[PATCH v2] kasan: test: Bypass __alloc_size checks

[Kees Cook]

Intentional overflows, as performed by the KASAN tests, are detected
at compile time[1] (instead of only at run-time) with the addition of
__alloc_size. Fix this by forcing the compiler into not being able to
trust the size used following the kmalloc()s.

[1] https://lore.kernel.org/lkml/20211005184717.65c6d8eb39350395e387b71f@xxxxxxxxxxxxxxxxxxxx

Cc: Andrey Ryabinin <ryabinin.a.a@xxxxxxxxx>
Cc: Alexander Potapenko <glider@xxxxxxxxxx>
Cc: Andrey Konovalov <andreyknvl@xxxxxxxxx>
Cc: Dmitry Vyukov <dvyukov@xxxxxxxxxx>
Cc: kasan-dev@xxxxxxxxxxxxxxxx
Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
---
v2: use OPTIMIZER_HIDE_VAR() (jann, mark)
v1: https://lore.kernel.org/lkml/20211006035522.539346-1-keescook@xxxxxxxxxxxx/
---
 lib/test_kasan.c        | 8 +++++++-
 lib/test_kasan_module.c | 2 ++
 2 files changed, 9 insertions(+), 1 deletion(-)

diff --git a/lib/test_kasan.c b/lib/test_kasan.c
index 8835e0784578..8a8a8133f4cd 100644
--- a/lib/test_kasan.c
+++ b/lib/test_kasan.c
@@ -440,6 +440,7 @@ static void kmalloc_oob_memset_2(struct kunit *test)
 	ptr = kmalloc(size, GFP_KERNEL);
 	KUNIT_ASSERT_NOT_ERR_OR_NULL(test, ptr);
 
+	OPTIMIZER_HIDE_VAR(size);
 	KUNIT_EXPECT_KASAN_FAIL(test, memset(ptr + size - 1, 0, 2));
 	kfree(ptr);
 }
@@ -452,6 +453,7 @@ static void kmalloc_oob_memset_4(struct kunit *test)
 	ptr = kmalloc(size, GFP_KERNEL);
 	KUNIT_ASSERT_NOT_ERR_OR_NULL(test, ptr);
 
+	OPTIMIZER_HIDE_VAR(size);
 	KUNIT_EXPECT_KASAN_FAIL(test, memset(ptr + size - 3, 0, 4));
 	kfree(ptr);
 }
@@ -464,6 +466,7 @@ static void kmalloc_oob_memset_8(struct kunit *test)
 	ptr = kmalloc(size, GFP_KERNEL);
 	KUNIT_ASSERT_NOT_ERR_OR_NULL(test, ptr);
 
+	OPTIMIZER_HIDE_VAR(size);
 	KUNIT_EXPECT_KASAN_FAIL(test, memset(ptr + size - 7, 0, 8));
 	kfree(ptr);
 }
@@ -476,6 +479,7 @@ static void kmalloc_oob_memset_16(struct kunit *test)
 	ptr = kmalloc(size, GFP_KERNEL);
 	KUNIT_ASSERT_NOT_ERR_OR_NULL(test, ptr);
 
+	OPTIMIZER_HIDE_VAR(size);
 	KUNIT_EXPECT_KASAN_FAIL(test, memset(ptr + size - 15, 0, 16));
 	kfree(ptr);
 }
@@ -488,6 +492,7 @@ static void kmalloc_oob_in_memset(struct kunit *test)
 	ptr = kmalloc(size, GFP_KERNEL);
 	KUNIT_ASSERT_NOT_ERR_OR_NULL(test, ptr);
 
+	OPTIMIZER_HIDE_VAR(size);
 	KUNIT_EXPECT_KASAN_FAIL(test,
 				memset(ptr, 0, size + KASAN_GRANULE_SIZE));
 	kfree(ptr);
@@ -497,7 +502,7 @@ static void kmalloc_memmove_invalid_size(struct kunit *test)
 {
 	char *ptr;
 	size_t size = 64;
-	volatile size_t invalid_size = -2;
+	size_t invalid_size = -2;
 
 	/*
 	 * Hardware tag-based mode doesn't check memmove for negative size.
@@ -510,6 +515,7 @@ static void kmalloc_memmove_invalid_size(struct kunit *test)
 	KUNIT_ASSERT_NOT_ERR_OR_NULL(test, ptr);
 
 	memset((char *)ptr, 0, 64);
+	OPTIMIZER_HIDE_VAR(invalid_size);
 	KUNIT_EXPECT_KASAN_FAIL(test,
 		memmove((char *)ptr, (char *)ptr + 4, invalid_size));
 	kfree(ptr);
diff --git a/lib/test_kasan_module.c b/lib/test_kasan_module.c
index 7ebf433edef3..b112cbc835e9 100644
--- a/lib/test_kasan_module.c
+++ b/lib/test_kasan_module.c
@@ -35,6 +35,8 @@ static noinline void __init copy_user_test(void)
 		return;
 	}
 
+	OPTIMIZER_HIDE_VAR(size);
+
 	pr_info("out-of-bounds in copy_from_user()\n");
 	unused = copy_from_user(kmem, usermem, size + 1);
 
-- 
2.30.2