Re: [PATCH v3] mm/kfence: Add a new kunit test test_use_after_free_read_nofault()

[Ritesh Harjani (IBM) <ritesh.list@gmail.com>]

Marco Elver <elver@google.com> writes:

> On Fri, 18 Oct 2024 at 19:46, Ritesh Harjani (IBM)
> <ritesh.list@gmail.com> wrote:
>>
>> From: Nirjhar Roy <nirjhar@linux.ibm.com>
>>
>> Faults from copy_from_kernel_nofault() needs to be handled by fixup
>> table and should not be handled by kfence. Otherwise while reading
>> /proc/kcore which uses copy_from_kernel_nofault(), kfence can generate
>> false negatives. This can happen when /proc/kcore ends up reading an
>> unmapped address from kfence pool.
>>
>> Let's add a testcase to cover this case.
>>
>> Co-developed-by: Ritesh Harjani (IBM) <ritesh.list@gmail.com>
>> Signed-off-by: Nirjhar Roy <nirjhar@linux.ibm.com>
>> Signed-off-by: Ritesh Harjani (IBM) <ritesh.list@gmail.com>
>> ---
>>
>> Will be nice if we can get some feedback on this.
>
> There was some discussion recently how sanitizers should behave around
> these nofault helpers when accessing invalid memory (including freed
> memory):
> https://lore.kernel.org/all/CANpmjNMAVFzqnCZhEity9cjiqQ9CVN1X7qeeeAp_6yKjwKo8iw@mail.gmail.com/
>
> It should be similar for KFENCE, i.e. no report should be generated.
> Definitely a good thing to test.
>
> Tested-by: Marco Elver <elver@google.com>
> Reviewed-by: Marco Elver <elver@google.com>
>

Gentle ping. Is this going into -next?

-ritesh

>> v2 -> v3:
>> =========
>> 1. Separated out this kfence kunit test from the larger powerpc+kfence+v3 series.
>> 2. Dropped RFC tag
>>
>> [v2]: https://lore.kernel.org/linuxppc-dev/cover.1728954719.git.ritesh.list@gmail.com
>> [powerpc+kfence+v3]: https://lore.kernel.org/linuxppc-dev/cover.1729271995.git.ritesh.list@gmail.com
>>
>>  mm/kfence/kfence_test.c | 17 +++++++++++++++++
>>  1 file changed, 17 insertions(+)
>>
>> diff --git a/mm/kfence/kfence_test.c b/mm/kfence/kfence_test.c
>> index 00fd17285285..f65fb182466d 100644
>> --- a/mm/kfence/kfence_test.c
>> +++ b/mm/kfence/kfence_test.c
>> @@ -383,6 +383,22 @@ static void test_use_after_free_read(struct kunit *test)
>>         KUNIT_EXPECT_TRUE(test, report_matches(&expect));
>>  }
>>
>> +static void test_use_after_free_read_nofault(struct kunit *test)
>> +{
>> +       const size_t size = 32;
>> +       char *addr;
>> +       char dst;
>> +       int ret;
>> +
>> +       setup_test_cache(test, size, 0, NULL);
>> +       addr = test_alloc(test, size, GFP_KERNEL, ALLOCATE_ANY);
>> +       test_free(addr);
>> +       /* Use after free with *_nofault() */
>> +       ret = copy_from_kernel_nofault(&dst, addr, 1);
>> +       KUNIT_EXPECT_EQ(test, ret, -EFAULT);
>> +       KUNIT_EXPECT_FALSE(test, report_available());
>> +}
>> +
>>  static void test_double_free(struct kunit *test)
>>  {
>>         const size_t size = 32;
>> @@ -780,6 +796,7 @@ static struct kunit_case kfence_test_cases[] = {
>>         KFENCE_KUNIT_CASE(test_out_of_bounds_read),
>>         KFENCE_KUNIT_CASE(test_out_of_bounds_write),
>>         KFENCE_KUNIT_CASE(test_use_after_free_read),
>> +       KFENCE_KUNIT_CASE(test_use_after_free_read_nofault),
>>         KFENCE_KUNIT_CASE(test_double_free),
>>         KFENCE_KUNIT_CASE(test_invalid_addr_free),
>>         KFENCE_KUNIT_CASE(test_corruption),
>> --
>> 2.46.0
>>