[PATCH 02/14] x86/hw_breakpoint: Prevent data breakpoints on direct GDT

[Peter Zijlstra]

From: Lai Jiangshan <laijs@xxxxxxxxxxxxxxxxx>

A data breakpoint on the GDT is terrifying and should be avoided.
The GDT on CPU entry area is already protected. The direct GDT
should be also protected, although it is seldom used and only
used for short time.

Signed-off-by: Lai Jiangshan <laijs@xxxxxxxxxxxxxxxxx>
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Link: https://lkml.kernel.org/r/20200526014221.2119-3-laijs@xxxxxxxxxxxxxxxxx
---
 arch/x86/kernel/hw_breakpoint.c |   30 ++++++++++++++++++++++--------
 1 file changed, 22 insertions(+), 8 deletions(-)

--- a/arch/x86/kernel/hw_breakpoint.c
+++ b/arch/x86/kernel/hw_breakpoint.c
@@ -32,6 +32,7 @@
 #include <asm/processor.h>
 #include <asm/debugreg.h>
 #include <asm/user.h>
+#include <asm/desc.h>
 
 /* Per cpu debug control register value */
 DEFINE_PER_CPU(unsigned long, cpu_dr7);
@@ -237,13 +238,26 @@ static inline bool within_area(unsigned
 }
 
 /*
- * Checks whether the range from addr to end, inclusive, overlaps the CPU
- * entry area range.
+ * Checks whether the range from addr to end, inclusive, overlaps the fixed
+ * mapped CPU entry area range or other ranges used for CPU entry.
  */
-static inline bool within_cpu_entry_area(unsigned long addr, unsigned long end)
+static inline bool within_cpu_entry(unsigned long addr, unsigned long end)
 {
-	return within_area(addr, end, CPU_ENTRY_AREA_BASE,
-			   CPU_ENTRY_AREA_TOTAL_SIZE);
+	int cpu;
+
+	/* CPU entry erea is always used for CPU entry */
+	if (within_area(addr, end, CPU_ENTRY_AREA_BASE,
+			CPU_ENTRY_AREA_TOTAL_SIZE))
+		return true;
+
+	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),
+				GDT_SIZE))
+			return true;
+	}
+
+	return false;
 }
 
 static int arch_build_bp_info(struct perf_event *bp,
@@ -257,12 +271,12 @@ static int arch_build_bp_info(struct per
 		return -EINVAL;
 
 	/*
-	 * Prevent any breakpoint of any type that overlaps the
-	 * cpu_entry_area.  This protects the IST stacks and also
+	 * Prevent any breakpoint of any type that overlaps the CPU
+	 * entry area and data.  This protects the IST stacks and also
 	 * reduces the chance that we ever find out what happens if
 	 * there's a data breakpoint on the GDT, IDT, or TSS.
 	 */
-	if (within_cpu_entry_area(attr->bp_addr, bp_end))
+	if (within_cpu_entry(attr->bp_addr, bp_end))
 		return -EINVAL;
 
 	hw->address = attr->bp_addr;