Re: [PATCH v8 7/9] selftests/landlock: Test FD passing from a Landlock-restricted to an unrestricted process

["Günther Noack" <gnoack3000@gmail.com>]

On Wed, Oct 05, 2022 at 08:57:12PM +0200, Mickaël Salaün wrote:
> Thanks for this refactoring.
> 
> We need a shorter subject though. ;)

Done, changed to:
"selftests/landlock: Test FD passing from restricted to unrestricted processes"

> On 01/10/2022 17:49, Günther Noack wrote:
> > A file descriptor created in a restricted process carries Landlock
> > restrictions with it which will apply even if the same opened file is
> > used from an unrestricted process.
> > 
> > This change extracts suitable FD-passing helpers from base_test.c and
> > moves them to common.h. We use the fixture variants from the ftruncate
> > fixture to exercise the same scenarios as in the open_and_ftruncate
> > test, but doing the Landlock restriction and open() in a different
> > process than the ftruncate() call.
> > 
> > Signed-off-by: Günther Noack <gnoack3000@gmail.com>
> > ---
> >   tools/testing/selftests/landlock/base_test.c | 36 +----------
> >   tools/testing/selftests/landlock/common.h    | 67 ++++++++++++++++++++
> >   tools/testing/selftests/landlock/fs_test.c   | 62 ++++++++++++++++++
> >   3 files changed, 132 insertions(+), 33 deletions(-)
> > 
> > diff --git a/tools/testing/selftests/landlock/base_test.c b/tools/testing/selftests/landlock/base_test.c
> > index 72cdae277b02..6d1b6eedb432 100644
> > --- a/tools/testing/selftests/landlock/base_test.c
> > +++ b/tools/testing/selftests/landlock/base_test.c
> > @@ -263,23 +263,6 @@ TEST(ruleset_fd_transfer)
> >   		.allowed_access = LANDLOCK_ACCESS_FS_READ_DIR,
> >   	};
> >   	int ruleset_fd_tx, dir_fd;
> > -	union {
> > -		/* Aligned ancillary data buffer. */
> > -		char buf[CMSG_SPACE(sizeof(ruleset_fd_tx))];
> > -		struct cmsghdr _align;
> > -	} cmsg_tx = {};
> > -	char data_tx = '.';
> > -	struct iovec io = {
> > -		.iov_base = &data_tx,
> > -		.iov_len = sizeof(data_tx),
> > -	};
> > -	struct msghdr msg = {
> > -		.msg_iov = &io,
> > -		.msg_iovlen = 1,
> > -		.msg_control = &cmsg_tx.buf,
> > -		.msg_controllen = sizeof(cmsg_tx.buf),
> > -	};
> > -	struct cmsghdr *cmsg;
> >   	int socket_fds[2];
> >   	pid_t child;
> >   	int status;
> > @@ -298,33 +281,20 @@ TEST(ruleset_fd_transfer)
> >   				    &path_beneath_attr, 0));
> >   	ASSERT_EQ(0, close(path_beneath_attr.parent_fd));
> > -	cmsg = CMSG_FIRSTHDR(&msg);
> > -	ASSERT_NE(NULL, cmsg);
> > -	cmsg->cmsg_len = CMSG_LEN(sizeof(ruleset_fd_tx));
> > -	cmsg->cmsg_level = SOL_SOCKET;
> > -	cmsg->cmsg_type = SCM_RIGHTS;
> > -	memcpy(CMSG_DATA(cmsg), &ruleset_fd_tx, sizeof(ruleset_fd_tx));
> > -
> >   	/* Sends the ruleset FD over a socketpair and then close it. */
> >   	ASSERT_EQ(0, socketpair(AF_UNIX, SOCK_STREAM | SOCK_CLOEXEC, 0,
> >   				socket_fds));
> > -	ASSERT_EQ(sizeof(data_tx), sendmsg(socket_fds[0], &msg, 0));
> > +	ASSERT_EQ(0, send_fd(socket_fds[0], ruleset_fd_tx));
> >   	ASSERT_EQ(0, close(socket_fds[0]));
> >   	ASSERT_EQ(0, close(ruleset_fd_tx));
> >   	child = fork();
> >   	ASSERT_LE(0, child);
> >   	if (child == 0) {
> > -		int ruleset_fd_rx;
> > +		int ruleset_fd_rx = recv_fd(socket_fds[1]);
> 
> We can now make it const.

Done.

> > -		*(char *)msg.msg_iov->iov_base = '\0';
> > -		ASSERT_EQ(sizeof(data_tx),
> > -			  recvmsg(socket_fds[1], &msg, MSG_CMSG_CLOEXEC));
> > -		ASSERT_EQ('.', *(char *)msg.msg_iov->iov_base);
> > +		ASSERT_LE(0, ruleset_fd_rx);
> >   		ASSERT_EQ(0, close(socket_fds[1]));
> > -		cmsg = CMSG_FIRSTHDR(&msg);
> > -		ASSERT_EQ(cmsg->cmsg_len, CMSG_LEN(sizeof(ruleset_fd_tx)));
> > -		memcpy(&ruleset_fd_rx, CMSG_DATA(cmsg), sizeof(ruleset_fd_tx));
> >   		/* Enforces the received ruleset on the child. */
> >   		ASSERT_EQ(0, prctl(PR_SET_NO_NEW_PRIVS, 1, 0, 0, 0));
> > diff --git a/tools/testing/selftests/landlock/common.h b/tools/testing/selftests/landlock/common.h
> > index 7d34592471db..15149d34e43b 100644
> > --- a/tools/testing/selftests/landlock/common.h
> > +++ b/tools/testing/selftests/landlock/common.h
> > @@ -10,6 +10,7 @@
> >   #include <errno.h>
> >   #include <linux/landlock.h>
> >   #include <sys/capability.h>
> > +#include <sys/socket.h>
> >   #include <sys/syscall.h>
> >   #include <sys/types.h>
> >   #include <sys/wait.h>
> > @@ -189,3 +190,69 @@ static void __maybe_unused clear_cap(struct __test_metadata *const _metadata,
> >   {
> >   	_effective_cap(_metadata, caps, CAP_CLEAR);
> >   }
> > +
> > +/* Receives an FD from a UNIX socket. Returns the received FD, -1 on error. */
> > +static int __maybe_unused recv_fd(int usock)
> > +{
> > +	int fd_rx;
> > +	union {
> > +		/* Aligned ancillary data buffer. */
> > +		char buf[CMSG_SPACE(sizeof(fd_rx))];
> > +		struct cmsghdr _align;
> > +	} cmsg_rx = {};
> > +	char data = '\0';
> > +	struct iovec io = {
> > +		.iov_base = &data,
> > +		.iov_len = sizeof(data),
> > +	};
> > +	struct msghdr msg = {
> > +		.msg_iov = &io,
> > +		.msg_iovlen = 1,
> > +		.msg_control = &cmsg_rx.buf,
> > +		.msg_controllen = sizeof(cmsg_rx.buf),
> > +	};
> > +	struct cmsghdr *cmsg;
> > +	int res;
> > +
> > +	res = recvmsg(usock, &msg, MSG_CMSG_CLOEXEC);
> > +	if (res < 0)
> > +		return -1;
> 
> It could be useful to return -errno for recv_fd() and send_fd(), and update
> the description accordingly. That would enable to quickly know the error
> with the ASSERT_*() calls.

Done, good idea.


> > +	cmsg = CMSG_FIRSTHDR(&msg);
> > +	if (cmsg->cmsg_len != CMSG_LEN(sizeof(fd_rx)))
> > +		return -1;

I made it return -EIO in the case where the other side sends invalid data.


> > +	memcpy(&fd_rx, CMSG_DATA(cmsg), sizeof(fd_rx));
> > +	return fd_rx;
> > +}
> > +
> > +/* Sends an FD on a UNIX socket. Returns 0 on success or -1 on error. */
> > +static int __maybe_unused send_fd(int usock, int fd_tx)
> > +{
> > +	union {
> > +		/* Aligned ancillary data buffer. */
> > +		char buf[CMSG_SPACE(sizeof(fd_tx))];
> > +		struct cmsghdr _align;
> > +	} cmsg_tx = {};
> > +	char data_tx = '.';
> > +	struct iovec io = {
> > +		.iov_base = &data_tx,
> > +		.iov_len = sizeof(data_tx),
> > +	};
> > +	struct msghdr msg = {
> > +		.msg_iov = &io,
> > +		.msg_iovlen = 1,
> > +		.msg_control = &cmsg_tx.buf,
> > +		.msg_controllen = sizeof(cmsg_tx.buf),
> > +	};
> > +	struct cmsghdr *cmsg = CMSG_FIRSTHDR(&msg);
> > +
> > +	cmsg->cmsg_len = CMSG_LEN(sizeof(fd_tx));
> > +	cmsg->cmsg_level = SOL_SOCKET;
> > +	cmsg->cmsg_type = SCM_RIGHTS;
> > +	memcpy(CMSG_DATA(cmsg), &fd_tx, sizeof(fd_tx));
> > +
> > +	if (sendmsg(usock, &msg, 0) < 0)
> > +		return -1;
> > +	return 0;
> > +}
> > diff --git a/tools/testing/selftests/landlock/fs_test.c b/tools/testing/selftests/landlock/fs_test.c
> > index 308f6f36e8c0..93ed80a25a74 100644
> > --- a/tools/testing/selftests/landlock/fs_test.c
> > +++ b/tools/testing/selftests/landlock/fs_test.c
> > @@ -3541,6 +3541,68 @@ TEST_F_FORK(ftruncate, open_and_ftruncate)
> >   	}
> >   }
> > +TEST_F_FORK(ftruncate, open_and_ftruncate_in_different_processes)
> > +{
> > +	int child, fd, status;
> > +	int socket_fds[2];
> > +
> > +	ASSERT_EQ(0, socketpair(AF_UNIX, SOCK_STREAM | SOCK_CLOEXEC, 0,
> > +				socket_fds));
> > +
> > +	child = fork();
> > +	ASSERT_LE(0, child);
> > +	if (child == 0) {
> > +		/*
> > +		 * Enable Landlock in the child process, open a file descriptor
> 
> "Enables"…

Done.

> > +		 * where truncation is forbidden and send it to the
> > +		 * non-landlocked parent process.
> > +		 */
> > +		const char *const path = file1_s1d1;
> > +		const struct rule rules[] = {
> > +			{
> > +				.path = path,
> > +				.access = variant->permitted,
> > +			},
> > +			{},
> > +		};
> > +		int fd, ruleset_fd;
> > +
> > +		/* Enable Landlock. */
> 
> This comment is not necessary.

Done.

> > +		ruleset_fd = create_ruleset(_metadata, variant->handled, rules);
> > +		ASSERT_LE(0, ruleset_fd);
> > +		enforce_ruleset(_metadata, ruleset_fd);
> > +		ASSERT_EQ(0, close(ruleset_fd));
> > +
> > +		fd = open(path, O_WRONLY);
> > +		ASSERT_EQ(variant->expected_open_result, (fd < 0 ? errno : 0));
> > +
> > +		if (fd >= 0) {
> > +			ASSERT_EQ(0, send_fd(socket_fds[0], fd));
> > +			ASSERT_EQ(0, close(fd));
> > +		}
> > +
> > +		ASSERT_EQ(0, close(socket_fds[0]));
> > +
> > +		_exit(_metadata->passed ? EXIT_SUCCESS : EXIT_FAILURE);
> 
> You might want to add a "return" here to help the compiler (and the reader).

Done.

> > +	}
> > +
> > +	if (variant->expected_open_result == 0) {
> > +		fd = recv_fd(socket_fds[1]);
> > +		ASSERT_LE(0, fd);
> > +
> > +		EXPECT_EQ(variant->expected_ftruncate_result,
> > +			  test_ftruncate(fd));
> > +		ASSERT_EQ(0, close(fd));
> > +	}
> > +
> > +	ASSERT_EQ(child, waitpid(child, &status, 0));
> > +	ASSERT_EQ(1, WIFEXITED(status));
> > +	ASSERT_EQ(EXIT_SUCCESS, WEXITSTATUS(status));
> > +
> > +	ASSERT_EQ(0, close(socket_fds[0]));
> > +	ASSERT_EQ(0, close(socket_fds[1]));
> > +}
> > +
> >   /* clang-format off */
> >   FIXTURE(layout1_bind) {};
> >   /* clang-format on */

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