KVM: x86: never write to memory from kvm_vcpu_check_block()

kvm_vcpu_check_block() is called while not in TASK_RUNNING, and therefore
it cannot sleep.  Writing to guest memory is therefore forbidden, but it
can happen on AMD processors if kvm_check_nested_events() causes a vmexit.

Fortunately, all events that are caught by kvm_check_nested_events() are
also recognized by kvm_vcpu_has_events() through vendor callbacks such as
kvm_x86_interrupt_allowed() or kvm_x86_ops.nested_ops->has_events(), so
remove the call and postpone the actual processing to vcpu_block().

Opportunistically honor the return of kvm_check_nested_events().  KVM
punted on the check in kvm_vcpu_running() because the only error path is
if vmx_complete_nested_posted_interrupt() fails, in which case KVM exits
to userspace with "internal error" i.e. the VM is likely dead anyways so
it wasn't worth overloading the return of kvm_vcpu_running().

Add the check mostly so that KVM is consistent with itself; the return of
the call via kvm_apic_accept_events()=>kvm_check_nested_events() that
immediately follows  _is_ checked.
Reported-by: Maxim Levitsky <mlevitsk@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
[sean: check and handle return of kvm_check_nested_events()]
Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20220921003201.1441511-11-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/x86.c

@@ -10817,6 +10817,17 @@ static inline int vcpu_block(struct kvm_vcpu *vcpu)
 			return 1;
 	}
 
+	/*
+	 * Evaluate nested events before exiting the halted state.  This allows
+	 * the halt state to be recorded properly in the VMCS12's activity
+	 * state field (AMD does not have a similar field and a VM-Exit always
+	 * causes a spurious wakeup from HLT).
+	 */
+	if (is_guest_mode(vcpu)) {
+		if (kvm_check_nested_events(vcpu) < 0)
+			return 0;
+	}
+
 	if (kvm_apic_accept_events(vcpu) < 0)
 		return 0;
 	switch(vcpu->arch.mp_state) {
@@ -10839,9 +10850,6 @@ static inline int vcpu_block(struct kvm_vcpu *vcpu)
 
 static inline bool kvm_vcpu_running(struct kvm_vcpu *vcpu)
 {
-	if (is_guest_mode(vcpu))
-		kvm_check_nested_events(vcpu);
-
 	return (vcpu->arch.mp_state == KVM_MP_STATE_RUNNABLE &&
 		!vcpu->arch.apf.halted);
 }