x86/entry, mce: Disallow #DB during #MC

#MC is fragile as heck, don't tempt fate.
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lkml.kernel.org/r/20200529213321.131187767@infradead.org

arch/x86/kernel/cpu/mce/core.c

@@ -1943,22 +1943,34 @@ static __always_inline void exc_machine_check_user(struct pt_regs *regs)
 /* MCE hit kernel mode */
 DEFINE_IDTENTRY_MCE(exc_machine_check)
 {
+	unsigned long dr7;
+
+	dr7 = local_db_save();
 	exc_machine_check_kernel(regs);
+	local_db_restore(dr7);
 }
 
 /* The user mode variant. */
 DEFINE_IDTENTRY_MCE_USER(exc_machine_check)
 {
+	unsigned long dr7;
+
+	dr7 = local_db_save();
 	exc_machine_check_user(regs);
+	local_db_restore(dr7);
 }
 #else
 /* 32bit unified entry point */
 DEFINE_IDTENTRY_MCE(exc_machine_check)
 {
+	unsigned long dr7;
+
+	dr7 = local_db_save();
 	if (user_mode(regs))
 		exc_machine_check_user(regs);
 	else
 		exc_machine_check_kernel(regs);
+	local_db_restore(dr7);
 }
 #endif