Re: [PATCH] kasan: test: Bypass __alloc_size checks

[Mark Rutland]

Hi Kees,

On Tue, Oct 05, 2021 at 08:55:22PM -0700, Kees Cook wrote:
> 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.

It might be better to use OPTIMIZER_HIDE_VAR(), since that's intended to
make the value opaque to the compiler, and volatile might not always do
that depending on how the compiler tracks the variable.

Thanks,
Mark.

> 
> [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>
> ---
>  lib/test_kasan.c        | 10 +++++-----
>  lib/test_kasan_module.c |  2 +-
>  2 files changed, 6 insertions(+), 6 deletions(-)
> 
> diff --git a/lib/test_kasan.c b/lib/test_kasan.c
> index 8835e0784578..0e1f8d5281b4 100644
> --- a/lib/test_kasan.c
> +++ b/lib/test_kasan.c
> @@ -435,7 +435,7 @@ static void kmalloc_uaf_16(struct kunit *test)
>  static void kmalloc_oob_memset_2(struct kunit *test)
>  {
>  	char *ptr;
> -	size_t size = 128 - KASAN_GRANULE_SIZE;
> +	volatile size_t size = 128 - KASAN_GRANULE_SIZE;
>  
>  	ptr = kmalloc(size, GFP_KERNEL);
>  	KUNIT_ASSERT_NOT_ERR_OR_NULL(test, ptr);
> @@ -447,7 +447,7 @@ static void kmalloc_oob_memset_2(struct kunit *test)
>  static void kmalloc_oob_memset_4(struct kunit *test)
>  {
>  	char *ptr;
> -	size_t size = 128 - KASAN_GRANULE_SIZE;
> +	volatile size_t size = 128 - KASAN_GRANULE_SIZE;
>  
>  	ptr = kmalloc(size, GFP_KERNEL);
>  	KUNIT_ASSERT_NOT_ERR_OR_NULL(test, ptr);
> @@ -459,7 +459,7 @@ static void kmalloc_oob_memset_4(struct kunit *test)
>  static void kmalloc_oob_memset_8(struct kunit *test)
>  {
>  	char *ptr;
> -	size_t size = 128 - KASAN_GRANULE_SIZE;
> +	volatile size_t size = 128 - KASAN_GRANULE_SIZE;
>  
>  	ptr = kmalloc(size, GFP_KERNEL);
>  	KUNIT_ASSERT_NOT_ERR_OR_NULL(test, ptr);
> @@ -471,7 +471,7 @@ static void kmalloc_oob_memset_8(struct kunit *test)
>  static void kmalloc_oob_memset_16(struct kunit *test)
>  {
>  	char *ptr;
> -	size_t size = 128 - KASAN_GRANULE_SIZE;
> +	volatile size_t size = 128 - KASAN_GRANULE_SIZE;
>  
>  	ptr = kmalloc(size, GFP_KERNEL);
>  	KUNIT_ASSERT_NOT_ERR_OR_NULL(test, ptr);
> @@ -483,7 +483,7 @@ static void kmalloc_oob_memset_16(struct kunit *test)
>  static void kmalloc_oob_in_memset(struct kunit *test)
>  {
>  	char *ptr;
> -	size_t size = 128 - KASAN_GRANULE_SIZE;
> +	volatile size_t size = 128 - KASAN_GRANULE_SIZE;
>  
>  	ptr = kmalloc(size, GFP_KERNEL);
>  	KUNIT_ASSERT_NOT_ERR_OR_NULL(test, ptr);
> diff --git a/lib/test_kasan_module.c b/lib/test_kasan_module.c
> index 7ebf433edef3..c8cc77b1dcf3 100644
> --- a/lib/test_kasan_module.c
> +++ b/lib/test_kasan_module.c
> @@ -19,7 +19,7 @@ static noinline void __init copy_user_test(void)
>  {
>  	char *kmem;
>  	char __user *usermem;
> -	size_t size = 128 - KASAN_GRANULE_SIZE;
> +	volatile size_t size = 128 - KASAN_GRANULE_SIZE;
>  	int __maybe_unused unused;
>  
>  	kmem = kmalloc(size, GFP_KERNEL);
> -- 
> 2.30.2
>