x86/debug: Prevent data breakpoints on cpu_dr7

local_db_save() is called at the start of exc_debug_kernel(), reads DR7 and
disables breakpoints to prevent recursion.

When running in a guest (X86_FEATURE_HYPERVISOR), local_db_save() reads the
per-cpu variable cpu_dr7 to check whether a breakpoint is active or not
before it accesses DR7.

A data breakpoint on cpu_dr7 therefore results in infinite #DB recursion.

Disallow data breakpoints on cpu_dr7 to prevent that.

Fixes: 84b6a349("x86/entry: Optimize local_db_save() for virt")
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/20210204152708.21308-2-jiangshanlai@gmail.com

arch/x86/kernel/hw_breakpoint.c

@@ -307,6 +307,14 @@ static inline bool within_cpu_entry(unsigned long addr, unsigned long end)
 				(unsigned long)&per_cpu(cpu_tlbstate, cpu),
 				sizeof(struct tlb_state)))
 			return true;
+
+		/*
+		 * When in guest (X86_FEATURE_HYPERVISOR), local_db_save()
+		 * will read per-cpu cpu_dr7 before clear dr7 register.
+		 */
+		if (within_area(addr, end, (unsigned long)&per_cpu(cpu_dr7, cpu),
+				sizeof(cpu_dr7)))
+			return true;
 	}
 
 	return false;