KVM: arm64: Fix read-side race on updates to vcpu reset state

KVM correctly serializes writes to a vCPU's reset state, however since
we do not take the KVM lock on the read side it is entirely possible to
read state from two different reset requests.

Cure the race for now by taking the KVM lock when reading the
reset_state structure.

Fixes: 358b28f0 ("arm/arm64: KVM: Allow a VCPU to fully reset itself")
Signed-off-by: Oliver Upton <oupton@google.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20210818202133.1106786-2-oupton@google.com

arch/arm64/kvm/reset.c

@@ -210,10 +210,16 @@ static bool vcpu_allowed_register_width(struct kvm_vcpu *vcpu)
  */
 int kvm_reset_vcpu(struct kvm_vcpu *vcpu)
 {
+	struct vcpu_reset_state reset_state;
 	int ret;
 	bool loaded;
 	u32 pstate;
 
+	mutex_lock(&vcpu->kvm->lock);
+	reset_state = vcpu->arch.reset_state;
+	WRITE_ONCE(vcpu->arch.reset_state.reset, false);
+	mutex_unlock(&vcpu->kvm->lock);
+
 	/* Reset PMU outside of the non-preemptible section */
 	kvm_pmu_vcpu_reset(vcpu);
 
@@ -276,8 +282,8 @@ int kvm_reset_vcpu(struct kvm_vcpu *vcpu)
 	 * Additional reset state handling that PSCI may have imposed on us.
 	 * Must be done after all the sys_reg reset.
 	 */
-	if (vcpu->arch.reset_state.reset) {
-		unsigned long target_pc = vcpu->arch.reset_state.pc;
+	if (reset_state.reset) {
+		unsigned long target_pc = reset_state.pc;
 
 		/* Gracefully handle Thumb2 entry point */
 		if (vcpu_mode_is_32bit(vcpu) && (target_pc & 1)) {
@@ -286,13 +292,11 @@ int kvm_reset_vcpu(struct kvm_vcpu *vcpu)
 		}
 
 		/* Propagate caller endianness */
-		if (vcpu->arch.reset_state.be)
+		if (reset_state.be)
 			kvm_vcpu_set_be(vcpu);
 
 		*vcpu_pc(vcpu) = target_pc;
-		vcpu_set_reg(vcpu, 0, vcpu->arch.reset_state.r0);
-
-		vcpu->arch.reset_state.reset = false;
+		vcpu_set_reg(vcpu, 0, reset_state.r0);
 	}
 
 	/* Reset timer */