Re: [PATCH v2 03/18] x86/reboot: Harden virtualization hooks for emergency reboot

[Chao Gao]

On Fri, Mar 10, 2023 at 01:42:17PM -0800, Sean Christopherson wrote:
>+void cpu_emergency_register_virt_callback(cpu_emergency_virt_cb *callback)
>+{
>+	if (WARN_ON_ONCE(rcu_access_pointer(cpu_emergency_virt_callback)))
>+		return;
>+
>+	rcu_assign_pointer(cpu_emergency_virt_callback, callback);

Was it intentional to not call synchronize_rcu() (in the original
code), different from the un-registration path?

>+}
>+EXPORT_SYMBOL_GPL(cpu_emergency_register_virt_callback);
>+
>+void cpu_emergency_unregister_virt_callback(cpu_emergency_virt_cb *callback)
>+{
>+	if (WARN_ON_ONCE(rcu_access_pointer(cpu_emergency_virt_callback) != callback))
>+		return;
>+
>+	rcu_assign_pointer(cpu_emergency_virt_callback, NULL);
>+	synchronize_rcu();
>+}
>+EXPORT_SYMBOL_GPL(cpu_emergency_unregister_virt_callback);
> 
> static inline void cpu_crash_vmclear_loaded_vmcss(void)
> {
>-	crash_vmclear_fn *do_vmclear_operation = NULL;
>+	cpu_emergency_virt_cb *callback;
> 
> 	rcu_read_lock();
>-	do_vmclear_operation = rcu_dereference(crash_vmclear_loaded_vmcss);
>-	if (do_vmclear_operation)
>-		do_vmclear_operation();
>+	callback = rcu_dereference(cpu_emergency_virt_callback);
>+	if (callback)
>+		callback();
> 	rcu_read_unlock();
> }
> #endif
>diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
>index 302086255be6..41095fc864f3 100644
>--- a/arch/x86/kvm/vmx/vmx.c
>+++ b/arch/x86/kvm/vmx/vmx.c
>@@ -8551,8 +8551,7 @@ static void __vmx_exit(void)
> {
> 	allow_smaller_maxphyaddr = false;
> 
>-	RCU_INIT_POINTER(crash_vmclear_loaded_vmcss, NULL);
>-	synchronize_rcu();
>+	cpu_emergency_unregister_virt_callback(crash_vmclear_local_loaded_vmcss);
> 
> 	vmx_cleanup_l1d_flush();
> }
>@@ -8602,8 +8601,7 @@ static int __init vmx_init(void)
> 		pi_init_cpu(cpu);
> 	}
> 
>-	rcu_assign_pointer(crash_vmclear_loaded_vmcss,
>-			   crash_vmclear_local_loaded_vmcss);
>+	cpu_emergency_register_virt_callback(crash_vmclear_local_loaded_vmcss);
> 
> 	vmx_check_vmcs12_offsets();
> 
>-- 
>2.40.0.rc1.284.g88254d51c5-goog
>