KVM: arm64: Handle PSCI resets before userspace touches vCPU state

The CPU_ON PSCI call takes a payload that KVM uses to configure a
destination vCPU to run. This payload is non-architectural state and not
exposed through any existing UAPI. Effectively, we have a race between
CPU_ON and userspace saving/restoring a guest: if the target vCPU isn't
ran again before the VMM saves its state, the requested PC and context
ID are lost. When restored, the target vCPU will be runnable and start
executing at its old PC.

We can avoid this race by making sure the reset payload is serviced
before userspace can access a vCPU's state.

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/20210818202133.1106786-3-oupton@google.com

arch/arm64/kvm/arm.c

@@ -1215,6 +1215,14 @@ long kvm_arch_vcpu_ioctl(struct file *filp,
 		if (copy_from_user(&reg, argp, sizeof(reg)))
 			break;
 
+		/*
+		 * We could owe a reset due to PSCI. Handle the pending reset
+		 * here to ensure userspace register accesses are ordered after
+		 * the reset.
+		 */
+		if (kvm_check_request(KVM_REQ_VCPU_RESET, vcpu))
+			kvm_reset_vcpu(vcpu);
+
 		if (ioctl == KVM_SET_ONE_REG)
 			r = kvm_arm_set_reg(vcpu, &reg);
 		else