Re: [PATCH v30 10/12] selftests/landlock: Add user space tests

Mickaël Salaün
2021-03-19T18:41:35Z

On 19/03/2021 18:56, Kees Cook wrote:
> On Tue, Mar 16, 2021 at 09:42:50PM +0100, Mickaël Salaün wrote:
>> From: Mickaël Salaün <mic@linux.microsoft.com>
>>
>> Test all Landlock system calls, ptrace hooks semantic and filesystem
>> access-control with multiple layouts.
>>
>> Test coverage for security/landlock/ is 93.6% of lines. The code not
>> covered only deals with internal kernel errors (e.g. memory allocation)
>> and race conditions.
>>
>> Cc: James Morris <jmorris@namei.org>
>> Cc: Jann Horn <jannh@google.com>
>> Cc: Kees Cook <keescook@chromium.org>
>> Cc: Serge E. Hallyn <serge@hallyn.com>
>> Cc: Shuah Khan <shuah@kernel.org>
>> Signed-off-by: Mickaël Salaün <mic@linux.microsoft.com>
>> Reviewed-by: Vincent Dagonneau <vincent.dagonneau@ssi.gouv.fr>
>> Link: https://lore.kernel.org/r/20210316204252.427806-11-mic@digikod.net
>
> This is terrific. I love the coverage. How did you measure this, BTW?

I used gcov: https://www.kernel.org/doc/html/latest/dev-tools/gcov.html

> To increase it into memory allocation failures, have you tried
> allocation fault injection:
> https://www.kernel.org/doc/html/latest/fault-injection/fault-injection.html

Yes, it is used by syzkaller, but I don't know how to extract this
specific coverage.

>
>> [...]
>> +TEST(inconsistent_attr) {
>> + const long page_size = sysconf(_SC_PAGESIZE);
>> + char *const buf = malloc(page_size + 1);
>> + struct landlock_ruleset_attr *const ruleset_attr = (void *)buf;
>> +
>> + ASSERT_NE(NULL, buf);
>> +
>> + /* Checks copy_from_user(). */
>> + ASSERT_EQ(-1, landlock_create_ruleset(ruleset_attr, 0, 0));
>> + /* The size if less than sizeof(struct landlock_attr_enforce). */
>> + ASSERT_EQ(EINVAL, errno);
>> + ASSERT_EQ(-1, landlock_create_ruleset(ruleset_attr, 1, 0));
>> + ASSERT_EQ(EINVAL, errno);
>
> Almost everywhere you're using ASSERT instead of EXPECT. Is this correct
> (in the sense than as soon as an ASSERT fails the rest of the test is
> skipped)? I do see you using EXPECT is some places, but I figured I'd
> ask about the intention here.

I intentionally use ASSERT as much as possible, but I use EXPECT when an
error could block a test or when it could stop a cleanup (i.e. teardown).

>
>> +/*
>> + * TEST_F_FORK() is useful when a test drop privileges but the corresponding
>> + * FIXTURE_TEARDOWN() requires them (e.g. to remove files from a directory
>> + * where write actions are denied). For convenience, FIXTURE_TEARDOWN() is
>> + * also called when the test failed, but not when FIXTURE_SETUP() failed. For
>> + * this to be possible, we must not call abort() but instead exit smoothly
>> + * (hence the step print).
>> + */
>
> Hm, interesting. I think this should be extracted into a separate patch
> and added to the test harness proper.

I agree, but it may require some modifications to fit nicely in
kselftest_harness.h . For now, it works well for my use case. I'll send
patches once Landlock is merged. In fact, I already made
kselftest_harness.h available for other users than seccomp. ;)

>
> Could this be solved with TEARDOWN being called on SETUP failure?

The goal of this helper is to still be able to call TEARDOWN when TEST
failed, not SETUP.

>
>> +#define TEST_F_FORK(fixture_name, test_name) \
>> + static void fixture_name##_##test_name##_child( \
>> + struct __test_metadata *_metadata, \
>> + FIXTURE_DATA(fixture_name) *self, \
>> + const FIXTURE_VARIANT(fixture_name) *variant); \
>> + TEST_F(fixture_name, test_name) \
>> + { \
>> + int status; \
>> + const pid_t child = fork(); \
>> + if (child < 0) \
>> + abort(); \
>> + if (child == 0) { \
>> + _metadata->no_print = 1; \
>> + fixture_name##_##test_name##_child(_metadata, self, variant); \
>> + if (_metadata->skip) \
>> + _exit(255); \
>> + if (_metadata->passed) \
>> + _exit(0); \
>> + _exit(_metadata->step); \
>> + } \
>> + if (child != waitpid(child, &status, 0)) \
>> + abort(); \
>> + if (WIFSIGNALED(status) || !WIFEXITED(status)) { \
>> + _metadata->passed = 0; \
>> + _metadata->step = 1; \
>> + return; \
>> + } \
>> + switch (WEXITSTATUS(status)) { \
>> + case 0: \
>> + _metadata->passed = 1; \
>> + break; \
>> + case 255: \
>> + _metadata->passed = 1; \
>> + _metadata->skip = 1; \
>> + break; \
>> + default: \
>> + _metadata->passed = 0; \
>> + _metadata->step = WEXITSTATUS(status); \
>> + break; \
>> + } \
>> + } \
>
> This looks like a subset of __wait_for_test()? Could __TEST_F_IMPL() be
> updated instead to do this? (Though the fork overhead might not be great
> for everyone.)

Yes, it will probably be my approach to update kselftest_harness.h .