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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>
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Feb 11, 2019
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/* 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. | ||
*/ | ||
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||
#ifndef _GNU_SOURCE | ||
#define _GNU_SOURCE 1 | ||
#endif | ||
#include <errno.h> | ||
#include <stdio.h> | ||
#include <stdlib.h> | ||
#include <string.h> | ||
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#include "config.h" | ||
#include "file_utils.h" | ||
#include "raw_syscalls.h" | ||
#include "string_utils.h" | ||
#include "syscall_wrappers.h" | ||
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#define LXC_MEMFD_REXEC_SEALS \ | ||
(F_SEAL_SEAL | F_SEAL_SHRINK | F_SEAL_GROW | F_SEAL_WRITE) | ||
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static int push_vargs(char *data, int data_length, char ***output) | ||
{ | ||
int num = 0; | ||
char *cur = data; | ||
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if (!data || *output) | ||
return -1; | ||
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*output = must_realloc(NULL, sizeof(**output)); | ||
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while (cur < data + data_length) { | ||
num++; | ||
*output = must_realloc(*output, (num + 1) * sizeof(**output)); | ||
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(*output)[num - 1] = cur; | ||
cur += strlen(cur) + 1; | ||
} | ||
(*output)[num] = NULL; | ||
return num; | ||
} | ||
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static int parse_exec_params(char ***argv, char ***envp) | ||
{ | ||
int ret; | ||
char *cmdline = NULL, *env = NULL; | ||
size_t cmdline_size, env_size; | ||
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cmdline = file_to_buf("/proc/self/cmdline", &cmdline_size); | ||
if (!cmdline) | ||
goto on_error; | ||
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env = file_to_buf("/proc/self/environ", &env_size); | ||
if (!env) | ||
goto on_error; | ||
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ret = push_vargs(cmdline, cmdline_size, argv); | ||
if (ret <= 0) | ||
goto on_error; | ||
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ret = push_vargs(env, env_size, envp); | ||
if (ret <= 0) | ||
goto on_error; | ||
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return 0; | ||
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on_error: | ||
free(env); | ||
free(cmdline); | ||
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return -1; | ||
} | ||
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static int is_memfd(void) | ||
{ | ||
int fd, saved_errno, seals; | ||
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fd = open("/proc/self/exe", O_RDONLY | O_CLOEXEC); | ||
if (fd < 0) | ||
return -ENOTRECOVERABLE; | ||
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seals = fcntl(fd, F_GET_SEALS); | ||
saved_errno = errno; | ||
close(fd); | ||
errno = saved_errno; | ||
if (seals < 0) | ||
return -EINVAL; | ||
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return seals == LXC_MEMFD_REXEC_SEALS; | ||
} | ||
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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; | ||
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memfd = memfd_create(memfd_name, MFD_ALLOW_SEALING | MFD_CLOEXEC); | ||
if (memfd < 0) | ||
return; | ||
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fd = open("/proc/self/exe", O_RDONLY | O_CLOEXEC); | ||
if (fd < 0) | ||
goto on_error; | ||
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/* 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; | ||
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if (fcntl(memfd, F_ADD_SEALS, LXC_MEMFD_REXEC_SEALS)) | ||
goto on_error; | ||
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fexecve(memfd, argv, envp); | ||
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on_error: | ||
saved_errno = errno; | ||
close(memfd); | ||
errno = saved_errno; | ||
} | ||
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static int lxc_rexec(const char *memfd_name) | ||
{ | ||
int ret; | ||
char **argv = NULL, **envp = NULL; | ||
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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; | ||
} | ||
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ret = parse_exec_params(&argv, &envp); | ||
if (ret < 0) { | ||
fprintf(stderr, | ||
"%s - Failed to parse command line parameters\n", | ||
strerror(errno)); | ||
return -1; | ||
} | ||
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lxc_rexec_as_memfd(argv, envp, memfd_name); | ||
fprintf(stderr, "%s - Failed to rexec as memfd\n", strerror(errno)); | ||
return -1; | ||
} | ||
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/** | ||
* 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); | ||
} | ||
} |
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