From f45ab7937e5b2bf1c112dc39e38c05cf73573213 Mon Sep 17 00:00:00 2001
From: Christian Brauner <christian.brauner@ubuntu.com>
Date: Sat, 26 Jan 2019 01:19:29 +0100
Subject: [PATCH] CVE-2019-5736 (runC): rexec callers as memfd
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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: Aleksa Sarai <asarai@suse.de>
Acked-by: Serge Hallyn <serge@hallyn.com>
---
 configure.ac        |  13 +++
 src/lxc/Makefile.am |  45 ++++----
 src/lxc/rexec.c     | 242 ++++++++++++++++++++++++++++++++++++++++++++
 3 files changed, 278 insertions(+), 22 deletions(-)
 create mode 100644 src/lxc/rexec.c

diff --git a/configure.ac b/configure.ac
index f8c008458..6ef4d9986 100644
--- a/configure.ac
+++ b/configure.ac
@@ -679,6 +679,18 @@ AC_ARG_ENABLE([thread-safety],
 	fi
 fi
 
+
+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
@@ -926,6 +938,7 @@ Security features:
  - seccomp: $enable_seccomp
  - SELinux: $enable_selinux
  - cgmanager: $enable_cgmanager
+ - memfd rexec: $enable_memfd_rexec
 
 Bindings:
  - lua: $enable_lua
diff --git a/src/lxc/Makefile.am b/src/lxc/Makefile.am
index 60400d401..96f139dac 100644
--- a/src/lxc/Makefile.am
+++ b/src/lxc/Makefile.am
@@ -146,24 +146,29 @@ if !HAVE_STRLCAT
 endif
 endif
 
-AM_CFLAGS=-DLXCROOTFSMOUNT=\"$(LXCROOTFSMOUNT)\" \
-	-DLXCPATH=\"$(LXCPATH)\" \
-	-DLXC_GLOBAL_CONF=\"$(LXC_GLOBAL_CONF)\" \
-	-DLXCINITDIR=\"$(LXCINITDIR)\" \
-	-DLIBEXECDIR=\"$(LIBEXECDIR)\" \
-	-DLXCTEMPLATEDIR=\"$(LXCTEMPLATEDIR)\" \
-	-DLXCTEMPLATECONFIG=\"$(LXCTEMPLATECONFIG)\" \
-	-DLOGPATH=\"$(LOGPATH)\" \
-	-DLXC_DEFAULT_CONFIG=\"$(LXC_DEFAULT_CONFIG)\" \
-	-DLXC_USERNIC_DB=\"$(LXC_USERNIC_DB)\" \
-	-DLXC_USERNIC_CONF=\"$(LXC_USERNIC_CONF)\" \
-	-DDEFAULT_CGROUP_PATTERN=\"$(DEFAULT_CGROUP_PATTERN)\" \
-	-DRUNTIME_PATH=\"$(RUNTIME_PATH)\" \
-	-DSBINDIR=\"$(SBINDIR)\" \
-	-I $(top_srcdir)/src \
-	-I $(top_srcdir)/src/lxc \
-	-I $(top_srcdir)/src/lxc/bdev \
-	-I $(top_srcdir)/src/lxc/cgroups
+if ENFORCE_MEMFD_REXEC
+liblxc_la_SOURCES += rexec.c
+endif
+
+AM_CFLAGS = -DLXCROOTFSMOUNT=\"$(LXCROOTFSMOUNT)\" \
+	    -DLXCPATH=\"$(LXCPATH)\" \
+	    -DLXC_GLOBAL_CONF=\"$(LXC_GLOBAL_CONF)\" \
+	    -DLXCINITDIR=\"$(LXCINITDIR)\" \
+	    -DLIBEXECDIR=\"$(LIBEXECDIR)\" \
+	    -DLXCTEMPLATEDIR=\"$(LXCTEMPLATEDIR)\" \
+	    -DLXCTEMPLATECONFIG=\"$(LXCTEMPLATECONFIG)\" \
+	    -DLOGPATH=\"$(LOGPATH)\" \
+	    -DLXC_DEFAULT_CONFIG=\"$(LXC_DEFAULT_CONFIG)\" \
+	    -DLXC_USERNIC_DB=\"$(LXC_USERNIC_DB)\" \
+	    -DLXC_USERNIC_CONF=\"$(LXC_USERNIC_CONF)\" \
+	    -DDEFAULT_CGROUP_PATTERN=\"$(DEFAULT_CGROUP_PATTERN)\" \
+	    -DRUNTIME_PATH=\"$(RUNTIME_PATH)\" \
+	    -DSBINDIR=\"$(SBINDIR)\" \
+	    -DAPPARMOR_CACHE_DIR=\"$(APPARMOR_CACHE_DIR)\" \
+	    -I $(top_srcdir)/src \
+	    -I $(top_srcdir)/src/lxc \
+	    -I $(top_srcdir)/src/lxc/bdev \
+	    -I $(top_srcdir)/src/lxc/cgroups
 
 if ENABLE_APPARMOR
 AM_CFLAGS += -DHAVE_APPARMOR
diff --git a/src/lxc/rexec.c b/src/lxc/rexec.c
new file mode 100644
index 000000000..05eaa75a9
--- /dev/null
+++ b/src/lxc/rexec.c
@@ -0,0 +1,242 @@
+/* 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 <fcntl.h>
+#include <limits.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <sys/sendfile.h>
+#include <sys/stat.h>
+#include <sys/types.h>
+#include <unistd.h>
+
+#include "config.h"
+#include "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 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;
+}
+
+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;
+}
+
+/* Maximum number of bytes sendfile() is able to send in one go. */
+#define LXC_SENDFILE_MAX 0x7ffff000
+static ssize_t lxc_sendfile_nointr(int out_fd, int in_fd, off_t *offset, size_t count)
+{
+	ssize_t ret;
+
+again:
+	ret = sendfile(out_fd, in_fd, offset, count);
+	if (ret < 0) {
+		if (errno == EINTR)
+			goto again;
+
+		return -1;
+	}
+
+	return ret;
+}
+
+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);
+	}
+}