CVE-2016-8649 (C, published 2017-05-01T06:59Z)
Description: value: lxc-attach in LXC before 1.0.9 and 2.x before 2.0.6 allows an attacker inside of an unprivileged container to use an inherited file descriptor, of the host's /proc, to access the rest of the host's filesystem via the openat() family of syscalls.
Severity: HIGH (CVSS v3 base score: 9.1)
Weakness: CWE-264 \u2014 Permissions, Privileges, and Access Controls
Weaknesses in this category are related to the management of permissions, privileges, and other security features that are used to perform access control.
Repository: https://github.com/lxc/lxc
Files changed:
src/lxc/attach.c
Commit message: attach: do not send procfd to attached process

So far, we opened a file descriptor refering to proc on the host inside the
host namespace and handed that fd to the attached process in
attach_child_main(). This was done to ensure that LSM labels were correctly
setup. However, by exploiting a potential kernel bug, ptrace could be used to
prevent the file descriptor from being closed which in turn could be used by an
unprivileged container to gain access to the host namespace. Aside from this
needing an upstream kernel fix, we should make sure that we don't pass the fd
for proc itself to the attached process. However, we cannot completely prevent
this, as the attached process needs to be able to change its apparmor profile
by writing to /proc/self/attr/exec or /proc/self/attr/current. To minimize the
attack surface, we only send the fd for /proc/self/attr/exec or
/proc/self/attr/current to the attached process. To do this we introduce a
little more IPC between the child and parent:

	 * IPC mechanism: (X is receiver)
	 *   initial process        intermediate          attached
	 *        X           <---  send pid of
	 *                          attached proc,
	 *                          then exit
	 *    send 0 ------------------------------------>    X
	 *                                              [do initialization]
	 *        X  <------------------------------------  send 1
	 *   [add to cgroup, ...]
	 *    send 2 ------------------------------------>    X
	 *						[set LXC_ATTACH_NO_NEW_PRIVS]
	 *        X  <------------------------------------  send 3
	 *   [open LSM label fd]
	 *    send 4 ------------------------------------>    X
	 *   						[set LSM label]
	 *   close socket                                 close socket
	 *                                                run program

The attached child tells the parent when it is ready to have its LSM labels set
up. The parent then opens an approriate fd for the child PID to
/proc/<pid>/attr/exec or /proc/<pid>/attr/current and sends it via SCM_RIGHTS
to the child. The child can then set its LSM laben. Both sides then close the
socket fds and the child execs the requested process.

Signed-off-by: Christian Brauner <christian.brauner@canonical.com>


Vulnerable code:
int lsm_set_label_at(int procfd, int on_exec, char* lsm_label) {
	int ret = 0;
	char* command = NULL;
		goto out;
		goto out;
		labelfd = openat(procfd, "self/attr/exec", O_RDWR);
	}
	else {
		labelfd = openat(procfd, "self/attr/current", O_RDWR);
		ret = -1;
		goto out;
			ret = -1;
			ret = -1;
		if (write(labelfd, command, size + 1) < 0) {
			SYSERROR("Unable to set LSM label");
			ret = -1;
	}
	else if (strcmp(name, "SELinux") == 0) {
		if (write(labelfd, lsm_label, strlen(lsm_label) + 1) < 0) {
			ret = -1;
	}
	else {
		ret = -1;
	if (labelfd != -1)
		close(labelfd);
	return ret;
	int procfd;
	int procfd;
				ERROR("error using IPC to receive notification from attached process (1)");
			ERROR("error using IPC to notify attached process for initialization (2)");
	procfd = open("/proc", O_DIRECTORY | O_RDONLY);
	if (procfd < 0) {
		SYSERROR("Unable to open /proc");
		shutdown(ipc_sockets[1], SHUT_RDWR);
		rexit(-1);
	}

			.procfd = procfd
	int procfd = payload->procfd;
		ERROR("error using IPC to receive notification from initial process (0)");
		ERROR("error using IPC to notify initial process for initialization (1)");
		ERROR("error using IPC to receive final notification from initial process (2)");
	shutdown(ipc_socket, SHUT_RDWR);
	close(ipc_socket);

	/* set new apparmor profile/selinux context */
		if (lsm_set_label_at(procfd, on_exec, init_ctx->lsm_label) < 0) {
	/* we don't need proc anymore */
	close(procfd);


Fixed code:
#include "af_unix.h"
static int lsm_openat(int procfd, pid_t pid, int on_exec)
{
	int ret = -1;
#define __LSMATTRLEN /* /proc */ (5 + /* /pid-to-str */ 21 + /* /current */ 7 + /* \0 */ 1)
	char path[__LSMATTRLEN];
		return 0;
		return 0;
		ret = snprintf(path, __LSMATTRLEN, "%d/attr/exec", pid);
		if (ret < 0 || ret >= __LSMATTRLEN)
			return -1;
		labelfd = openat(procfd, path, O_RDWR);
	} else {
		ret = snprintf(path, __LSMATTRLEN, "%d/attr/current", pid);
		if (ret < 0 || ret >= __LSMATTRLEN)
			return -1;
		labelfd = openat(procfd, path, O_RDWR);
		return -1;
	return labelfd;
}

static int lsm_set_label_at(int lsm_labelfd, int on_exec, char *lsm_label)
{
	int fret = -1;
	const char* name;
	char *command = NULL;

	name = lsm_name();

	if (strcmp(name, "nop") == 0)
		return 0;

	if (strcmp(name, "none") == 0)
		return 0;

	/* We don't support on-exec with AppArmor */
	if (strcmp(name, "AppArmor") == 0)
		on_exec = 0;

		if (write(lsm_labelfd, command, size + 1) < 0) {
			SYSERROR("Unable to set LSM label: %s.", command);
		INFO("Set LSM label to: %s.", command);
	} else if (strcmp(name, "SELinux") == 0) {
		if (write(lsm_labelfd, lsm_label, strlen(lsm_label) + 1) < 0) {
		INFO("Set LSM label to: %s.", lsm_label);
	} else {
	fret = 0;
	if (lsm_labelfd != -1)
		close(lsm_labelfd);
	return fret;
	 *						[set LXC_ATTACH_NO_NEW_PRIVS]
	 *        X  <------------------------------------  send 3
	 *   [open LSM label fd]
	 *    send 4 ------------------------------------>    X
	 *   						[set LSM label]
		int procfd = -1;
		/* Open /proc before setns() to the containers namespace so we
		 * don't rely on any information from inside the container.
		 */
		procfd = open("/proc", O_DIRECTORY | O_RDONLY | O_CLOEXEC);
		if (procfd < 0) {
			SYSERROR("Unable to open /proc.");
			goto cleanup_error;
		}

				ERROR("error using IPC to receive notification "
				      "from attached process (1)");
			ERROR("Error using IPC to notify attached process for "
			      "initialization (2): %s.", strerror(errno));
		/* Wait for the (grand)child to tell us that it's ready to set
		 * up its LSM labels.
		 */
		expected = 3;
		ret = lxc_read_nointr_expect(ipc_sockets[0], &status, sizeof(status), &expected);
		if (ret <= 0) {
			ERROR("Error using IPC for the child to tell us to open LSM fd (3): %s.",
			      strerror(errno));
			goto cleanup_error;
		}

		/* Open LSM fd and send it to child. */
		if ((options->namespaces & CLONE_NEWNS) && (options->attach_flags & LXC_ATTACH_LSM) && init_ctx->lsm_label) {
			int on_exec, labelfd;
			on_exec = options->attach_flags & LXC_ATTACH_LSM_EXEC ? 1 : 0;
			/* Open fd for the LSM security module. */
			labelfd = lsm_openat(procfd, attached_pid, on_exec);
			if (labelfd < 0)
				goto cleanup_error;

			/* Send child fd of the LSM security module to write to. */
			ret = lxc_abstract_unix_send_fd(ipc_sockets[0], labelfd, NULL, 0);
			if (ret <= 0) {
				ERROR("Error using IPC to send child LSM fd (4): %s.",
						strerror(errno));
				goto cleanup_error;
			}
		}

		if (procfd >= 0)
			close(procfd);
	int lsm_labelfd;
		ERROR("Error using IPC to receive notification from initial process (0): %s.", strerror(errno));
		ERROR("Error using IPC to notify initial process for initialization (1): %s.", strerror(errno));
		ERROR("Error using IPC to receive message from initial process "
		      "that it is done pre-initializing (2): %s",
		      strerror(errno));
			shutdown(ipc_socket, SHUT_RDWR);
	/* Tell the (grand)parent to send us LSM label fd. */
	status = 3;
	ret = lxc_write_nointr(ipc_socket, &status, sizeof(status));
	if (ret <= 0) {
		ERROR("Error using IPC to tell parent to set up LSM labels (3): %s.", strerror(errno));
		shutdown(ipc_socket, SHUT_RDWR);
		rexit(-1);
	}

		/* Receive fd for LSM security module. */
		ret = lxc_abstract_unix_recv_fd(ipc_socket, &lsm_labelfd, NULL, 0);
		if (ret <= 0) {
			ERROR("Error using IPC for parent to tell us LSM label fd (4): %s.", strerror(errno));
			shutdown(ipc_socket, SHUT_RDWR);
			rexit(-1);
		}
		/* Change into our new LSM profile. */
		if (lsm_set_label_at(lsm_labelfd, on_exec, init_ctx->lsm_label) < 0) {
			SYSERROR("Failed to set LSM label.");
			shutdown(ipc_socket, SHUT_RDWR);
			close(lsm_labelfd);
		close(lsm_labelfd);

		shutdown(ipc_socket, SHUT_RDWR);

	shutdown(ipc_socket, SHUT_RDWR);
	close(ipc_socket);