--- /dev/null
+From 0000000000000000000000000000000000000000 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
+MIME-Version: 1.0
+Content-Type: text/plain; charset=UTF-8
+Content-Transfer-Encoding: 8bit
+
+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>
+(cherry picked from commit 6400238d08cdf1ca20d49bafb85f4e224348bf9d)
+---
+ configure.ac | 12 +++
+ src/lxc/Makefile.am | 4 +
+ src/lxc/file_utils.c | 41 +++++++++-
+ src/lxc/file_utils.h | 1 +
+ src/lxc/rexec.c | 181 +++++++++++++++++++++++++++++++++++++++++++++
+ src/lxc/syscall_wrappers.h | 14 ++++
+ 6 files changed, 252 insertions(+), 1 deletion(-)
+ create mode 100644 src/lxc/rexec.c
+
+diff --git a/configure.ac b/configure.ac
+index 9f3b8fb3..3177f7da 100644
+--- a/configure.ac
++++ b/configure.ac
+@@ -727,6 +727,17 @@ 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
+@@ -956,6 +967,7 @@ Security features:
+ - Linux capabilities: $enable_capabilities
+ - seccomp: $enable_seccomp
+ - SELinux: $enable_selinux
++ - memfd rexec: $enable_memfd_rexec
+
+ PAM:
+ - PAM module: $enable_pam
+diff --git a/src/lxc/Makefile.am b/src/lxc/Makefile.am
+index 95b0a2f7..865d341f 100644
+--- a/src/lxc/Makefile.am
++++ b/src/lxc/Makefile.am
+@@ -175,6 +175,10 @@ if !HAVE_STRLCAT
+ liblxc_la_SOURCES += ../include/strlcat.c ../include/strlcat.h
+ endif
+
++if ENFORCE_MEMFD_REXEC
++liblxc_la_SOURCES += rexec.c
++endif
++
+ AM_CFLAGS = -DLXCROOTFSMOUNT=\"$(LXCROOTFSMOUNT)\" \
+ -DLXCPATH=\"$(LXCPATH)\" \
+ -DLXC_GLOBAL_CONF=\"$(LXC_GLOBAL_CONF)\" \
+diff --git a/src/lxc/file_utils.c b/src/lxc/file_utils.c
+index f89aa638..930fd738 100644
+--- a/src/lxc/file_utils.c
++++ b/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 @@ again:
+
+ 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;
++}
+diff --git a/src/lxc/file_utils.h b/src/lxc/file_utils.h
+index 6361557a..518a61af 100644
+--- a/src/lxc/file_utils.h
++++ b/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 */
+diff --git a/src/lxc/rexec.c b/src/lxc/rexec.c
+new file mode 100644
+index 00000000..396bd617
+--- /dev/null
++++ b/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);
++ }
++}
+diff --git a/src/lxc/syscall_wrappers.h b/src/lxc/syscall_wrappers.h
+index 42d94db2..dca4d157 100644
+--- a/src/lxc/syscall_wrappers.h
++++ b/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
+--
+2.11.0
+
projects / lxc.git / commitdiff