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

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Message-ID: <202103191207.E12FD4E51@keescook>
Date:   Fri, 19 Mar 2021 12:11:39 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Mickaël Salaün <mic@...ikod.net>
Cc:     James Morris <jmorris@...ei.org>, Jann Horn <jannh@...gle.com>,
        "Serge E . Hallyn" <serge@...lyn.com>,
        Al Viro <viro@...iv.linux.org.uk>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Andy Lutomirski <luto@...capital.net>,
        Anton Ivanov <anton.ivanov@...bridgegreys.com>,
        Arnd Bergmann <arnd@...db.de>,
        Casey Schaufler <casey@...aufler-ca.com>,
        David Howells <dhowells@...hat.com>,
        Jeff Dike <jdike@...toit.com>,
        Jonathan Corbet <corbet@....net>,
        Michael Kerrisk <mtk.manpages@...il.com>,
        Richard Weinberger <richard@....at>,
        Shuah Khan <shuah@...nel.org>,
        Vincent Dagonneau <vincent.dagonneau@....gouv.fr>,
        kernel-hardening@...ts.openwall.com, linux-api@...r.kernel.org,
        linux-arch@...r.kernel.org, linux-doc@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-kselftest@...r.kernel.org,
        linux-security-module@...r.kernel.org, x86@...nel.org,
        Mickaël Salaün <mic@...ux.microsoft.com>,
        Dmitry Vyukov <dvyukov@...gle.com>
Subject: Re: [PATCH v30 10/12] selftests/landlock: Add user space tests

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@...ux.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@...ei.org>
> >> Cc: Jann Horn <jannh@...gle.com>
> >> Cc: Kees Cook <keescook@...omium.org>
> >> Cc: Serge E. Hallyn <serge@...lyn.com>
> >> Cc: Shuah Khan <shuah@...nel.org>
> >> Signed-off-by: Mickaël Salaün <mic@...ux.microsoft.com>
> >> Reviewed-by: Vincent Dagonneau <vincent.dagonneau@....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