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Subject[tip: x86/urgent] x86/debug: Prevent data breakpoints on __per_cpu_offset
The following commit has been merged into the x86/urgent branch of tip:

Commit-ID: c4bed4b96918ff1d062ee81fdae4d207da4fa9b0
Gitweb: https://git.kernel.org/tip/c4bed4b96918ff1d062ee81fdae4d207da4fa9b0
Author: Lai Jiangshan <laijs@linux.alibaba.com>
AuthorDate: Thu, 04 Feb 2021 23:27:06 +08:00
Committer: Thomas Gleixner <tglx@linutronix.de>
CommitterDate: Fri, 05 Feb 2021 20:13:11 +01:00

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

---
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 6694c0f..012ed82 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),
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