Re: [PATCH 1/2] kasan: stop tests being eliminated as dead code with FORTIFY_SOURCE

[Dmitry Vyukov]

On Wed, Jan 15, 2020 at 7:37 AM Daniel Axtens <dja@xxxxxxxxxx> wrote:
>
> 3 KASAN self-tests fail on a kernel with both KASAN and FORTIFY_SOURCE:
> memchr, memcmp and strlen.
>
> When FORTIFY_SOURCE is on, a number of functions are replaced with
> fortified versions, which attempt to check the sizes of the operands.
> However, these functions often directly invoke __builtin_foo() once they
> have performed the fortify check. The compiler can detect that the results
> of these functions are not used, and knows that they have no other side
> effects, and so can eliminate them as dead code.
>
> Why are only memchr, memcmp and strlen affected?
> ================================================
>
> Of string and string-like functions, kasan_test tests:
>
>  * strchr  ->  not affected, no fortified version
>  * strrchr ->  likewise
>  * strcmp  ->  likewise
>  * strncmp ->  likewise
>
>  * strnlen ->  not affected, the fortify source implementation calls the
>                underlying strnlen implementation which is instrumented, not
>                a builtin
>
>  * strlen  ->  affected, the fortify souce implementation calls a __builtin
>                version which the compiler can determine is dead.
>
>  * memchr  ->  likewise
>  * memcmp  ->  likewise
>
>  * memset ->   not affected, the compiler knows that memset writes to its
>                first argument and therefore is not dead.
>
> Why does this not affect the functions normally?
> ================================================
>
> In string.h, these functions are not marked as __pure, so the compiler
> cannot know that they do not have side effects. If relevant functions are
> marked as __pure in string.h, we see the following warnings and the
> functions are elided:
>
> lib/test_kasan.c: In function âkasan_memchrâ:
> lib/test_kasan.c:606:2: warning: statement with no effect [-Wunused-value]
>   memchr(ptr, '1', size + 1);
>   ^~~~~~~~~~~~~~~~~~~~~~~~~~
> lib/test_kasan.c: In function âkasan_memcmpâ:
> lib/test_kasan.c:622:2: warning: statement with no effect [-Wunused-value]
>   memcmp(ptr, arr, size+1);
>   ^~~~~~~~~~~~~~~~~~~~~~~~
> lib/test_kasan.c: In function âkasan_stringsâ:
> lib/test_kasan.c:645:2: warning: statement with no effect [-Wunused-value]
>   strchr(ptr, '1');
>   ^~~~~~~~~~~~~~~~
> ...
>
> This annotation would make sense to add and could be added at any point, so
> the behaviour of test_kasan.c should change.
>
> The fix
> =======
>
> Make all the functions that are pure write their results to a global,
> which makes them live. The strlen and memchr tests now pass.
>
> The memcmp test still fails to trigger, which is addressed in the next
> patch.
>
> Cc: Daniel Micay <danielmicay@xxxxxxxxx>
> Cc: Andrey Ryabinin <aryabinin@xxxxxxxxxxxxx>
> Cc: Alexander Potapenko <glider@xxxxxxxxxx>
> Cc: Dmitry Vyukov <dvyukov@xxxxxxxxxx>
> Fixes: 0c96350a2d2f ("lib/test_kasan.c: add tests for several string/memory API functions")
> Signed-off-by: Daniel Axtens <dja@xxxxxxxxxx>
> ---
>  lib/test_kasan.c | 30 +++++++++++++++++++-----------
>  1 file changed, 19 insertions(+), 11 deletions(-)
>
> diff --git a/lib/test_kasan.c b/lib/test_kasan.c
> index 328d33beae36..58a8cef0d7a2 100644
> --- a/lib/test_kasan.c
> +++ b/lib/test_kasan.c
> @@ -23,6 +23,14 @@
>
>  #include <asm/page.h>
>
> +/*
> + * We assign some test results to these globals to make sure the tests
> + * are not eliminated as dead code.
> + */
> +
> +int int_result;
> +void *ptr_result;

These are globals, but are not static and don't have kasan_ prefix.
But I guess this does not matter for modules?
Otherwise:

Reviewed-by: Dmitry Vyukov <dvyukov@xxxxxxxxxx>

> +
>  /*
>   * Note: test functions are marked noinline so that their names appear in
>   * reports.
> @@ -603,7 +611,7 @@ static noinline void __init kasan_memchr(void)
>         if (!ptr)
>                 return;
>
> -       memchr(ptr, '1', size + 1);
> +       ptr_result = memchr(ptr, '1', size + 1);
>         kfree(ptr);
>  }
>
> @@ -618,8 +626,7 @@ static noinline void __init kasan_memcmp(void)
>         if (!ptr)
>                 return;
>
> -       memset(arr, 0, sizeof(arr));
> -       memcmp(ptr, arr, size+1);
> +       int_result = memcmp(ptr, arr, size + 1);
>         kfree(ptr);
>  }
>
> @@ -642,22 +649,22 @@ static noinline void __init kasan_strings(void)
>          * will likely point to zeroed byte.
>          */
>         ptr += 16;
> -       strchr(ptr, '1');
> +       ptr_result = strchr(ptr, '1');
>
>         pr_info("use-after-free in strrchr\n");
> -       strrchr(ptr, '1');
> +       ptr_result = strrchr(ptr, '1');
>
>         pr_info("use-after-free in strcmp\n");
> -       strcmp(ptr, "2");
> +       int_result = strcmp(ptr, "2");
>
>         pr_info("use-after-free in strncmp\n");
> -       strncmp(ptr, "2", 1);
> +       int_result = strncmp(ptr, "2", 1);
>
>         pr_info("use-after-free in strlen\n");
> -       strlen(ptr);
> +       int_result = strlen(ptr);
>
>         pr_info("use-after-free in strnlen\n");
> -       strnlen(ptr, 1);
> +       int_result = strnlen(ptr, 1);
>  }
>
>  static noinline void __init kasan_bitops(void)
> @@ -724,11 +731,12 @@ static noinline void __init kasan_bitops(void)
>         __test_and_change_bit(BITS_PER_LONG + BITS_PER_BYTE, bits);
>
>         pr_info("out-of-bounds in test_bit\n");
> -       (void)test_bit(BITS_PER_LONG + BITS_PER_BYTE, bits);
> +       int_result = test_bit(BITS_PER_LONG + BITS_PER_BYTE, bits);
>
>  #if defined(clear_bit_unlock_is_negative_byte)
>         pr_info("out-of-bounds in clear_bit_unlock_is_negative_byte\n");
> -       clear_bit_unlock_is_negative_byte(BITS_PER_LONG + BITS_PER_BYTE, bits);
> +       int_result = clear_bit_unlock_is_negative_byte(BITS_PER_LONG +
> +               BITS_PER_BYTE, bits);
>  #endif
>         kfree(bits);
>  }
> --
> 2.20.1
>