x86/fpu: Fix irq_fpu_usable() to return false during CPU onlining

index 948b4f5fad99c..6495259a23962 100644
@@ -43,11 +43,11 @@ struct fpu_state_config fpu_user_cfg __ro_after_init;
  * depending on the FPU hardware format:
  */
 struct fpstate init_fpstate __ro_after_init;
 
 /* Track in-kernel FPU usage */
-static DEFINE_PER_CPU(bool, in_kernel_fpu);
+static DEFINE_PER_CPU(bool, kernel_fpu_allowed) = true;
 
 /*
  * Track which context is using the FPU on the CPU:
  */
 DEFINE_PER_CPU(struct fpu *, fpu_fpregs_owner_ctx);

@@ -75,11 +75,11 @@ bool irq_fpu_usable(void)
 	 * In kernel FPU usage already active?  This detects any explicitly
 	 * nested usage in task or softirq context, which is unsupported.  It
 	 * also detects attempted usage in a hardirq that has interrupted a
 	 * kernel-mode FPU section.
 	 */
-	if (this_cpu_read(in_kernel_fpu)) {
+	if (!this_cpu_read(kernel_fpu_allowed)) {
 		WARN_ON_FPU(!in_hardirq());
 		return false;
 	}
 
 	/*

@@ -437,13 +437,14 @@ void kernel_fpu_begin_mask(unsigned int kfpu_mask)
 {
 	if (!irqs_disabled())
 		fpregs_lock();
 
 	WARN_ON_FPU(!irq_fpu_usable());
-	WARN_ON_FPU(this_cpu_read(in_kernel_fpu));
 
-	this_cpu_write(in_kernel_fpu, true);
+	/* Toggle kernel_fpu_allowed to false: */
+	WARN_ON_FPU(!this_cpu_read(kernel_fpu_allowed));
+	this_cpu_write(kernel_fpu_allowed, false);
 
 	if (!(current->flags & (PF_KTHREAD | PF_USER_WORKER)) &&
 	    !test_thread_flag(TIF_NEED_FPU_LOAD)) {
 		set_thread_flag(TIF_NEED_FPU_LOAD);
 		save_fpregs_to_fpstate(x86_task_fpu(current));

@@ -459,13 +460,14 @@ void kernel_fpu_begin_mask(unsigned int kfpu_mask)
 }
 EXPORT_SYMBOL_GPL(kernel_fpu_begin_mask);
 
 void kernel_fpu_end(void)
 {
-	WARN_ON_FPU(!this_cpu_read(in_kernel_fpu));
+	/* Toggle kernel_fpu_allowed back to true: */
+	WARN_ON_FPU(this_cpu_read(kernel_fpu_allowed));
+	this_cpu_write(kernel_fpu_allowed, true);
 
-	this_cpu_write(in_kernel_fpu, false);
 	if (!irqs_disabled())
 		fpregs_unlock();
 }
 EXPORT_SYMBOL_GPL(kernel_fpu_end);
 

index 8e6848f55dcdb..2983acd95f5de 100644
@@ -116,10 +116,11 @@ extern void fpu_reset_from_exception_fixup(void);
 /* Boot, hotplug and resume */
 extern void fpu__init_cpu(void);
 extern void fpu__init_system(void);
 extern void fpu__init_check_bugs(void);
 extern void fpu__resume_cpu(void);
+extern void fpu__disable_cpu(void);
 
 #ifdef CONFIG_MATH_EMULATION
 extern void fpstate_init_soft(struct swregs_state *soft);
 #else
 static inline void fpstate_init_soft(struct swregs_state *soft) {}

index 6495259a23962..ea138583dd92a 100644
@@ -42,12 +42,15 @@ struct fpu_state_config fpu_user_cfg __ro_after_init;
  * Represents the initial FPU state. It's mostly (but not completely) zeroes,
  * depending on the FPU hardware format:
  */
 struct fpstate init_fpstate __ro_after_init;
 
-/* Track in-kernel FPU usage */
-static DEFINE_PER_CPU(bool, kernel_fpu_allowed) = true;
+/*
+ * Track FPU initialization and kernel-mode usage. 'true' means the FPU is
+ * initialized and is not currently being used by the kernel:
+ */
+DEFINE_PER_CPU(bool, kernel_fpu_allowed);
 
 /*
  * Track which context is using the FPU on the CPU:
  */
 DEFINE_PER_CPU(struct fpu *, fpu_fpregs_owner_ctx);

@@ -70,19 +73,22 @@ bool irq_fpu_usable(void)
 {
 	if (WARN_ON_ONCE(in_nmi()))
 		return false;
 
 	/*
-	 * In kernel FPU usage already active?  This detects any explicitly
-	 * nested usage in task or softirq context, which is unsupported.  It
-	 * also detects attempted usage in a hardirq that has interrupted a
-	 * kernel-mode FPU section.
+	 * Return false in the following cases:
+	 *
+	 * - FPU is not yet initialized. This can happen only when the call is
+	 *   coming from CPU onlining, for example for microcode checksumming.
+	 * - The kernel is already using the FPU, either because of explicit
+	 *   nesting (which should never be done), or because of implicit
+	 *   nesting when a hardirq interrupted a kernel-mode FPU section.
+	 *
+	 * The single boolean check below handles both cases:
 	 */
-	if (!this_cpu_read(kernel_fpu_allowed)) {
-		WARN_ON_FPU(!in_hardirq());
+	if (!this_cpu_read(kernel_fpu_allowed))
 		return false;
-	}
 
 	/*
 	 * When not in NMI or hard interrupt context, FPU can be used in:
 	 *
 	 * - Task context except from within fpregs_lock()'ed critical

index 6bb3e35c40e24..c581a3e452dfd 100644
@@ -49,10 +49,23 @@ static void fpu__init_cpu_generic(void)
  */
 void fpu__init_cpu(void)
 {
 	fpu__init_cpu_generic();
 	fpu__init_cpu_xstate();
+
+	/* Start allowing kernel-mode FPU: */
+	WARN_ON_FPU(this_cpu_read(kernel_fpu_allowed));
+	this_cpu_write(kernel_fpu_allowed, true);
+}
+
+/*
+ * Stop allowing kernel-mode FPU. Called when a CPU is brought offline:
+ */
+void fpu__disable_cpu(void)
+{
+	WARN_ON_FPU(!this_cpu_read(kernel_fpu_allowed));
+	this_cpu_write(kernel_fpu_allowed, false);
 }
 
 static bool __init fpu__probe_without_cpuid(void)
 {
 	unsigned long cr0;

index 975de070c9c98..9782152d609c7 100644
@@ -2,10 +2,12 @@
 #ifndef __X86_KERNEL_FPU_INTERNAL_H
 #define __X86_KERNEL_FPU_INTERNAL_H
 
 extern struct fpstate init_fpstate;
 
+DECLARE_PER_CPU(bool, kernel_fpu_allowed);
+
 /* CPU feature check wrappers */
 static __always_inline __pure bool use_xsave(void)
 {
 	return cpu_feature_enabled(X86_FEATURE_XSAVE);
 }

index d7d61b3de2bf6..cf42a7632dd49 100644
@@ -1186,10 +1186,16 @@ void cpu_disable_common(void)
 {
 	int cpu = smp_processor_id();
 
 	remove_siblinginfo(cpu);
 
+	/*
+	 * Stop allowing kernel-mode FPU. This is needed so that if the CPU is
+	 * brought online again, the initial state is not allowed:
+	 */
+	fpu__disable_cpu();
+
 	/* It's now safe to remove this processor from the online map */
 	lock_vector_lock();
 	remove_cpu_from_maps(cpu);
 	unlock_vector_lock();
 	fixup_irqs();