CVE-2016-8649 (published 2017-05-01T06:59Z)

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 v2 base score: 9.0 (AV:N/AC:L/Au:S/C:C/I:C/A:C)
CVSS v3 base score: 9.1, CRITICAL (CVSS:3.0/AV:N/AC:L/PR:H/UI:N/S:C/C:H/I:H/A:H)

Weakness information:
cwe_id: CWE-264
cwe_name: Permissions, Privileges, and Access Controls
description: Weaknesses in this category are related to the management of permissions, privileges, and other security features that are used to perform access control.

Fix information:
repo_url: https://github.com/lxc/lxc
commit_details: repo_url: https://github.com/lxc/lxc
author: Christian Brauner
author_date: 2016-11-08 19:21:19+01:00
msg: 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>
repository: repo_url: https://github.com/lxc/lxc
repo_name: lxc/lxc
description: LXC - Linux Containers
repo_language: C
owner: lxc
file_changes: filename: attach.c
change_type: ModificationType.MODIFY
diff: @@ -53,6 +53,7 @@
 
 #include "namespace.h"
 #include "log.h"
+#include "af_unix.h"
 #include "attach.h"
 #include "caps.h"
 #include "config.h"
@@ -84,80 +85,103 @@
 
 lxc_log_define(lxc_attach, lxc);
 
-int lsm_set_label_at(int procfd, int on_exec, char* lsm_label) {
+static int lsm_openat(int procfd, pid_t pid, int on_exec)
+{
+	int ret = -1;
 	int labelfd = -1;
-	int ret = 0;
 	const char* name;
-	char* command = NULL;
+#define __LSMATTRLEN /* /proc */ (5 + /* /pid-to-str */ 21 + /* /current */ 7 + /* \0 */ 1)
+	char path[__LSMATTRLEN];
 
 	name = lsm_name();
 
 	if (strcmp(name, "nop") == 0)
-		goto out;
+		return 0;
 
 	if (strcmp(name, "none") == 0)
-		goto out;
+		return 0;
 
 	/* We don't support on-exec with AppArmor */
 	if (strcmp(name, "AppArmor") == 0)
 		on_exec = 0;
 
 	if (on_exec) {
-		labelfd = openat(procfd, "self/attr/exec", O_RDWR);
-	}
-	else {
-		labelfd = openat(procfd, "self/attr/current", O_RDWR);
+		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);
 	}
 
 	if (labelfd < 0) {
 		SYSERROR("Unable to open LSM label");
-		ret = -1;
-		goto out;
+		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 (strcmp(name, "AppArmor") == 0) {
 		int size;
 
 		command = malloc(strlen(lsm_label) + strlen("changeprofile ") + 1);
 		if (!command) {
 			SYSERROR("Failed to write apparmor profile");
-			ret = -1;
 			goto out;
 		}
 
 		size = sprintf(command, "changeprofile %s", lsm_label);
 		if (size < 0) {
 			SYSERROR("Failed to write apparmor profile");
-			ret = -1;
 			goto out;
 		}
 
-		if (write(labelfd, command, size + 1) < 0) {
-			SYSERROR("Unable to set LSM label");
-			ret = -1;
+		if (write(lsm_labelfd, command, size + 1) < 0) {
+			SYSERROR("Unable to set LSM label: %s.", command);
 			goto out;
 		}
-	}
-	else if (strcmp(name, "SELinux") == 0) {
-		if (write(labelfd, lsm_label, strlen(lsm_label) + 1) < 0) {
+		INFO("Set LSM label to: %s.", command);
+	} else if (strcmp(name, "SELinux") == 0) {
+		if (write(lsm_labelfd, lsm_label, strlen(lsm_label) + 1) < 0) {
 			SYSERROR("Unable to set LSM label");
-			ret = -1;
 			goto out;
 		}
-	}
-	else {
+		INFO("Set LSM label to: %s.", lsm_label);
+	} else {
 		ERROR("Unable to restore label for unknown LSM: %s", name);
-		ret = -1;
 		goto out;
 	}
+	fret = 0;
 
 out:
 	free(command);
 
-	if (labelfd != -1)
-		close(labelfd);
+	if (lsm_labelfd != -1)
+		close(lsm_labelfd);
 
-	return ret;
+	return fret;
 }
 
 static struct lxc_proc_context_info *lxc_proc_get_context_info(pid_t pid)
@@ -646,7 +670,6 @@ struct attach_clone_payload {
 	struct lxc_proc_context_info* init_ctx;
 	lxc_attach_exec_t exec_function;
 	void* exec_payload;
-	int procfd;
 };
 
 static int attach_child_main(void* data);
@@ -744,7 +767,6 @@ int lxc_attach(const char* name, const char* lxcpath, lxc_attach_exec_t exec_fun
 	char* cwd;
 	char* new_cwd;
 	int ipc_sockets[2];
-	int procfd;
 	signed long personality;
 
 	if (!options)
@@ -821,6 +843,11 @@ int lxc_attach(const char* name, const char* lxcpath, lxc_attach_exec_t exec_fun
 	 *        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
 	 */
@@ -854,6 +881,7 @@ int lxc_attach(const char* name, const char* lxcpath, lxc_attach_exec_t exec_fun
 	}
 
 	if (pid) {
+		int procfd = -1;
 		pid_t to_cleanup_pid = pid;
 
 		/* initial thread, we close the socket that is for the
@@ -868,6 +896,15 @@ int lxc_attach(const char* name, const char* lxcpath, lxc_attach_exec_t exec_fun
 				goto cleanup_error;
 		}
 
+		/* 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;
+		}
+
 		/* Let the child process know to go ahead */
 		status = 0;
 		ret = lxc_write_nointr(ipc_sockets[0], &status, sizeof(status));
@@ -911,7 +948,8 @@ int lxc_attach(const char* name, const char* lxcpath, lxc_attach_exec_t exec_fun
 		ret = lxc_read_nointr_expect(ipc_sockets[0], &status, sizeof(status), &expected);
 		if (ret <= 0) {
 			if (ret != 0)
-				ERROR("error using IPC to receive notification from attached process (1)");
+				ERROR("error using IPC to receive notification "
+				      "from attached process (1)");
 			goto cleanup_error;
 		}
 
@@ -919,10 +957,40 @@ int lxc_attach(const char* name, const char* lxcpath, lxc_attach_exec_t exec_fun
 		status = 2;
 		ret = lxc_write_nointr(ipc_sockets[0], &status, sizeof(status));
 		if (ret <= 0) {
-			ERROR("error using IPC to notify attached process for initialization (2)");
+			ERROR("Error using IPC to notify attached process for "
+			      "initialization (2): %s.", strerror(errno));
 			goto cleanup_error;
 		}
 
+		/* 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;
+			}
+		}
+
 		/* now shut down communication with child, we're done */
 		shutdown(ipc_sockets[0], SHUT_RDWR);
 		close(ipc_sockets[0]);
@@ -940,6 +1008,8 @@ int lxc_attach(const char* name, const char* lxcpath, lxc_attach_exec_t exec_fun
 		/* first shut down the socket, then wait for the pid,
 		 * otherwise the pid we're waiting for may never exit
 		 */
+		if (procfd >= 0)
+			close(procfd);
 		shutdown(ipc_sockets[0], SHUT_RDWR);
 		close(ipc_sockets[0]);
 		if (to_cleanup_pid)
@@ -966,13 +1036,6 @@ int lxc_attach(const char* name, const char* lxcpath, lxc_attach_exec_t exec_fun
 	if ((options->attach_flags & LXC_ATTACH_MOVE_TO_CGROUP) && cgns_supported())
 		options->namespaces |= CLONE_NEWCGROUP;
 
-	procfd = open("/proc", O_DIRECTORY | O_RDONLY);
-	if (procfd < 0) {
-		SYSERROR("Unable to open /proc");
-		shutdown(ipc_sockets[1], SHUT_RDWR);
-		rexit(-1);
-	}
-
 	/* attach now, create another subprocess later, since pid namespaces
 	 * only really affect the children of the current process
 	 */
@@ -1001,7 +1064,6 @@ int lxc_attach(const char* name, const char* lxcpath, lxc_attach_exec_t exec_fun
 			.init_ctx = init_ctx,
 			.exec_function = exec_function,
 			.exec_payload = exec_payload,
-			.procfd = procfd
 		};
 		/* We use clone_parent here to make this subprocess a direct child of
 		 * the initial process. Then this intermediate process can exit and
@@ -1039,7 +1101,6 @@ static int attach_child_main(void* data)
 {
 	struct attach_clone_payload* payload = (struct attach_clone_payload*)data;
 	int ipc_socket = payload->ipc_socket;
-	int procfd = payload->procfd;
 	lxc_attach_options_t* options = payload->options;
 	struct lxc_proc_context_info* init_ctx = payload->init_ctx;
 #if HAVE_SYS_PERSONALITY_H
@@ -1050,6 +1111,7 @@ static int attach_child_main(void* data)
 	int expected;
 	long flags;
 	int fd;
+	int lsm_labelfd;
 	uid_t new_uid;
 	gid_t new_gid;
 
@@ -1060,7 +1122,7 @@ static int attach_child_main(void* data)
 	status = -1;
 	ret = lxc_read_nointr_expect(ipc_socket, &status, sizeof(status), &expected);
 	if (ret <= 0) {
-		ERROR("error using IPC to receive notification from initial process (0)");
+		ERROR("Error using IPC to receive notification from initial process (0): %s.", strerror(errno));
 		shutdown(ipc_socket, SHUT_RDWR);
 		rexit(-1);
 	}
@@ -1159,7 +1221,7 @@ static int attach_child_main(void* data)
 	status = 1;
 	ret = lxc_write_nointr(ipc_socket, &status, sizeof(status));
 	if (ret != sizeof(status)) {
-		ERROR("error using IPC to notify initial process for initialization (1)");
+		ERROR("Error using IPC to notify initial process for initialization (1): %s.", strerror(errno));
 		shutdown(ipc_socket, SHUT_RDWR);
 		rexit(-1);
 	}
@@ -1171,14 +1233,13 @@ static int attach_child_main(void* data)
 	status = -1;
 	ret = lxc_read_nointr_expect(ipc_socket, &status, sizeof(status), &expected);
 	if (ret <= 0) {
-		ERROR("error using IPC to receive final notification from initial process (2)");
+		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);
 		rexit(-1);
 	}
 
-	shutdown(ipc_socket, SHUT_RDWR);
-	close(ipc_socket);
-
 	if ((init_ctx->container && init_ctx->container->lxc_conf &&
 	     init_ctx->container->lxc_conf->no_new_privs) ||
 	    (options->attach_flags & LXC_ATTACH_NO_NEW_PRIVS)) {
@@ -1186,27 +1247,53 @@ static int attach_child_main(void* data)
 			SYSERROR("PR_SET_NO_NEW_PRIVS could not be set. "
 				 "Process can use execve() gainable "
 				 "privileges.");
+			shutdown(ipc_socket, SHUT_RDWR);
 			rexit(-1);
 		}
 		INFO("PR_SET_NO_NEW_PRIVS is set. Process cannot use execve() "
 		     "gainable privileges.");
 	}
 
-	/* set new apparmor profile/selinux context */
+	/* 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);
+	}
+
 	if ((options->namespaces & CLONE_NEWNS) && (options->attach_flags & LXC_ATTACH_LSM) && init_ctx->lsm_label) {
 		int on_exec;
+		/* 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. */
 		on_exec = options->attach_flags & LXC_ATTACH_LSM_EXEC ? 1 : 0;
-		if (lsm_set_label_at(procfd, on_exec, init_ctx->lsm_label) < 0) {
+		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);
 			rexit(-1);
 		}
+		close(lsm_labelfd);
 	}
+
 	if (init_ctx->container && init_ctx->container->lxc_conf &&
 	    init_ctx->container->lxc_conf->seccomp &&
 	    (lxc_seccomp_load(init_ctx->container->lxc_conf) != 0)) {
 		ERROR("Loading seccomp policy");
+		shutdown(ipc_socket, SHUT_RDWR);
 		rexit(-1);
 	}
+
+	shutdown(ipc_socket, SHUT_RDWR);
+	close(ipc_socket);
 	lxc_proc_put_context_info(init_ctx);
 
 	/* The following is done after the communication socket is
@@ -1245,9 +1332,6 @@ static int attach_child_main(void* data)
 				SYSERROR("Unable to clear CLOEXEC from fd");
 	}
 
-	/* we don't need proc anymore */
-	close(procfd);
-
 	/* we're done, so we can now do whatever the user intended us to do */
 	rexit(payload->exec_function(payload->exec_payload));
 }

programming_language: C