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>

1 files changed, 24 insertions, 6 deletions

diff --git a/arch/x86/kernel/reboot.c b/arch/x86/kernel/reboot.c
index 3fa4c6717a1d..62ccedeb5e2b 100644
--- a/arch/x86/kernel/reboot.c
+++ b/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();
 }