Re: [PATCH RFC v2] kasan: add atomic tests

[Marco Elver]

On Wed, 31 Jan 2024 at 22:01, Paul Heidekrüger <paul.heidekrueger@xxxxxx> wrote:
>
> Hi!
>
> This RFC patch adds tests that detect whether KASan is able to catch
> unsafe atomic accesses.
>
> Since v1, which can be found on Bugzilla (see "Closes:" tag), I've made
> the following suggested changes:
>
> * Adjust size of allocations to make kasan_atomics() work with all KASan modes
> * Remove comments and move tests closer to the bitops tests
> * For functions taking two addresses as an input, test each address in a separate function call.
> * Rename variables for clarity
> * Add tests for READ_ONCE(), WRITE_ONCE(), smp_load_acquire() and smp_store_release()
>
> I'm still uncelar on which kinds of atomic accesses we should be testing
> though. The patch below only covers a subset, and I don't know if it
> would be feasible to just manually add all atomics of interest. Which
> ones would those be exactly?

The atomics wrappers are generated by a script. An exhaustive test
case would, if generated by hand, be difficult to keep in sync if some
variants are removed or renamed (although that's probably a relatively
rare occurrence).

I would probably just cover some of the most common ones that all
architectures (that support KASAN) provide. I think you are already
covering some of the most important ones, and I'd just say it's good
enough for the test.

> As Andrey pointed out on Bugzilla, if we
> were to include all of the atomic64_* ones, that would make a lot of
> function calls.

Just include a few atomic64_ cases, similar to the ones you already
include for atomic_. Although beware that the atomic64_t helpers are
likely not available on 32-bit architectures, so you need an #ifdef
CONFIG_64BIT.

Alternatively, there is also atomic_long_t, which (on 64-bit
architectures) just wraps atomic64_t helpers, and on 32-bit the
atomic_t ones. I'd probably opt for the atomic_long_t variants, just
to keep it simpler and get some additional coverage on 32-bit
architectures.

> Also, the availability of atomics varies between architectures; I did my
> testing on arm64. Is something like gen-atomic-instrumented.sh required?

I would not touch gen-atomic-instrumented.sh for the test.

> Many thanks,
> Paul
>
> CC: Marco Elver <elver@xxxxxxxxxx>
> CC: Andrey Konovalov <andreyknvl@xxxxxxxxx>
> Closes: https://bugzilla.kernel.org/show_bug.cgi?id=214055
> Signed-off-by: Paul Heidekrüger <paul.heidekrueger@xxxxxx>
> ---
>  mm/kasan/kasan_test.c | 50 +++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 50 insertions(+)
>
> diff --git a/mm/kasan/kasan_test.c b/mm/kasan/kasan_test.c
> index 8281eb42464b..1ab4444fe4a0 100644
> --- a/mm/kasan/kasan_test.c
> +++ b/mm/kasan/kasan_test.c
> @@ -1150,6 +1150,55 @@ static void kasan_bitops_tags(struct kunit *test)
>         kfree(bits);
>  }
>
> +static void kasan_atomics_helper(struct kunit *test, void *unsafe, void *safe)
> +{
> +       int *i_safe = (int *)safe;
> +       int *i_unsafe = (int *)unsafe;
> +
> +       KUNIT_EXPECT_KASAN_FAIL(test, READ_ONCE(*i_unsafe));
> +       KUNIT_EXPECT_KASAN_FAIL(test, WRITE_ONCE(*i_unsafe, 42));
> +       KUNIT_EXPECT_KASAN_FAIL(test, smp_load_acquire(i_unsafe));
> +       KUNIT_EXPECT_KASAN_FAIL(test, smp_store_release(i_unsafe, 42));
> +       KUNIT_EXPECT_KASAN_FAIL(test, atomic_read(unsafe));
> +       KUNIT_EXPECT_KASAN_FAIL(test, atomic_set(unsafe, 42));
> +       KUNIT_EXPECT_KASAN_FAIL(test, atomic_add(42, unsafe));
> +       KUNIT_EXPECT_KASAN_FAIL(test, atomic_sub(42, unsafe));
> +       KUNIT_EXPECT_KASAN_FAIL(test, atomic_inc(unsafe));
> +       KUNIT_EXPECT_KASAN_FAIL(test, atomic_dec(unsafe));
> +       KUNIT_EXPECT_KASAN_FAIL(test, atomic_and(42, unsafe));
> +       KUNIT_EXPECT_KASAN_FAIL(test, atomic_andnot(42, unsafe));
> +       KUNIT_EXPECT_KASAN_FAIL(test, atomic_or(42, unsafe));
> +       KUNIT_EXPECT_KASAN_FAIL(test, atomic_xor(42, unsafe));
> +       KUNIT_EXPECT_KASAN_FAIL(test, atomic_xchg(unsafe, 42));
> +       KUNIT_EXPECT_KASAN_FAIL(test, atomic_cmpxchg(unsafe, 21, 42));
> +       KUNIT_EXPECT_KASAN_FAIL(test, atomic_try_cmpxchg(unsafe, safe, 42));
> +       KUNIT_EXPECT_KASAN_FAIL(test, atomic_try_cmpxchg(safe, unsafe, 42));
> +       KUNIT_EXPECT_KASAN_FAIL(test, atomic_sub_and_test(42, unsafe));
> +       KUNIT_EXPECT_KASAN_FAIL(test, atomic_dec_and_test(unsafe));
> +       KUNIT_EXPECT_KASAN_FAIL(test, atomic_inc_and_test(unsafe));
> +       KUNIT_EXPECT_KASAN_FAIL(test, atomic_add_negative(42, unsafe));
> +       KUNIT_EXPECT_KASAN_FAIL(test, atomic_add_unless(unsafe, 21, 42));
> +       KUNIT_EXPECT_KASAN_FAIL(test, atomic_inc_not_zero(unsafe));
> +       KUNIT_EXPECT_KASAN_FAIL(test, atomic_inc_unless_negative(unsafe));
> +       KUNIT_EXPECT_KASAN_FAIL(test, atomic_dec_unless_positive(unsafe));
> +       KUNIT_EXPECT_KASAN_FAIL(test, atomic_dec_if_positive(unsafe));
> +}
> +
> +static void kasan_atomics(struct kunit *test)
> +{
> +       int *a1, *a2;

If you're casting it to void* below and never using as an int* in this
function, just make these void* (the sizeof can just be sizeof(int)).

> +       a1 = kzalloc(48, GFP_KERNEL);
> +       KUNIT_ASSERT_NOT_ERR_OR_NULL(test, a1);
> +       a2 = kzalloc(sizeof(*a1), GFP_KERNEL);
> +       KUNIT_ASSERT_NOT_ERR_OR_NULL(test, a1);
> +
> +       kasan_atomics_helper(test, (void *)a1 + 48, (void *)a2);

We try to ensure (where possible) that the KASAN tests are not
destructive to the rest of the kernel. I think the size of "48" was
chosen to fall into the 64-byte size class, similar to the bitops. I
would just copy that comment, so nobody attempts to change it in
future. :-)

> +       kfree(a1);
> +       kfree(a2);

Thanks,
-- Marco