[PATCH 1/2] x86/hw_breakpoint: Prevent data breakpoints on __per_cpu_offset

[Lai Jiangshan <jiangshanlai@gmail.com>]

From: Lai Jiangshan <laijs@linux.alibaba.com>

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

When data breakpoint is set on __per_cpu_offset[cpu] (read-write
operation), the specific cpu will be stuck in the infinite #DB loop.
RCU will try to send NMI to the specific cpu, but it is not working
either since NMI also relies on paranoid_entry().

Fixes: eaad981291ee3("x86/entry/64: Introduce the FIND_PERCPU_BASE macro")
Signed-off-by: Lai Jiangshan <laijs@linux.alibaba.com>
---
 arch/x86/kernel/hw_breakpoint.c | 14 ++++++++++++++
 1 file changed, 14 insertions(+)

diff --git a/arch/x86/kernel/hw_breakpoint.c b/arch/x86/kernel/hw_breakpoint.c
index 03aa33b58165..bc7493a0736f 100644
--- 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),
-- 
2.19.1.6.gb485710b