On Fri, May 29, 2020 at 12:31:37AM -0700, Kees Cook wrote:

On Thu, May 28, 2020 at 04:08:57AM -0700, Sargun Dhillon wrote:

quoted

This adds a seccomp notifier ioctl which allows for the listener to "add"
file descriptors to a process which originated a seccomp user
notification. This allows calls like mount, and mknod to be "implemented",
as the return value, and the arguments are data in memory. On the other
hand, calls like connect can be "implemented" using pidfd_getfd.

Unfortunately, there are calls which return file descriptors, like
open, which are vulnerable to TOC-TOU attacks, and require that the
more privileged supervisor can inspect the argument, and perform the
syscall on behalf of the process generating the notifiation. This
allows the file descriptor generated from that open call to be
returned to the calling process.

In addition, there is funcitonality to allow for replacement of
specific file descriptors, following dup2-like semantics.

Signed-off-by: Sargun Dhillon <redacted>
Suggested-by: Matt Denton <redacted>

This looks mostly really clean. When I've got more brain tomorrow I want to
double-check the locking, but I think the use of notify_lock and being
in the ioctl fully protects everything from any use-after-free-like
issues.

Notes below...

quoted

+/* valid flags for seccomp_notif_addfd */
+#define SECCOMP_ADDFD_FLAG_SETFD	(1UL << 0) /* Specify remote fd */

Nit: please use BIT()

quoted

@@ -735,6 +770,41 @@ static u64 seccomp_next_notify_id(struct seccomp_filter *filter)
 	return filter->notif->next_id++;
 }
 
+static void seccomp_handle_addfd(struct seccomp_kaddfd *addfd)
+{
+	struct socket *sock;
+	int ret, err;
+
+	/*
+	 * Remove the notification, and reset the list pointers, indicating
+	 * that it has been handled.
+	 */
+	list_del_init(&addfd->list);
+
+	ret = security_file_receive(addfd->file);
+	if (ret)
+		goto out;
+
+	if (addfd->fd == -1) {
+		ret = get_unused_fd_flags(addfd->flags);
+		if (ret >= 0)
+			fd_install(ret, get_file(addfd->file));
+	} else {
+		ret = replace_fd(addfd->fd, addfd->file, addfd->flags);
+	}
+
+	/* These are the semantics from copying FDs via SCM_RIGHTS */
+	sock = sock_from_file(addfd->file, &err);
+	if (sock) {
+		sock_update_netprioidx(&sock->sk->sk_cgrp_data);
+		sock_update_classid(&sock->sk->sk_cgrp_data);
+	}

This made my eye twitch. ;) I see this is borrowed from
scm_detach_fds()... this really feels like the kind of thing that will
quickly go out of sync. I think this "receive an fd" logic needs to be
lifted out of scm_detach_fds() so it and seccomp can share it. I'm not
sure how to parameterize it quite right, though. Perhaps:

int file_receive(int fd, unsigned long flags, struct file *file)
{
	struct socket *sock;
	int ret;

	ret = security_file_receive(file);
	if (ret)
		return ret;

	/* Install the file. */
	if (fd == -1) {
		ret = get_unused_fd_flags(flags);
		if (ret >= 0)
			fd_install(ret, get_file(file));
	} else {
		ret = replace_fd(fd, file, flags);
	}

	/* Bump the usage count. */
	sock = sock_from_file(addfd->file, &err);
	if (sock) {
		sock_update_netprioidx(&sock->sk->sk_cgrp_data);
		sock_update_classid(&sock->sk->sk_cgrp_data);
	}

	return ret;
}


static void seccomp_handle_addfd(struct seccomp_kaddfd *addfd)
{
	/*
	 * Remove the notification, and reset the list pointers, indicating
	 * that it has been handled.
	 */
	list_del_init(&addfd->list);
	addfd->ret = file_receive(addfd->fd, addfd->flags, addfd->file);
	complete(&addfd->completion);
}

scm_detach_fds()
	...
	for (i=0, cmfptr=(__force int __user *)CMSG_DATA(cm); i<fdmax;
             i++, cmfptr++)
	{

		err = file_receive(-1, MSG_CMSG_CLOEXEC & msg->msg_flags
                                          ? O_CLOEXEC : 0, fp[i]);
		if (err < 0)
			break;
		err = put_user(err, cmfptr);
		if (err)
			/* wat */
	}
	...

I'm not sure on the put_user() failure, though. We could check early
for faults with a put_user(0, cmfptr) before the file_receive() call, or
we could just ignore it? I'm not sure what SCM does here. I guess
worst-case:

int file_receive(int fd, unsigned long flags, struct file *file,
		 int __user *fdptr)
{
		...
		ret = get_unused_fd_flags(flags);
		if (ret >= 0) {
			if (cmfptr) {
				int err;

	                	err = put_user(ret, cmfptr);
				if (err) {
					put_unused_fd(ret);
					return err;
				}
			}
			fd_install(ret, get_file(file));
		}
		...
}

What about:

/*
 * File Receive - Retrieve a file from another process
 *
 * It can either replace an existing fd, or use a newly allocated fd. If you
 * intend on using an existing fd, replace should be false, and flags will
 * be ignored. The fd should be allocated using get_unused_fd_flags with the
 * flags that you want. It does not consume the reference to file.
 *
 * Returns 0 upon success
 */
static int __file_receive(int fd, unsigned int flags, struct file *file,
			  bool replace)
{
	struct socket *sock;
	int err;

	err = security_file_receive(file);
	if (err)
		return err;

	/* Is this an existing FD? */
	if (replace) {
		err = replace_fd(fd, file, flags);
		if (err)
			return err;
	} else {
		fd_install(fd, get_file(file));
	}

	sock = sock_from_file(file, &err);
	if (sock) {
		sock_update_netprioidx(&sock->sk->sk_cgrp_data);
		sock_update_classid(&sock->sk->sk_cgrp_data);
	}

	return 0;
}

int file_receive_replace(int fd, unsigned int flags, struct file *file)
{
	return __file_receive(fd, flags, file, true);
}

int file_receive(int fd, struct file *file)
{
	return __file_receive(fd, 0, file, false);
}


// And then SCM reads:
	for (i=0, cmfptr=(__force int __user *)CMSG_DATA(cm); i<fdmax;
	     i++, cmfptr++)
	{
		int new_fd;
		err = get_unused_fd_flags(MSG_CMSG_CLOEXEC & msg->msg_flags
					  ? O_CLOEXEC : 0);
		if (err < 0)
			break;
		new_fd = err;
		err = put_user(new_fd, cmfptr);
		if (err) {
			put_unused_fd(new_fd);
			break;
		}

		err = file_receive(new_fd, fp[i]);
		if (err) {
			put_unused_fd(new_fd);
			break;
		}
	}

And our code reads:


static void seccomp_handle_addfd(struct seccomp_kaddfd *addfd)
{
	int ret, err;

	/*
	 * Remove the notification, and reset the list pointers, indicating
	 * that it has been handled.
	 */
	list_del_init(&addfd->list);

	if (addfd->fd == -1) {
		ret = get_unused_fd_flags(addfd->flags);
		if (ret < 0)
			goto err;

		err = file_receive(ret, addfd->file);
		if (err) {
			put_unused_fd(ret);
			ret = err;
		}
	} else {
		ret = file_receive_replace(addfd->fd, addfd->flags,
					   addfd->file);
	}

err:
	addfd->ret = ret;
	complete(&addfd->completion);
}


And the pidfd getfd code reads:

static int pidfd_getfd(struct pid *pid, int fd)
{
	struct task_struct *task;
	struct file *file;
	int ret, err;

	task = get_pid_task(pid, PIDTYPE_PID);
	if (!task)
		return -ESRCH;

	file = __pidfd_fget(task, fd);
	put_task_struct(task);
	if (IS_ERR(file))
		return PTR_ERR(file);

	ret = get_unused_fd_flags(O_CLOEXEC);
	if (ret >= 0) {
		err = file_receive(ret, file);
		if (err) {
			put_unused_fd(ret);
			ret = err;
		}
	}

	fput(file);
	return ret;
}