const struct kvm_vcpu *vcpu)
 	return 0;
 }
 
+static s64 kvm_arm64_ftr_safe_value(u32 id, const struct arm64_ftr_bits *ftrp,
+				    s64 new, s64 cur)
+{
+	struct arm64_ftr_bits kvm_ftr = *ftrp;
+
+	/* Some features have different safe value type in KVM than host features */
+	switch (id) {
+	case SYS_ID_AA64DFR0_EL1:
+		if (kvm_ftr.shift == ID_AA64DFR0_EL1_PMUVer_SHIFT)
+			kvm_ftr.type = FTR_LOWER_SAFE;
+		break;
+	case SYS_ID_DFR0_EL1:
+		if (kvm_ftr.shift == ID_DFR0_EL1_PerfMon_SHIFT)
+			kvm_ftr.type = FTR_LOWER_SAFE;
+		break;
+	}
+
+	return arm64_ftr_safe_value(&kvm_ftr, new, cur);
+}
+
+/**
+ * arm64_check_features() - Check if a feature register value constitutes
+ * a subset of features indicated by the idreg's KVM sanitised limit.
+ *
+ * This function will check if each feature field of @val is the "safe" value
+ * against idreg's KVM sanitised limit return from reset() callback.
+ * If a field value in @val is the same as the one in limit, it is always
+ * considered the safe value regardless For register fields that are not in
+ * writable, only the value in limit is considered the safe value.
+ *
+ * Return: 0 if all the fields are safe. Otherwise, return negative errno.
+ */
+static int arm64_check_features(struct kvm_vcpu *vcpu,
+				const struct sys_reg_desc *rd,
+				u64 val)
+{
+	const struct arm64_ftr_reg *ftr_reg;
+	const struct arm64_ftr_bits *ftrp = NULL;
+	u32 id = reg_to_encoding(rd);
+	u64 writable_mask = rd->val;
+	u64 limit = rd->reset(vcpu, rd);
+	u64 mask = 0;
+
+	/*
+	 * Hidden and unallocated ID registers may not have a corresponding
+	 * struct arm64_ftr_reg. Of course, if the register is RAZ we know the
+	 * only safe value is 0.
+	 */
+	if (sysreg_visible_as_raz(vcpu, rd))
+		return val ? -E2BIG : 0;
+
+	ftr_reg = get_arm64_ftr_reg(id);
+	if (!ftr_reg)
+		return -EINVAL;
+
+	ftrp = ftr_reg->ftr_bits;
+
+	for (; ftrp && ftrp->width; ftrp++) {
+		s64 f_val, f_lim, safe_val;
+		u64 ftr_mask;
+
+		ftr_mask = arm64_ftr_mask(ftrp);
+		if ((ftr_mask & writable_mask) != ftr_mask)
+			continue;
+
+		f_val = arm64_ftr_value(ftrp, val);
+		f_lim = arm64_ftr_value(ftrp, limit);
+		mask |= ftr_mask;
+
+		if (f_val == f_lim)
+			safe_val = f_val;
+		else
+			safe_val = kvm_arm64_ftr_safe_value(id, ftrp, f_val, f_lim);
+
+		if (safe_val != f_val)
+			return -E2BIG;
+	}
+
+	/* For fields that are not writable, values in limit are the safe values. */
+	if ((val & ~mask) != (limit & ~mask))
+		return -E2BIG;
+
+	return 0;
+}
+
 static u8 perfmon_to_pmuver(u8 perfmon)
 {
 	switch (perfmon) {
@@ -1528,11 +1613,41 @@ static int get_id_reg(struct kvm_vcpu *vcpu, const struct sys_reg_desc *rd,
 static int set_id_reg(struct kvm_vcpu *vcpu, const struct sys_reg_desc *rd,
 		      u64 val)
 {
-	/* This is what we mean by invariant: you can't change it. */
-	if (val != read_id_reg(vcpu, rd))
-		return -EINVAL;
+	u32 id = reg_to_encoding(rd);
+	int ret;
 
-	return 0;
+	mutex_lock(&vcpu->kvm->arch.config_lock);
+
+	/*
+	 * Once the VM has started the ID registers are immutable. Reject any
+	 * write that does not match the final register value.
+	 */
+	if (kvm_vm_has_ran_once(vcpu->kvm)) {
+		if (val != read_id_reg(vcpu, rd))
+			ret = -EBUSY;
+		else
+			ret = 0;
+
+		mutex_unlock(&vcpu->kvm->arch.config_lock);
+		return ret;
+	}
+
+	ret = arm64_check_features(vcpu, rd, val);
+	if (!ret)
+		IDREG(vcpu->kvm, id) = val;
+
+	mutex_unlock(&vcpu->kvm->arch.config_lock);
+
+	/*
+	 * arm64_check_features() returns -E2BIG to indicate the register's
+	 * feature set is a superset of the maximally-allowed register value.
+	 * While it would be nice to precisely describe this to userspace, the
+	 * existing UAPI for KVM_SET_ONE_REG has it that invalid register
+	 * writes return -EINVAL.
+	 */
+	if (ret == -E2BIG)
+		ret = -EINVAL;
+	return ret;
 }
 
 static int get_raz_reg(struct kvm_vcpu *vcpu, const struct sys_reg_desc *rd,