[PATCH 2/2] x86/hw_breakpoint: Prevent data breakpoints on cpu_dr7

[Lai Jiangshan <jiangshanlai@gmail.com>]

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

When in guest (X86_FEATURE_HYPERVISOR), local_db_save() will read
per-cpu cpu_dr7 before clear dr7 register.

local_db_save() is called at the start of exc_debug_kernel().
To avoid recursive #DB, we have to disallow data breakpoints
on cpu_dr7.

Fixes: 84b6a3491567a("x86/entry: Optimize local_db_save() for virt")
Signed-off-by: Lai Jiangshan <laijs@linux.alibaba.com>
---
 arch/x86/kernel/hw_breakpoint.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/arch/x86/kernel/hw_breakpoint.c b/arch/x86/kernel/hw_breakpoint.c
index bc7493a0736f..de34dd49d317 100644
--- a/arch/x86/kernel/hw_breakpoint.c
+++ b/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;
-- 
2.19.1.6.gb485710b