])
])
+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
- Linux capabilities: $enable_capabilities
- seccomp: $enable_seccomp
- SELinux: $enable_selinux
+ - memfd rexec: $enable_memfd_rexec
PAM:
- PAM module: $enable_pam
--- /dev/null
+/* 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);
+ }
+}
projects / mirror_lxc.git / commitdiff