From 6400238d08cdf1ca20d49bafb85f4e224348bf9d Mon Sep 17 00:00:00 2001 From: Christian Brauner 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/ 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 Co-Developed-by: Alesa Sarai Acked-by: Serge Hallyn Signed-off-by: Christian Brauner --- 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 8313b18d1..d43dabc0d 100644 --- a/configure.ac +++ b/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: $enable_pam diff --git a/src/lxc/Makefile.am b/src/lxc/Makefile.am index 6ba9ecad2..dc8aa3d77 100644 --- a/src/lxc/Makefile.am +++ b/src/lxc/Makefile.am @@ -177,6 +177,10 @@ if !HAVE_STRLCAT liblxc_la_SOURCES += ../include/strlcat.c ../include/strlcat.h endif +if ENFORCE_MEMFD_REXEC +liblxc_la_SOURCES += rexec.c +endif + AM_CFLAGS = -DLXCROOTFSMOUNT=\"$(LXCROOTFSMOUNT)\" \ -DLXCPATH=\"$(LXCPATH)\" \ -DLXC_GLOBAL_CONF=\"$(LXC_GLOBAL_CONF)\" \ diff --git a/src/lxc/file_utils.c b/src/lxc/file_utils.c index f89aa638d..930fd738a 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 @@ 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; +} diff --git a/src/lxc/file_utils.h b/src/lxc/file_utils.h index 6361557a0..518a61af3 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 000000000..396bd617f --- /dev/null +++ b/src/lxc/rexec.c @@ -0,0 +1,181 @@ +/* liblxcapi + * + * Copyright © 2019 Christian Brauner . + * 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 +#include +#include +#include + +#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 42d94db28..dca4d1571 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