x86/reboot: Harden virtualization hooks for emergency reboot

Provide dedicated helpers to (un)register virt hooks used during an
emergency crash/reboot, and WARN if there is an attempt to overwrite
the registered callback, or an attempt to do an unpaired unregister.

Opportunsitically use rcu_assign_pointer() instead of RCU_INIT_POINTER(),
mainly so that the set/unset paths are more symmetrical, but also because
any performance gains from using RCU_INIT_POINTER() are meaningless for
this code.

Reviewed-by: Kai Huang <kai.huang@intel.com>
Link: https://lore.kernel.org/r/20230721201859.2307736-3-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

arch/x86/include/asm/reboot.h

@@ -25,8 +25,9 @@ void __noreturn machine_real_restart(unsigned int type);
 #define MRR_BIOS	0
 #define MRR_APM		1
 
-typedef void crash_vmclear_fn(void);
-extern crash_vmclear_fn __rcu *crash_vmclear_loaded_vmcss;
+typedef void (cpu_emergency_virt_cb)(void);
+void cpu_emergency_register_virt_callback(cpu_emergency_virt_cb *callback);
+void cpu_emergency_unregister_virt_callback(cpu_emergency_virt_cb *callback);
 void cpu_emergency_disable_virtualization(void);
 
 typedef void (*nmi_shootdown_cb)(int, struct pt_regs*);

arch/x86/kernel/reboot.c

@@ -794,17 +794,35 @@ void machine_crash_shutdown(struct pt_regs *regs)
  *
  * protected by rcu.
  */
-crash_vmclear_fn __rcu *crash_vmclear_loaded_vmcss;
-EXPORT_SYMBOL_GPL(crash_vmclear_loaded_vmcss);
+static cpu_emergency_virt_cb __rcu *cpu_emergency_virt_callback;
+
+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);
+}
+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();
 }
 

arch/x86/kvm/vmx/vmx.c

@@ -8571,8 +8571,7 @@ 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();
 }
@@ -8620,8 +8619,7 @@ 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();