[PATCH 2/2] lib/test_kasan: Add stack overflow test

[Marco Elver]

Adds a simple stack overflow test, to check the error being reported on
an overflow. Without CONFIG_STACK_GUARD_PAGE, the result is typically
some seemingly unrelated KASAN error message due to accessing random
other memory.

Signed-off-by: Marco Elver <elver@xxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: Borislav Petkov <bp@xxxxxxxxx>
Cc: "H. Peter Anvin" <hpa@xxxxxxxxx>
Cc: Andrey Ryabinin <aryabinin@xxxxxxxxxxxxx>
Cc: Alexander Potapenko <glider@xxxxxxxxxx>
Cc: Dmitry Vyukov <dvyukov@xxxxxxxxxx>
Cc: Andrey Konovalov <andreyknvl@xxxxxxxxxx>
Cc: Mark Rutland <mark.rutland@xxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: x86@xxxxxxxxxx
Cc: linux-kernel@xxxxxxxxxxxxxxx
Cc: kasan-dev@xxxxxxxxxxxxxxxx
---
 lib/test_kasan.c | 36 ++++++++++++++++++++++++++++++++++++
 1 file changed, 36 insertions(+)

diff --git a/lib/test_kasan.c b/lib/test_kasan.c
index b63b367a94e8..3092ec01189d 100644
--- a/lib/test_kasan.c
+++ b/lib/test_kasan.c
@@ -15,6 +15,7 @@
 #include <linux/mman.h>
 #include <linux/module.h>
 #include <linux/printk.h>
+#include <linux/sched/task_stack.h>
 #include <linux/slab.h>
 #include <linux/string.h>
 #include <linux/uaccess.h>
@@ -709,6 +710,32 @@ static noinline void __init kmalloc_double_kzfree(void)
 	kzfree(ptr);
 }
 
+#ifdef CONFIG_STACK_GUARD_PAGE
+static noinline void __init stack_overflow_via_recursion(void)
+{
+	volatile int n = 512;
+
+	BUILD_BUG_ON(IS_ENABLED(CONFIG_STACK_GROWSUP));
+
+	/* About to overflow: overflow via alloca'd array and try to write. */
+	if (!object_is_on_stack((void *)&n - n)) {
+		volatile char overflow[n];
+
+		overflow[0] = overflow[0];
+		return;
+	}
+
+	stack_overflow_via_recursion();
+}
+
+static noinline void __init kasan_stack_overflow(void)
+{
+	pr_info("stack overflow begin\n");
+	stack_overflow_via_recursion();
+	pr_info("stack overflow end\n");
+}
+#endif
+
 static int __init kmalloc_tests_init(void)
 {
 	/*
@@ -753,6 +780,15 @@ static int __init kmalloc_tests_init(void)
 	kasan_bitops();
 	kmalloc_double_kzfree();
 
+#ifdef CONFIG_STACK_GUARD_PAGE
+	/*
+	 * Only test with CONFIG_STACK_GUARD_PAGE, as without we get other
+	 * random KASAN violations, due to accessing other random memory (we
+	 * want to avoid actually corrupting memory in these tests).
+	 */
+	kasan_stack_overflow();
+#endif
+
 	kasan_restore_multi_shot(multishot);
 
 	return -EAGAIN;
-- 
2.22.0.657.g960e92d24f-goog