kasan: support use-after-scope detection

Gcc revision 241896 implements use-after-scope detection.  Will be
available in gcc 7.  Support it in KASAN.

Gcc emits 2 new callbacks to poison/unpoison large stack objects when
they go in/out of scope.  Implement the callbacks and add a test.

[dvyukov@google.com: v3]
  Link: http://lkml.kernel.org/r/1479998292-144502-1-git-send-email-dvyukov@google.com
Link: http://lkml.kernel.org/r/1479226045-145148-1-git-send-email-dvyukov@google.com
Signed-off-by: Dmitry Vyukov <dvyukov@google.com>
Acked-by: Andrey Ryabinin <aryabinin@virtuozzo.com>
Cc: Alexander Potapenko <glider@google.com>
Cc: <stable@vger.kernel.org>	[4.0+]
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/kasan/kasan.c

@@ -764,6 +764,25 @@ EXPORT_SYMBOL(__asan_storeN_noabort);
 void __asan_handle_no_return(void) {}
 EXPORT_SYMBOL(__asan_handle_no_return);
 
+/* Emitted by compiler to poison large objects when they go out of scope. */
+void __asan_poison_stack_memory(const void *addr, size_t size)
+{
+	/*
+	 * Addr is KASAN_SHADOW_SCALE_SIZE-aligned and the object is surrounded
+	 * by redzones, so we simply round up size to simplify logic.
+	 */
+	kasan_poison_shadow(addr, round_up(size, KASAN_SHADOW_SCALE_SIZE),
+			    KASAN_USE_AFTER_SCOPE);
+}
+EXPORT_SYMBOL(__asan_poison_stack_memory);
+
+/* Emitted by compiler to unpoison large objects when they go into scope. */
+void __asan_unpoison_stack_memory(const void *addr, size_t size)
+{
+	kasan_unpoison_shadow(addr, size);
+}
+EXPORT_SYMBOL(__asan_unpoison_stack_memory);
+
 #ifdef CONFIG_MEMORY_HOTPLUG
 static int kasan_mem_notifier(struct notifier_block *nb,
 			unsigned long action, void *data)

mm/kasan/kasan.h

@@ -21,6 +21,7 @@
 #define KASAN_STACK_MID         0xF2
 #define KASAN_STACK_RIGHT       0xF3
 #define KASAN_STACK_PARTIAL     0xF4
+#define KASAN_USE_AFTER_SCOPE   0xF8
 
 /* Don't break randconfig/all*config builds */
 #ifndef KASAN_ABI_VERSION

mm/kasan/report.c

@@ -90,6 +90,9 @@ static void print_error_description(struct kasan_access_info *info)
 	case KASAN_KMALLOC_FREE:
 		bug_type = "use-after-free";
 		break;
+	case KASAN_USE_AFTER_SCOPE:
+		bug_type = "use-after-scope";
+		break;
 	}
 
 	pr_err("BUG: KASAN: %s in %pS at addr %p\n",