x86/hw_breakpoint: Prevent data breakpoints on cpu_entry_area

A data breakpoint near the top of an IST stack will cause unrecoverable
recursion.  A data breakpoint on the GDT, IDT, or TSS is terrifying.
Prevent either of these from happening.
Co-developed-by: NPeter Zijlstra <peterz@infradead.org>
Signed-off-by: NAndy Lutomirski <luto@kernel.org>
Signed-off-by: NPeter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: NThomas Gleixner <tglx@linutronix.de>
Reviewed-by: NBorislav Petkov <bp@suse.de>
Reviewed-by: NLai Jiangshan <jiangshanlai@gmail.com>
Reviewed-by: NAlexandre Chartre <alexandre.chartre@oracle.com>
Link: https://lkml.kernel.org/r/20200505134058.272448010@linutronix.de

arch/x86/kernel/hw_breakpoint.c

@@ -227,10 +227,35 @@ int arch_check_bp_in_kernelspace(struct arch_hw_breakpoint *hw)
 	return (va >= TASK_SIZE_MAX) || ((va + len - 1) >= TASK_SIZE_MAX);
 }
 
+/*
+ * Checks whether the range from addr to end, inclusive, overlaps the CPU
+ * entry area range.
+ */
+static inline bool within_cpu_entry_area(unsigned long addr, unsigned long end)
+{
+	return end >= CPU_ENTRY_AREA_BASE &&
+	       addr < (CPU_ENTRY_AREA_BASE + CPU_ENTRY_AREA_TOTAL_SIZE);
+}
+
 static int arch_build_bp_info(struct perf_event *bp,
 			      const struct perf_event_attr *attr,
 			      struct arch_hw_breakpoint *hw)
 {
+	unsigned long bp_end;
+
+	bp_end = attr->bp_addr + attr->bp_len - 1;
+	if (bp_end < attr->bp_addr)
+		return -EINVAL;
+
+	/*
+	 * Prevent any breakpoint of any type that overlaps the
+	 * cpu_entry_area.  This protects the IST stacks and also
+	 * reduces the chance that we ever find out what happens if
+	 * there's a data breakpoint on the GDT, IDT, or TSS.
+	 */
+	if (within_cpu_entry_area(attr->bp_addr, bp_end))
+		return -EINVAL;
+
 	hw->address = attr->bp_addr;
 	hw->mask = 0;