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: 84b6a3491567a("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

diff --git a/arch/x86/kernel/hw_breakpoint.c b/arch/x86/kernel/hw_breakpoint.c
index 012ed82e5bd324d491ba83f666bfc0088dd63df3..668a4a6533d923c7eddef457f3f2ebac0040a00b 100644 (file)
--- 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;