CVE-2019-5736 (runC): rexec callers as memfd

Adam Iwaniuk and Borys Popławski discovered that an attacker can compromise the
runC host binary from inside a privileged runC container. As a result, this
could be exploited to gain root access on the host. runC is used as the default
runtime for containers with Docker, containerd, Podman, and CRI-O.

The attack can be made when attaching to a running container or when starting a
container running a specially crafted image.  For example, when runC attaches
to a container the attacker can trick it into executing itself. This could be
done by replacing the target binary inside the container with a custom binary
pointing back at the runC binary itself. As an example, if the target binary
was /bin/bash, this could be replaced with an executable script specifying the
interpreter path #!/proc/self/exe (/proc/self/exec is a symbolic link created
by the kernel for every process which points to the binary that was executed
for that process). As such when /bin/bash is executed inside the container,
instead the target of /proc/self/exe will be executed - which will point to the
runc binary on the host. The attacker can then proceed to write to the target
of /proc/self/exe to try and overwrite the runC binary on the host. However in
general, this will not succeed as the kernel will not permit it to be
overwritten whilst runC is executing. To overcome this, the attacker can
instead open a file descriptor to /proc/self/exe using the O_PATH flag and then
proceed to reopen the binary as O_WRONLY through /proc/self/fd/<nr> and try to
write to it in a busy loop from a separate process. Ultimately it will succeed
when the runC binary exits. After this the runC binary is compromised and can
be used to attack other containers or the host itself.

This attack is only possible with privileged containers since it requires root
privilege on the host to overwrite the runC binary. Unprivileged containers
with a non-identity ID mapping do not have the permission to write to the host
binary and therefore are unaffected by this attack.

LXC is also impacted in a similar manner by this vulnerability, however as the
LXC project considers privileged containers to be unsafe no CVE has been
assigned for this issue for LXC. Quoting from the
https://linuxcontainers.org/lxc/security/ project's Security information page:

"As privileged containers are considered unsafe, we typically will not consider
new container escape exploits to be security issues worthy of a CVE and quick
fix. We will however try to mitigate those issues so that accidental damage to
the host is prevented."

To prevent this attack, LXC has been patched to create a temporary copy of the
calling binary itself when it starts or attaches to containers. To do this LXC
creates an anonymous, in-memory file using the memfd_create() system call and
copies itself into the temporary in-memory file, which is then sealed to
prevent further modifications. LXC then executes this sealed, in-memory file
instead of the original on-disk binary. Any compromising write operations from
a privileged container to the host LXC binary will then write to the temporary
in-memory binary and not to the host binary on-disk, preserving the integrity
of the host LXC binary. Also as the temporary, in-memory LXC binary is sealed,
writes to this will also fail.

Note: memfd_create() was added to the Linux kernel in the 3.17 release.

Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>
Co-Developed-by: Alesa Sarai <asarai@suse.de>
Acked-by: Serge Hallyn <serge@hallyn.com>
Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>

configure.ac

@@ -746,6 +746,17 @@ AM_COND_IF([ENABLE_DLOG],
 		])
 	])
 
+AC_ARG_ENABLE([memfd-rexec],
+	[AC_HELP_STRING([--enable-memfd-rexec], [enforce liblxc as a memfd to protect against certain symlink attacks [default=yes]])],
+	[], [enable_memfd_rexec=yes])
+AM_CONDITIONAL([ENFORCE_MEMFD_REXEC], [test "x$enable_memfd_rexec" = "xyes"])
+if test "x$enable_memfd_rexec" = "xyes"; then
+	AC_DEFINE([ENFORCE_MEMFD_REXEC], 1, [Rexec liblxc as memfd])
+	AC_MSG_RESULT([yes])
+else
+	AC_MSG_RESULT([no])
+fi
+
 # Files requiring some variable expansion
 AC_CONFIG_FILES([
 	Makefile
@@ -974,6 +985,7 @@ Security features:
  - Linux capabilities: $enable_capabilities
  - seccomp: $enable_seccomp
  - SELinux: $enable_selinux
+ - memfd rexec: $enable_memfd_rexec
 
 PAM:
  - PAM module: $enable_pam

src/lxc/Makefile.am

@@ -177,6 +177,10 @@ if !HAVE_STRLCAT
 liblxc_la_SOURCES += ../include/strlcat.c ../include/strlcat.h
 endif
 
+if ENFORCE_MEMFD_REXEC
+liblxc_la_SOURCES += rexec.c
+endif
+
 AM_CFLAGS = -DLXCROOTFSMOUNT=\"$(LXCROOTFSMOUNT)\" \
 	    -DLXCPATH=\"$(LXCPATH)\" \
 	    -DLXC_GLOBAL_CONF=\"$(LXC_GLOBAL_CONF)\" \

src/lxc/file_utils.c

@@ -31,7 +31,7 @@
 #include "config.h"
 #include "file_utils.h"
 #include "macro.h"
-#include "string.h"
+#include "string_utils.h"
 
 int lxc_write_to_file(const char *filename, const void *buf, size_t count,
 		      bool add_newline, mode_t mode)
@@ -327,3 +327,42 @@ ssize_t lxc_sendfile_nointr(int out_fd, int in_fd, off_t *offset, size_t count)
 
 	return ret;
 }
+
+char *file_to_buf(char *path, size_t *length)
+{
+	int fd;
+	char buf[PATH_MAX];
+	char *copy = NULL;
+
+	if (!length)
+		return NULL;
+
+	fd = open(path, O_RDONLY | O_CLOEXEC);
+	if (fd < 0)
+		return NULL;
+
+	*length = 0;
+	for (;;) {
+		int n;
+		char *old = copy;
+
+		n = lxc_read_nointr(fd, buf, sizeof(buf));
+		if (n < 0)
+			goto on_error;
+		if (!n)
+			break;
+
+		copy = must_realloc(old, (*length + n) * sizeof(*old));
+		memcpy(copy + *length, buf, n);
+		*length += n;
+	}
+
+	close(fd);
+	return copy;
+
+on_error:
+	close(fd);
+	free(copy);
+
+	return NULL;
+}

src/lxc/file_utils.h

@@ -55,5 +55,6 @@ extern bool is_fs_type(const struct statfs *fs, fs_type_magic magic_val);
 extern FILE *fopen_cloexec(const char *path, const char *mode);
 extern ssize_t lxc_sendfile_nointr(int out_fd, int in_fd, off_t *offset,
 				   size_t count);
+extern char *file_to_buf(char *path, size_t *length);
 
 #endif /* __LXC_FILE_UTILS_H */

src/lxc/rexec.c

@@ -0,0 +1,181 @@
+/* liblxcapi
+ *
+ * Copyright © 2019 Christian Brauner <christian.brauner@ubuntu.com>.
+ * Copyright © 2019 Canonical Ltd.
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License version 2, as
+ * published by the Free Software Foundation.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License along
+ * with this program; if not, write to the Free Software Foundation, Inc.,
+ * 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
+ */
+
+#ifndef _GNU_SOURCE
+#define _GNU_SOURCE 1
+#endif
+#include <errno.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+
+#include "config.h"
+#include "file_utils.h"
+#include "raw_syscalls.h"
+#include "string_utils.h"
+#include "syscall_wrappers.h"
+
+#define LXC_MEMFD_REXEC_SEALS \
+	(F_SEAL_SEAL | F_SEAL_SHRINK | F_SEAL_GROW | F_SEAL_WRITE)
+
+static int push_vargs(char *data, int data_length, char ***output)
+{
+	int num = 0;
+	char *cur = data;
+
+	if (!data || *output)
+		return -1;
+
+	*output = must_realloc(NULL, sizeof(**output));
+
+	while (cur < data + data_length) {
+		num++;
+		*output = must_realloc(*output, (num + 1) * sizeof(**output));
+
+		(*output)[num - 1] = cur;
+		cur += strlen(cur) + 1;
+	}
+	(*output)[num] = NULL;
+	return num;
+}
+
+static int parse_exec_params(char ***argv, char ***envp)
+{
+	int ret;
+	char *cmdline = NULL, *env = NULL;
+	size_t cmdline_size, env_size;
+
+	cmdline = file_to_buf("/proc/self/cmdline", &cmdline_size);
+	if (!cmdline)
+		goto on_error;
+
+	env = file_to_buf("/proc/self/environ", &env_size);
+	if (!env)
+		goto on_error;
+
+	ret = push_vargs(cmdline, cmdline_size, argv);
+	if (ret <= 0)
+		goto on_error;
+
+	ret = push_vargs(env, env_size, envp);
+	if (ret <= 0)
+		goto on_error;
+
+	return 0;
+
+on_error:
+	free(env);
+	free(cmdline);
+
+	return -1;
+}
+
+static int is_memfd(void)
+{
+	int fd, saved_errno, seals;
+
+	fd = open("/proc/self/exe", O_RDONLY | O_CLOEXEC);
+	if (fd < 0)
+		return -ENOTRECOVERABLE;
+
+	seals = fcntl(fd, F_GET_SEALS);
+	saved_errno = errno;
+	close(fd);
+	errno = saved_errno;
+	if (seals < 0)
+		return -EINVAL;
+
+	return seals == LXC_MEMFD_REXEC_SEALS;
+}
+
+static void lxc_rexec_as_memfd(char **argv, char **envp, const char *memfd_name)
+{
+	int saved_errno;
+	ssize_t bytes_sent;
+	int fd = -1, memfd = -1;
+
+	memfd = memfd_create(memfd_name, MFD_ALLOW_SEALING | MFD_CLOEXEC);
+	if (memfd < 0)
+		return;
+
+	fd = open("/proc/self/exe", O_RDONLY | O_CLOEXEC);
+	if (fd < 0)
+		goto on_error;
+
+	/* sendfile() handles up to 2GB. */
+	bytes_sent = lxc_sendfile_nointr(memfd, fd, NULL, LXC_SENDFILE_MAX);
+	saved_errno = errno;
+	close(fd);
+	errno = saved_errno;
+	if (bytes_sent < 0)
+		goto on_error;
+
+	if (fcntl(memfd, F_ADD_SEALS, LXC_MEMFD_REXEC_SEALS))
+		goto on_error;
+
+	fexecve(memfd, argv, envp);
+
+on_error:
+	saved_errno = errno;
+	close(memfd);
+	errno = saved_errno;
+}
+
+static int lxc_rexec(const char *memfd_name)
+{
+	int ret;
+	char **argv = NULL, **envp = NULL;
+
+	ret = is_memfd();
+	if (ret < 0 && ret == -ENOTRECOVERABLE) {
+		fprintf(stderr,
+			"%s - Failed to determine whether this is a memfd\n",
+			strerror(errno));
+		return -1;
+	} else if (ret > 0) {
+		return 0;
+	}
+
+	ret = parse_exec_params(&argv, &envp);
+	if (ret < 0) {
+		fprintf(stderr,
+			"%s - Failed to parse command line parameters\n",
+			strerror(errno));
+		return -1;
+	}
+
+	lxc_rexec_as_memfd(argv, envp, memfd_name);
+	fprintf(stderr, "%s - Failed to rexec as memfd\n", strerror(errno));
+	return -1;
+}
+
+/**
+ * This function will copy any binary that calls liblxc into a memory file and
+ * will use the memfd to rexecute the binary. This is done to prevent attacks
+ * through the /proc/self/exe symlink to corrupt the host binary when host and
+ * container are in the same user namespace or have set up an identity id
+ * mapping: CVE-2019-5736.
+ */
+__attribute__((constructor)) static void liblxc_rexec(void)
+{
+	if (lxc_rexec("liblxc")) {
+		fprintf(stderr, "Failed to re-execute liblxc via memory file descriptor\n");
+		_exit(EXIT_FAILURE);
+	}
+}

src/lxc/syscall_wrappers.h

@@ -58,6 +58,20 @@ static inline long __keyctl(int cmd, unsigned long arg2, unsigned long arg3,
 #define keyctl __keyctl
 #endif
 
+#ifndef F_LINUX_SPECIFIC_BASE
+#define F_LINUX_SPECIFIC_BASE 1024
+#endif
+#ifndef F_ADD_SEALS
+#define F_ADD_SEALS (F_LINUX_SPECIFIC_BASE + 9)
+#define F_GET_SEALS (F_LINUX_SPECIFIC_BASE + 10)
+#endif
+#ifndef F_SEAL_SEAL
+#define F_SEAL_SEAL 0x0001
+#define F_SEAL_SHRINK 0x0002
+#define F_SEAL_GROW 0x0004
+#define F_SEAL_WRITE 0x0008
+#endif
+
 #ifndef HAVE_MEMFD_CREATE
 static inline int memfd_create(const char *name, unsigned int flags) {
 	#ifndef __NR_memfd_create