x86/fpu: Add debug check to kernel_fpu_disable()

We are not supposed to call kernel_fpu_disable() if we have not
previously enabled it.

Also use kernel_fpu_disable()/enable() in the __kernel_fpu_begin/end()
primitives, instead of writing to in_kernel_fpu directly,
so that we get the debugging checks.
Reviewed-by: NBorislav Petkov <bp@alien8.de>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Fenghua Yu <fenghua.yu@intel.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: NIngo Molnar <mingo@kernel.org>

arch/x86/kernel/fpu/core.c

@@ -17,6 +17,7 @@ static void kernel_fpu_disable(void)
 
 static void kernel_fpu_enable(void)
 {
+	WARN_ON_ONCE(!this_cpu_read(in_kernel_fpu));
 	this_cpu_write(in_kernel_fpu, false);
 }
 
@@ -77,7 +78,7 @@ void __kernel_fpu_begin(void)
 {
 	struct task_struct *me = current;
 
-	this_cpu_write(in_kernel_fpu, true);
+	kernel_fpu_disable();
 
 	if (__thread_has_fpu(me)) {
 		__save_init_fpu(me);
@@ -100,7 +101,7 @@ void __kernel_fpu_end(void)
 		stts();
 	}
 
-	this_cpu_write(in_kernel_fpu, false);
+	kernel_fpu_enable();
 }
 EXPORT_SYMBOL(__kernel_fpu_end);