CVE-2017-5985 (C, published 2017-03-14T17:59Z)
Description: value: lxc-user-nic in Linux Containers (LXC) allows local users with a lxc-usernet allocation to create network interfaces on the host and choose the name of those interfaces by leveraging lack of netns ownership check.
Severity: LOW (CVSS v3 base score: 3.3)
Weakness: CWE-862 \u2014 Missing Authorization
The product does not perform an authorization check when an actor attempts to access a resource or perform an action.
Repository: https://github.com/lxc/lxc
Files changed:
src/lxc/lxc_user_nic.c
Commit message: CVE-2017-5985: Ensure target netns is caller-owned

Before this commit, lxc-user-nic could potentially have been tricked into
operating on a network namespace over which the caller did not hold privilege.

This commit ensures that the caller is privileged over the network namespace by
temporarily dropping privilege.

Launchpad: https://bugs.launchpad.net/ubuntu/+source/lxc/+bug/1654676
Reported-by: Jann Horn <jannh@google.com>
Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>


Vulnerable code:

	int fd = -1, ofd = -1, ret, ifindex = -1;
		fprintf(stderr, "Failed opening network namespace path for '%d'.", getpid());
		return -1;
		fprintf(stderr, "Failed opening network namespace path for '%d'.", pid);
		return -1;
	if (setns(fd, 0) < 0) {
		fprintf(stderr, "setns to container network namespace\n");
		goto out_err;
	close(fd); fd = -1;
		if (!(ifindex = if_nametoindex(oldname))) {
			fprintf(stderr, "failed to get netdev index\n");
			goto out_err;
	if ((ret = lxc_netdev_rename_by_name(oldname, *newnamep)) < 0) {
		fprintf(stderr, "Error %d renaming netdev %s to %s in container\n", ret, oldname, *newnamep);
		goto out_err;
		char ifname[IFNAMSIZ], *namep = ifname;
			fprintf(stderr, "Failed to get new netdev name\n");
			goto out_err;
			goto out_err;
	if (setns(ofd, 0) < 0) {
		fprintf(stderr, "Error returning to original netns\n");
		close(ofd);
		return -1;
	close(ofd);
	return 0;
out_err:
	if (ofd >= 0)
		close(ofd);
	if (setns(ofd, 0) < 0)
		fprintf(stderr, "Error returning to original network namespace\n");
	return -1;

Fixed code:
#define usernic_debug_stream(stream, format, ...)                              \
	do {                                                                   \
		fprintf(stream, "%s: %d: %s: " format, __FILE__, __LINE__,     \
			__func__, __VA_ARGS__);                                \
	} while (false)

#define usernic_error(format, ...) usernic_debug_stream(stderr, format, __VA_ARGS__)

	uid_t ruid, suid, euid;
	int fret = -1;
	int fd = -1, ifindex = -1, ofd = -1, ret;
		usernic_error("Failed opening network namespace path for '%d'.", getpid());
		return fret;
		usernic_error("Failed opening network namespace path for '%d'.", pid);
		goto do_partial_cleanup;
	}

	ret = getresuid(&ruid, &euid, &suid);
	if (ret < 0) {
		usernic_error("Failed to retrieve real, effective, and saved "
			      "user IDs: %s\n",
			      strerror(errno));
		goto do_partial_cleanup;
	}

	ret = setns(fd, CLONE_NEWNET);
	close(fd);
	fd = -1;
	if (ret < 0) {
		usernic_error("Failed to setns() to the network namespace of "
			      "the container with PID %d: %s.\n",
			      pid, strerror(errno));
		goto do_partial_cleanup;
	ret = setresuid(ruid, ruid, 0);
	if (ret < 0) {
		usernic_error("Failed to drop privilege by setting effective "
			      "user id and real user id to %d, and saved user "
			      "ID to 0: %s.\n",
			      ruid, strerror(errno));
		// COMMENT(brauner): It's ok to jump to do_full_cleanup here
		// since setresuid() will succeed when trying to set real,
		// effective, and saved to values they currently have.
		goto do_full_cleanup;


		ifindex = if_nametoindex(oldname);
		if (!ifindex) {
			usernic_error("Failed to get netdev index: %s.\n", strerror(errno));
			goto do_full_cleanup;

	ret = lxc_netdev_rename_by_name(oldname, *newnamep);
	if (ret < 0) {
		usernic_error("Error %d renaming netdev %s to %s in container.\n", ret, oldname, *newnamep);
		goto do_full_cleanup;

		char ifname[IFNAMSIZ];
		char *namep = ifname;

			usernic_error("Failed to get new netdev name: %s.\n", strerror(errno));
			goto do_full_cleanup;

			goto do_full_cleanup;

	fret = 0;

do_full_cleanup:
	ret = setresuid(ruid, euid, suid);
	if (ret < 0) {
		usernic_error("Failed to restore privilege by setting effective "
			      "user id to %d, real user id to %d, and saved user "
			      "ID to %d: %s.\n",
			      ruid, euid, suid, strerror(errno));
		fret = -1;
		// COMMENT(brauner): setns() should fail if setresuid() doesn't
		// succeed but there's no harm in falling through; keeps the
		// code cleaner.
	ret = setns(ofd, CLONE_NEWNET);
	if (ret < 0) {
		usernic_error("Failed to setns() to original network namespace "
			      "of PID %d: %s.\n",
			      ofd, strerror(errno));
		fret = -1;
	}
do_partial_cleanup:
	close(ofd);

	return fret;