x86/debug: Prevent data breakpoints on __per_cpu_offset

When FSGSBASE is enabled, paranoid_entry() fetches the per-CPU GSBASE value
via __per_cpu_offset or pcpu_unit_offsets.

When a data breakpoint is set on __per_cpu_offset[cpu] (read-write
operation), the specific CPU will be stuck in an infinite #DB loop.

RCU will try to send an NMI to the specific CPU, but it is not working
either since NMI also relies on paranoid_entry(). Which means it's
undebuggable.

Fixes: eaad981291ee3("x86/entry/64: Introduce the FIND_PERCPU_BASE macro")
Signed-off-by: Lai Jiangshan <laijs@linux.alibaba.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: stable@vger.kernel.org
Link: https://lore.kernel.org/r/20210204152708.21308-1-jiangshanlai@gmail.com

diff --git a/arch/x86/kernel/hw_breakpoint.c b/arch/x86/kernel/hw_breakpoint.c
index 6694c0f..012ed82 100644 (file)
--- a/arch/x86/kernel/hw_breakpoint.c
+++ b/arch/x86/kernel/hw_breakpoint.c
@@ -269,6 +269,20 @@ static inline bool within_cpu_entry(unsigned long addr, unsigned long end)
                        CPU_ENTRY_AREA_TOTAL_SIZE))
                return true;
 
+       /*
+        * When FSGSBASE is enabled, paranoid_entry() fetches the per-CPU
+        * GSBASE value via __per_cpu_offset or pcpu_unit_offsets.
+        */
+#ifdef CONFIG_SMP
+       if (within_area(addr, end, (unsigned long)__per_cpu_offset,
+                       sizeof(unsigned long) * nr_cpu_ids))
+               return true;
+#else
+       if (within_area(addr, end, (unsigned long)&pcpu_unit_offsets,
+                       sizeof(pcpu_unit_offsets)))
+               return true;
+#endif
+
        for_each_possible_cpu(cpu) {
                /* The original rw GDT is being used after load_direct_gdt() */
                if (within_area(addr, end, (unsigned long)get_cpu_gdt_rw(cpu),