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

["Paul Heidekrüger" <paul.heidekrueger@tum.de>]

On 01.02.2024 10:38, Marco Elver wrote:
> On Wed, 31 Jan 2024 at 22:01, Paul Heidekrüger <paul.heidekrueger@tum.de> 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.

If I were to add some atomic_long_* cases, e.g. atomic_long_read() or 
atomic_long_write(), in addition to the test cases I already have, wouldn't that 
mean that on 32-bit architectures we would have the same test case twice because 
atomic_read() and long_atomic_read() both boil down to raw_atomic_read() and 
raw_atomic_write() respectively? Or did I misunderstand and I should only be 
covering long_atomic_* functions whose atomic_* counterpart doesn't exist in the 
test cases already?

> > 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@google.com>
> > CC: Andrey Konovalov <andreyknvl@gmail.com>
> > Closes: https://bugzilla.kernel.org/show_bug.cgi?id=214055
> > Signed-off-by: Paul Heidekrüger <paul.heidekrueger@tum.de>
> > ---
> >  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. :-)

And yes to all the rest - thanks for the feedback!

> > +       kfree(a1);
> > +       kfree(a2);
> 
> Thanks,
> -- Marco

Many thanks,
Paul