[PATCH 1/3] x86/perf/hw_breakpoint: Disallow kernel breakpoints unless kprobe-safe

[Andy Lutomirski <luto@kernel.org>]

Code on the kprobe blacklist doesn't want unexpected int3
exceptions.  It probably doesn't want unexpected debug exceptions
either.  Be safe: disallow breakpoints in nokprobes code.

On non-CONFIG_KPROBES kernels, there is no kprobe blacklist.  In
that case, disallow kernel breakpoints entirely.

It will be particularly important to keep hw breakpoints out of the
entry and NMI code once we move debug exceptions off the IST stack.

Signed-off-by: Andy Lutomirski <luto@kernel.org>
---
 arch/x86/kernel/hw_breakpoint.c | 15 +++++++++++++++
 include/linux/kprobes.h         |  2 ++
 kernel/kprobes.c                |  2 +-
 3 files changed, 18 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kernel/hw_breakpoint.c b/arch/x86/kernel/hw_breakpoint.c
index 7114ba220fd4..78f3e90c5659 100644
--- a/arch/x86/kernel/hw_breakpoint.c
+++ b/arch/x86/kernel/hw_breakpoint.c
@@ -32,6 +32,7 @@
 #include <linux/irqflags.h>
 #include <linux/notifier.h>
 #include <linux/kallsyms.h>
+#include <linux/kprobes.h>
 #include <linux/percpu.h>
 #include <linux/kdebug.h>
 #include <linux/kernel.h>
@@ -243,6 +244,20 @@ static int arch_build_bp_info(struct perf_event *bp)
 		info->type = X86_BREAKPOINT_RW;
 		break;
 	case HW_BREAKPOINT_X:
+		/*
+		 * We don't allow kernel breakpoints in places that are not
+		 * acceptable for kprobes.  On non-kprobes kernels, we don't
+		 * allow kernel breakpoints at all.
+		 */
+		if (bp->attr.bp_addr >= TASK_SIZE_MAX) {
+#ifdef CONFIG_KPROBES
+			if (within_kprobe_blacklist(bp->attr.bp_addr))
+				return -EINVAL;
+#else
+			return -EINVAL;
+#endif
+		}
+
 		info->type = X86_BREAKPOINT_EXECUTE;
 		/*
 		 * x86 inst breakpoints need to have a specific undefined len.
diff --git a/include/linux/kprobes.h b/include/linux/kprobes.h
index 1ab54754a86d..8f6849084248 100644
--- a/include/linux/kprobes.h
+++ b/include/linux/kprobes.h
@@ -267,6 +267,8 @@ extern void show_registers(struct pt_regs *regs);
 extern void kprobes_inc_nmissed_count(struct kprobe *p);
 extern bool arch_within_kprobe_blacklist(unsigned long addr);
 
+extern bool within_kprobe_blacklist(unsigned long addr);
+
 struct kprobe_insn_cache {
 	struct mutex mutex;
 	void *(*alloc)(void);	/* allocate insn page */
diff --git a/kernel/kprobes.c b/kernel/kprobes.c
index c90e417bb963..d10ab6b9b5e0 100644
--- a/kernel/kprobes.c
+++ b/kernel/kprobes.c
@@ -1332,7 +1332,7 @@ bool __weak arch_within_kprobe_blacklist(unsigned long addr)
 	       addr < (unsigned long)__kprobes_text_end;
 }
 
-static bool within_kprobe_blacklist(unsigned long addr)
+bool within_kprobe_blacklist(unsigned long addr)
 {
 	struct kprobe_blacklist_entry *ent;
 
-- 
2.4.3