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

Kees Cook
2021-03-19T19:12:35Z

On Fri, Mar 19, 2021 at 07:41:00PM +0100, Mickaël Salaün wrote:
>
> 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).

Okay. Does the test suite run sanely when landlock is missing from the
kernel?

> >
> >> +/*
> >> + * 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. ;)

Fair points.

> >
> > 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 .

It seems like this would be named better as TEST_DROPS_PRIVS or something,
which describes the reason for the fork.

--
Kees Cook