kasan: test: avoid corrupting memory in copy_user_test

copy_user_test() does writes past the allocated object.  As the result, it
corrupts kernel memory, which might lead to crashes with the HW_TAGS mode,
as it neither uses quarantine nor redzones.

(Technically, this test can't yet be enabled with the HW_TAGS mode, but
this will be implemented in the future.)

Adjust the test to only write memory within the aligned kmalloc object.

Link: https://lkml.kernel.org/r/19bf3a5112ee65b7db88dc731643b657b816c5e8.1628779805.git.andreyknvl@gmail.com
Signed-off-by: Andrey Konovalov <andreyknvl@gmail.com>
Reviewed-by: Marco Elver <elver@google.com>
Cc: Alexander Potapenko <glider@google.com>
Cc: Andrey Ryabinin <aryabinin@virtuozzo.com>
Cc: Dmitry Vyukov <dvyukov@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

lib/test_kasan_module.c

@@ -15,13 +15,11 @@
 
 #include "../mm/kasan/kasan.h"
 
-#define OOB_TAG_OFF (IS_ENABLED(CONFIG_KASAN_GENERIC) ? 0 : KASAN_GRANULE_SIZE)
-
 static noinline void __init copy_user_test(void)
 {
 	char *kmem;
 	char __user *usermem;
-	size_t size = 10;
+	size_t size = 128 - KASAN_GRANULE_SIZE;
 	int __maybe_unused unused;
 
 	kmem = kmalloc(size, GFP_KERNEL);
@@ -38,25 +36,25 @@ static noinline void __init copy_user_test(void)
 	}
 
 	pr_info("out-of-bounds in copy_from_user()\n");
-	unused = copy_from_user(kmem, usermem, size + 1 + OOB_TAG_OFF);
+	unused = copy_from_user(kmem, usermem, size + 1);
 
 	pr_info("out-of-bounds in copy_to_user()\n");
-	unused = copy_to_user(usermem, kmem, size + 1 + OOB_TAG_OFF);
+	unused = copy_to_user(usermem, kmem, size + 1);
 
 	pr_info("out-of-bounds in __copy_from_user()\n");
-	unused = __copy_from_user(kmem, usermem, size + 1 + OOB_TAG_OFF);
+	unused = __copy_from_user(kmem, usermem, size + 1);
 
 	pr_info("out-of-bounds in __copy_to_user()\n");
-	unused = __copy_to_user(usermem, kmem, size + 1 + OOB_TAG_OFF);
+	unused = __copy_to_user(usermem, kmem, size + 1);
 
 	pr_info("out-of-bounds in __copy_from_user_inatomic()\n");
-	unused = __copy_from_user_inatomic(kmem, usermem, size + 1 + OOB_TAG_OFF);
+	unused = __copy_from_user_inatomic(kmem, usermem, size + 1);
 
 	pr_info("out-of-bounds in __copy_to_user_inatomic()\n");
-	unused = __copy_to_user_inatomic(usermem, kmem, size + 1 + OOB_TAG_OFF);
+	unused = __copy_to_user_inatomic(usermem, kmem, size + 1);
 
 	pr_info("out-of-bounds in strncpy_from_user()\n");
-	unused = strncpy_from_user(kmem, usermem, size + 1 + OOB_TAG_OFF);
+	unused = strncpy_from_user(kmem, usermem, size + 1);
 
 	vm_munmap((unsigned long)usermem, PAGE_SIZE);
 	kfree(kmem);