[PATCH] x86/hw_breakpoint: Prevent data breakpoints on cpu_entry_area

[Andy Lutomirski]

A data breakpoint near the top of an IST stack will cause unresoverable
recursion.  A data breakpoint on the GDT, IDT, or TSS is terrifying.
Prevent either of these from happening.

Co-developed-by: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Signed-off-by: Andy Lutomirski <luto@xxxxxxxxxx>
---

The rest of my series is still in progress -- as we all know, idtentry
is a morass.  But this is self-contained and is an obvious fix.

arch/x86/include/asm/cpu_entry_area.h | 10 ++++++++++
 arch/x86/kernel/hw_breakpoint.c       | 17 +++++++++++++++++
 2 files changed, 27 insertions(+)

diff --git a/arch/x86/include/asm/cpu_entry_area.h b/arch/x86/include/asm/cpu_entry_area.h
index e23e2d9a92d7..3f50d4738487 100644
--- a/arch/x86/include/asm/cpu_entry_area.h
+++ b/arch/x86/include/asm/cpu_entry_area.h
@@ -126,6 +126,16 @@ static inline struct entry_stack *cpu_entry_stack(int cpu)
 	return &get_cpu_entry_area(cpu)->entry_stack_page.stack;
 }
 
+/*
+ * Checks whether the range from addr to end, inclusive, overlaps the CPU
+ * entry area range.
+ */
+static inline bool within_cpu_entry_area(unsigned long addr, unsigned long end)
+{
+	return end >= CPU_ENTRY_AREA_PER_CPU &&
+		addr < (CPU_ENTRY_AREA_PER_CPU + CPU_ENTRY_AREA_TOT_SIZE);
+}
+
 #define __this_cpu_ist_top_va(name)					\
 	CEA_ESTACK_TOP(__this_cpu_read(cea_exception_stacks), name)
 
diff --git a/arch/x86/kernel/hw_breakpoint.c b/arch/x86/kernel/hw_breakpoint.c
index 218c8917118e..dc4581fe4b4e 100644
--- a/arch/x86/kernel/hw_breakpoint.c
+++ b/arch/x86/kernel/hw_breakpoint.c
@@ -231,6 +231,23 @@ static int arch_build_bp_info(struct perf_event *bp,
 			      const struct perf_event_attr *attr,
 			      struct arch_hw_breakpoint *hw)
 {
+	unsigned long bp_end;
+
+	/* Ensure that bp_end does not oveflow. */
+	if (attr->bp_len >= ULONG_MAX - attr->bp_addr)
+		return -EINVAL;
+
+	bp_end = attr->bp_addr + attr->bp_len - 1;
+
+	/*
+	 * Prevent any breakpoint of any type that overlaps the
+	 * cpu_entry_area.  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))
+		return -EINVAL;
+
 	hw->address = attr->bp_addr;
 	hw->mask = 0;
 
-- 
2.21.0