x86/hw_breakpoint: Prevent data breakpoints on per_cpu cpu_tss_rw

cpu_tss_rw is not directly referenced by hardware, but cpu_tss_rw is
accessed in CPU entry code, especially when #DB shifts its stacks.

If a data breakpoint would be set on cpu_tss_rw.x86_tss.ist[IST_INDEX_DB],
it would cause recursive #DB ending up in a double fault.

Add it to the list of protected items.
Signed-off-by: Lai Jiangshan <laijs@linux.alibaba.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lkml.kernel.org/r/20200526014221.2119-4-laijs@linux.alibaba.com
Link: https://lkml.kernel.org/r/20200529213320.897976479@infradead.org

arch/x86/kernel/hw_breakpoint.c

@@ -255,6 +255,15 @@ static inline bool within_cpu_entry(unsigned long addr, unsigned long end)
 		if (within_area(addr, end, (unsigned long)get_cpu_gdt_rw(cpu),
 				GDT_SIZE))
 			return true;
+
+		/*
+		 * cpu_tss_rw is not directly referenced by hardware, but
+		 * cpu_tss_rw is also used in CPU entry code,
+		 */
+		if (within_area(addr, end,
+				(unsigned long)&per_cpu(cpu_tss_rw, cpu),
+				sizeof(struct tss_struct)))
+			return true;
 	}
 
 	return false;