kasan: test: avoid writing invalid memory

Multiple KASAN tests do writes past the allocated objects or writes to
freed memory.  Turn these writes into reads to avoid corrupting memory.
Otherwise, these tests might lead to crashes with the HW_TAGS mode, as it
neither uses quarantine nor redzones.

Link: https://lkml.kernel.org/r/c3cd2a383e757e27dd9131635fc7d09a48a49cf9.1628779805.git.andreyknvl@gmail.comSigned-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.c

@@ -167,7 +167,7 @@ static void kmalloc_node_oob_right(struct kunit *test)
 	ptr = kmalloc_node(size, GFP_KERNEL, 0);
 	KUNIT_ASSERT_NOT_ERR_OR_NULL(test, ptr);
 
-	KUNIT_EXPECT_KASAN_FAIL(test, ptr[size] = 0);
+	KUNIT_EXPECT_KASAN_FAIL(test, ptr[0] = ptr[size]);
 	kfree(ptr);
 }
 
@@ -203,7 +203,7 @@ static void kmalloc_pagealloc_uaf(struct kunit *test)
 	KUNIT_ASSERT_NOT_ERR_OR_NULL(test, ptr);
 	kfree(ptr);
 
-	KUNIT_EXPECT_KASAN_FAIL(test, ptr[0] = 0);
+	KUNIT_EXPECT_KASAN_FAIL(test, ((volatile char *)ptr)[0]);
 }
 
 static void kmalloc_pagealloc_invalid_free(struct kunit *test)
@@ -237,7 +237,7 @@ static void pagealloc_oob_right(struct kunit *test)
 	ptr = page_address(pages);
 	KUNIT_ASSERT_NOT_ERR_OR_NULL(test, ptr);
 
-	KUNIT_EXPECT_KASAN_FAIL(test, ptr[size] = 0);
+	KUNIT_EXPECT_KASAN_FAIL(test, ptr[0] = ptr[size]);
 	free_pages((unsigned long)ptr, order);
 }
 
@@ -252,7 +252,7 @@ static void pagealloc_uaf(struct kunit *test)
 	KUNIT_ASSERT_NOT_ERR_OR_NULL(test, ptr);
 	free_pages((unsigned long)ptr, order);
 
-	KUNIT_EXPECT_KASAN_FAIL(test, ptr[0] = 0);
+	KUNIT_EXPECT_KASAN_FAIL(test, ((volatile char *)ptr)[0]);
 }
 
 static void kmalloc_large_oob_right(struct kunit *test)
@@ -514,7 +514,7 @@ static void kmalloc_uaf(struct kunit *test)
 	KUNIT_ASSERT_NOT_ERR_OR_NULL(test, ptr);
 
 	kfree(ptr);
-	KUNIT_EXPECT_KASAN_FAIL(test, *(ptr + 8) = 'x');
+	KUNIT_EXPECT_KASAN_FAIL(test, ((volatile char *)ptr)[8]);
 }
 
 static void kmalloc_uaf_memset(struct kunit *test)
@@ -553,7 +553,7 @@ static void kmalloc_uaf2(struct kunit *test)
 		goto again;
 	}
 
-	KUNIT_EXPECT_KASAN_FAIL(test, ptr1[40] = 'x');
+	KUNIT_EXPECT_KASAN_FAIL(test, ((volatile char *)ptr1)[40]);
 	KUNIT_EXPECT_PTR_NE(test, ptr1, ptr2);
 
 	kfree(ptr2);
@@ -700,7 +700,7 @@ static void ksize_unpoisons_memory(struct kunit *test)
 	ptr[size] = 'x';
 
 	/* This one must. */
-	KUNIT_EXPECT_KASAN_FAIL(test, ptr[real_size] = 'y');
+	KUNIT_EXPECT_KASAN_FAIL(test, ((volatile char *)ptr)[real_size]);
 
 	kfree(ptr);
 }