[PATCH v2 1/2] KVM: arm64: mixed-width check should be skipped for uninitialized vCPUs

[Reiji Watanabe]

KVM allows userspace to configure either all 32bit or 64bit vCPUs
for a guest.  At vCPU reset, vcpu_allowed_register_width() checks
if the vcpu's register width is consistent with all other vCPUs'.
Since the checking is done even against vCPUs that are not initialized
(KVM_ARM_VCPU_INIT has not been done) yet, the uninitialized vCPUs
are erroneously treated as 64bit vCPU, which causes the function to
incorrectly detect a mixed-width VM.

Introduce a new flag (el1_reg_width) in kvm_arch to indicates that
the guest needs to be configured with all 32bit or 64bit vCPUs,
and initialize it at the first KVM_ARM_VCPU_INIT for the guest.
Check vcpu's register width against the flag at the vcpu's
KVM_ARM_VCPU_INIT (instead of against other vCPUs' register width).

Fixes: 66e94d5cafd4 ("KVM: arm64: Prevent mixed-width VM creation")
Signed-off-by: Reiji Watanabe <reijiw at google.com>
---
 arch/arm64/include/asm/kvm_host.h | 13 +++++++++++++
 arch/arm64/kvm/arm.c              | 30 ++++++++++++++++++++++++++++++
 arch/arm64/kvm/reset.c            |  8 --------
 3 files changed, 43 insertions(+), 8 deletions(-)

diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h
index 2a5f7f38006f..c02b7caf2c82 100644
--- a/arch/arm64/include/asm/kvm_host.h
+++ b/arch/arm64/include/asm/kvm_host.h
@@ -102,6 +102,12 @@ struct kvm_s2_mmu {
 struct kvm_arch_memory_slot {
 };
 
+enum kvm_el1_reg_width {
+	EL1_WIDTH_UNINITIALIZED = 0,
+	EL1_32BIT,
+	EL1_64BIT,
+};
+
 struct kvm_arch {
 	struct kvm_s2_mmu mmu;
 
@@ -137,6 +143,13 @@ struct kvm_arch {
 
 	/* Memory Tagging Extension enabled for the guest */
 	bool mte_enabled;
+
+	/*
+	 * EL1 register width for the guest.
+	 * This is set at the first KVM_ARM_VCPU_INIT for the guest based
+	 * on whether the vcpu has KVM_ARM_VCPU_EL1_32BIT or not.
+	 */
+	enum kvm_el1_reg_width el1_reg_width;
 };
 
 struct kvm_vcpu_fault_info {
diff --git a/arch/arm64/kvm/arm.c b/arch/arm64/kvm/arm.c
index e4727dc771bf..54ae8bf9d187 100644
--- a/arch/arm64/kvm/arm.c
+++ b/arch/arm64/kvm/arm.c
@@ -1058,6 +1058,32 @@ int kvm_vm_ioctl_irq_line(struct kvm *kvm, struct kvm_irq_level *irq_level,
 	return -EINVAL;
 }
 
+/*
+ * A guest can have either all 32bit or 64bit vcpus only.
+ * Either one the guest has is indicated in kvm->arch.el1_reg_width.
+ * Check if the vcpu's register width is consistent with
+ * kvm->arch.el1_reg_width.  If kvm->arch.el1_reg_width is not set yet,
+ * set it based on the vcpu's KVM_ARM_VCPU_EL1_32BIT configuration.
+ */
+static int kvm_register_width_check_or_init(struct kvm_vcpu *vcpu)
+{
+	bool is32bit;
+	bool allowed = true;
+	struct kvm *kvm = vcpu->kvm;
+
+	is32bit = vcpu_has_feature(vcpu, KVM_ARM_VCPU_EL1_32BIT);
+
+	mutex_lock(&kvm->lock);
+
+	if (kvm->arch.el1_reg_width == EL1_WIDTH_UNINITIALIZED)
+		kvm->arch.el1_reg_width = is32bit ? EL1_32BIT : EL1_64BIT;
+	else
+		allowed = (is32bit == (kvm->arch.el1_reg_width == EL1_32BIT));
+
+	mutex_unlock(&kvm->lock);
+	return allowed ? 0 : -EINVAL;
+}
+
 static int kvm_vcpu_set_target(struct kvm_vcpu *vcpu,
 			       const struct kvm_vcpu_init *init)
 {
@@ -1097,6 +1123,10 @@ static int kvm_vcpu_set_target(struct kvm_vcpu *vcpu,
 
 	/* Now we know what it is, we can reset it. */
 	ret = kvm_reset_vcpu(vcpu);
+
+	if (!ret)
+		ret = kvm_register_width_check_or_init(vcpu);
+
 	if (ret) {
 		vcpu->arch.target = -1;
 		bitmap_zero(vcpu->arch.features, KVM_VCPU_MAX_FEATURES);
diff --git a/arch/arm64/kvm/reset.c b/arch/arm64/kvm/reset.c
index 426bd7fbc3fd..dbf2939a6a96 100644
--- a/arch/arm64/kvm/reset.c
+++ b/arch/arm64/kvm/reset.c
@@ -168,9 +168,7 @@ static int kvm_vcpu_enable_ptrauth(struct kvm_vcpu *vcpu)
 
 static bool vcpu_allowed_register_width(struct kvm_vcpu *vcpu)
 {
-	struct kvm_vcpu *tmp;
 	bool is32bit;
-	int i;
 
 	is32bit = vcpu_has_feature(vcpu, KVM_ARM_VCPU_EL1_32BIT);
 	if (!cpus_have_const_cap(ARM64_HAS_32BIT_EL1) && is32bit)
@@ -180,12 +178,6 @@ static bool vcpu_allowed_register_width(struct kvm_vcpu *vcpu)
 	if (kvm_has_mte(vcpu->kvm) && is32bit)
 		return false;
 
-	/* Check that the vcpus are either all 32bit or all 64bit */
-	kvm_for_each_vcpu(i, tmp, vcpu->kvm) {
-		if (vcpu_has_feature(tmp, KVM_ARM_VCPU_EL1_32BIT) != is32bit)
-			return false;
-	}
-
 	return true;
 }
 

base-commit: 37144b2c855f9311c72f292125061d4a52d02856
-- 
2.34.1.703.g22d0c6ccf7-goog