Re: [PATCH v5] mm, kasan, kmsan: copy_from/to_kernel_nofault

[Andrey Konovalov]

On Thu, Oct 10, 2024 at 3:10 PM Sabyrzhan Tasbolatov
<snovitoll@xxxxxxxxx> wrote:
>
> diff --git a/mm/kasan/kasan_test_c.c b/mm/kasan/kasan_test_c.c
> index a181e4780d9d..cb6ad84641ec 100644
> --- a/mm/kasan/kasan_test_c.c
> +++ b/mm/kasan/kasan_test_c.c
> @@ -1954,6 +1954,42 @@ static void rust_uaf(struct kunit *test)
>         KUNIT_EXPECT_KASAN_FAIL(test, kasan_test_rust_uaf());
>  }
>
> +static void copy_to_kernel_nofault_oob(struct kunit *test)
> +{
> +       char *ptr;
> +       char buf[128];
> +       size_t size = sizeof(buf);
> +
> +       /* This test currently fails with the HW_TAGS mode.
> +        * The reason is unknown and needs to be investigated. */
> +       ptr = kmalloc(size - KASAN_GRANULE_SIZE, GFP_KERNEL);
> +       KUNIT_ASSERT_NOT_ERR_OR_NULL(test, ptr);
> +       OPTIMIZER_HIDE_VAR(ptr);
> +
> +       if (IS_ENABLED(CONFIG_KASAN_SW_TAGS)) {
> +               /* Check that the returned pointer is tagged. */
> +               KUNIT_EXPECT_GE(test, (u8)get_tag(ptr), (u8)KASAN_TAG_MIN);
> +               KUNIT_EXPECT_LT(test, (u8)get_tag(ptr), (u8)KASAN_TAG_KERNEL);
> +       }

It appears you deleted a wrong check. I meant the checks above, not
the CONFIG_KASAN_HW_TAGS one.

> +
> +       /*
> +       * We test copy_to_kernel_nofault() to detect corrupted memory that is
> +       * being written into the kernel. In contrast, copy_from_kernel_nofault()
> +       * is primarily used in kernel helper functions where the source address
> +       * might be random or uninitialized. Applying KASAN instrumentation to
> +       * copy_from_kernel_nofault() could lead to false positives.
> +       * By focusing KASAN checks only on copy_to_kernel_nofault(),
> +       * we ensure that only valid memory is written to the kernel,
> +       * minimizing the risk of kernel corruption while avoiding
> +       * false positives in the reverse case.
> +       */
> +       KUNIT_EXPECT_KASAN_FAIL(test,
> +               copy_to_kernel_nofault(&buf[0], ptr, size));
> +       KUNIT_EXPECT_KASAN_FAIL(test,
> +               copy_to_kernel_nofault(ptr, &buf[0], size));

Nit: empty line before kfree.

> +       kfree(ptr);
> +}