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From:   Greg Kroah-Hartman <gregkh@linuxfoundation.org>
To:     linux-kernel@vger.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        stable@vger.kernel.org, Lai Jiangshan <laijs@linux.alibaba.com>,
        Thomas Gleixner <tglx@linutronix.de>
Subject: [PATCH 5.10 108/120] x86/debug: Prevent data breakpoints on __per_cpu_offset
Date:   Mon,  8 Feb 2021 16:01:35 +0100
Message-Id: <20210208145822.689200218@linuxfoundation.org>
In-Reply-To: <20210208145818.395353822@linuxfoundation.org>
References: <20210208145818.395353822@linuxfoundation.org>
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From: Lai Jiangshan <laijs@linux.alibaba.com>

commit c4bed4b96918ff1d062ee81fdae4d207da4fa9b0 upstream.

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
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
 arch/x86/kernel/hw_breakpoint.c |   14 ++++++++++++++
 1 file changed, 14 insertions(+)

--- a/arch/x86/kernel/hw_breakpoint.c
+++ b/arch/x86/kernel/hw_breakpoint.c
@@ -269,6 +269,20 @@ static inline bool within_cpu_entry(unsi
 			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),