[PATCH v4 17/21] mm/ksw: add silent corruption test case

[Jinchao Wang <wangjinchao600@gmail.com>]

Introduce a new test scenario to simulate silent stack corruption:

- silent_corruption_buggy():
  exposes a local variable address globally without resetting it.
- silent_corruption_unwitting():
  reads the exposed pointer and modifies the memory, simulating a routine
  that unknowingly writes to another stack frame.
- silent_corruption_victim():
  demonstrates the effect of silent corruption on unrelated local variables.

Signed-off-by: Jinchao Wang <wangjinchao600@gmail.com>
---
 mm/kstackwatch/test.c | 96 ++++++++++++++++++++++++++++++++++++++++++-
 1 file changed, 95 insertions(+), 1 deletion(-)

diff --git a/mm/kstackwatch/test.c b/mm/kstackwatch/test.c
index ab1a3f92b5e8..2b196f72ffd7 100644
--- a/mm/kstackwatch/test.c
+++ b/mm/kstackwatch/test.c
@@ -20,6 +20,9 @@ static struct proc_dir_entry *test_proc;
 #define BUFFER_SIZE 4
 #define MAX_DEPTH 6
 
+/* global variables for Silent corruption test */
+static u64 *g_corrupt_ptr;
+
 /*
  * Test Case 0: Write to the canary position directly (Canary Test)
  * use a u64 buffer array to ensure the canary will be placed
@@ -61,6 +64,92 @@ static void canary_test_overflow(void)
 	pr_info("canary overflow test completed\n");
 }
 
+static void do_something(int min_ms, int max_ms)
+{
+	u32 rand;
+
+	get_random_bytes(&rand, sizeof(rand));
+	rand = min_ms + rand % (max_ms - min_ms + 1);
+	msleep(rand);
+}
+
+static void silent_corruption_buggy(int i)
+{
+	u64 local_var;
+
+	pr_info("starting %s\n", __func__);
+
+	pr_info("%s %d local_var addr: 0x%lx\n", __func__, i,
+		(unsigned long)&local_var);
+	WRITE_ONCE(g_corrupt_ptr, &local_var);
+
+	do_something(50, 150);
+	//buggy: return without resetting g_corrupt_ptr
+}
+
+static void silent_corruption_victim(int i)
+{
+	u64 local_var;
+
+	local_var = 0xdeadbeef;
+	pr_info("starting %s %dth\n", __func__, i);
+	pr_info("%s local_var addr: 0x%lx\n", __func__,
+		(unsigned long)&local_var);
+
+	do_something(50, 150);
+
+	if (local_var != 0)
+		pr_info("%s %d happy with 0x%llx\n", __func__, i, local_var);
+	else
+		pr_info("%s %d unhappy with 0x%llx\n", __func__, i, local_var);
+}
+
+static int silent_corruption_unwitting(void *data)
+{
+	u64 *local_ptr;
+
+	pr_info("starting %s\n", __func__);
+
+	do {
+		local_ptr = READ_ONCE(g_corrupt_ptr);
+		do_something(500, 1000);
+	} while (!local_ptr);
+
+	local_ptr[0] = 0;
+
+	return 0;
+}
+
+/*
+ * Test Case 2: Silent Corruption
+ * buggy() does not protect its local var correctly
+ * unwitting() simply does its intended work
+ * victim() is unaware know what happened
+ */
+static void silent_corruption_test(void)
+{
+	struct task_struct *unwitting;
+
+	pr_info("starting %s\n", __func__);
+	WRITE_ONCE(g_corrupt_ptr, NULL);
+
+	unwitting = kthread_run(silent_corruption_unwitting, NULL, "unwitting");
+	if (IS_ERR(unwitting)) {
+		pr_err("failed to create thread2\n");
+		return;
+	}
+
+	silent_corruption_buggy(0);
+
+	/*
+	 * An iteration-based bug: The unwitting thread corrupts the victim's
+	 * stack. In a twist of fate, the victim's subsequent repetitions ensure
+	 * the corruption is contained, protecting the caller's stack.
+	 */
+	for (int i = 0; i < 20; i++)
+		silent_corruption_victim(i);
+}
+
 static ssize_t test_proc_write(struct file *file, const char __user *buffer,
 			       size_t count, loff_t *pos)
 {
@@ -88,6 +177,10 @@ static ssize_t test_proc_write(struct file *file, const char __user *buffer,
 			pr_info("triggering canary overflow test\n");
 			canary_test_overflow();
 			break;
+		case 2:
+			pr_info("triggering silent corruption test\n");
+			silent_corruption_test();
+			break;
 		default:
 			pr_err("Unknown test number %d\n", test_num);
 			return -EINVAL;
@@ -108,7 +201,8 @@ static ssize_t test_proc_read(struct file *file, char __user *buffer,
 		"==================================\n"
 		"Usage:\n"
 		"  echo 'test0' > /proc/kstackwatch_test  - Canary write test\n"
-		"  echo 'test1' > /proc/kstackwatch_test  - Canary overflow test\n";
+		"  echo 'test1' > /proc/kstackwatch_test  - Canary overflow test\n"
+		"  echo 'test2' > /proc/kstackwatch_test  - Silent corruption test\n";
 
 	return simple_read_from_buffer(buffer, count, pos, usage,
 				       strlen(usage));
-- 
2.43.0