If run as root we were leaking mounts to the parent namespace, which lead to an error when removing the temporary mountroot. To fix this we remount the whole tree as private as soon as we created the new mountenamespace.
170 lines
4.5 KiB
C
170 lines
4.5 KiB
C
#define _GNU_SOURCE
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#include <glib.h>
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#include <glib/gstdio.h>
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#include <errno.h>
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#include <sched.h>
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#include <unistd.h>
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#include <sys/mount.h>
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#include <sys/stat.h>
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#include <sys/types.h>
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#include <sys/wait.h>
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#include <sys/syscall.h>
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#define fail(s, err) g_error("%s: %s: %s", __func__, s, g_strerror(err))
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#define fail_if(expr) \
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if (expr) \
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fail(#expr, errno);
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const gchar *bind_blacklist[] = {"bin", "etc", "host", "real-host", "usr", "lib", "lib64", "lib32", "sbin", NULL};
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int pivot_root(const char *new_root, const char *put_old) {
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return syscall(SYS_pivot_root, new_root, put_old);
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}
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void mount_tmpfs(const gchar *target) {
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fail_if(mount("none", target, "tmpfs", 0, NULL));
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}
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void bind_mount(const gchar *source, const gchar *target) {
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fail_if(g_mkdir(target, 0755));
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fail_if(mount(source, target, NULL, MS_BIND | MS_REC, NULL));
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}
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const gchar *create_tmpdir() {
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gchar *prefix =
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g_build_filename(g_get_tmp_dir(), "chrootenvXXXXXX", NULL);
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fail_if(!g_mkdtemp_full(prefix, 0755));
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return prefix;
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}
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void pivot_host(const gchar *guest) {
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g_autofree gchar *point = g_build_filename(guest, "host", NULL);
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fail_if(g_mkdir(point, 0755));
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fail_if(pivot_root(guest, point));
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}
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void bind_mount_item(const gchar *host, const gchar *guest, const gchar *name) {
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g_autofree gchar *source = g_build_filename(host, name, NULL);
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g_autofree gchar *target = g_build_filename(guest, name, NULL);
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if (G_LIKELY(g_file_test(source, G_FILE_TEST_IS_DIR)))
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bind_mount(source, target);
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}
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void bind(const gchar *host, const gchar *guest) {
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mount_tmpfs(guest);
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pivot_host(guest);
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g_autofree gchar *host_dir = g_build_filename("/host", host, NULL);
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g_autoptr(GError) err = NULL;
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g_autoptr(GDir) dir = g_dir_open(host_dir, 0, &err);
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if (err != NULL)
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fail("g_dir_open", errno);
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const gchar *item;
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while ((item = g_dir_read_name(dir)))
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if (!g_strv_contains(bind_blacklist, item))
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bind_mount_item(host_dir, "/", item);
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}
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void spit(const char *path, char *fmt, ...) {
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va_list args;
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va_start(args, fmt);
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FILE *f = g_fopen(path, "w");
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if (f == NULL)
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fail("g_fopen", errno);
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g_vfprintf(f, fmt, args);
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fclose(f);
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}
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int main(gint argc, gchar **argv) {
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const gchar *self = *argv++;
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if (argc < 2) {
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g_message("%s command [arguments...]", self);
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return 1;
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}
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g_autofree const gchar *prefix = create_tmpdir();
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pid_t cpid = fork();
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if (cpid < 0)
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fail("fork", errno);
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else if (cpid == 0) {
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uid_t uid = getuid();
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gid_t gid = getgid();
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int namespaces = CLONE_NEWNS;
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if (uid != 0) {
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namespaces |= CLONE_NEWUSER;
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}
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if (unshare(namespaces) < 0) {
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int unshare_errno = errno;
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g_message("Requires Linux version >= 3.19 built with CONFIG_USER_NS");
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if (g_file_test("/proc/sys/kernel/unprivileged_userns_clone",
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G_FILE_TEST_EXISTS))
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g_message("Run: sudo sysctl -w kernel.unprivileged_userns_clone=1");
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fail("unshare", unshare_errno);
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}
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// hide all mounts we do from the parent
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fail_if(mount(0, "/", 0, MS_PRIVATE | MS_REC, 0));
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if (uid != 0) {
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spit("/proc/self/setgroups", "deny");
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spit("/proc/self/uid_map", "%d %d 1", uid, uid);
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spit("/proc/self/gid_map", "%d %d 1", gid, gid);
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}
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// If there is a /host directory, assume this is nested chrootenv and use it as host instead.
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gboolean nested_host = g_file_test("/host", G_FILE_TEST_EXISTS | G_FILE_TEST_IS_DIR);
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g_autofree const gchar *host = nested_host ? "/host" : "/";
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bind(host, prefix);
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// Replace /host by an actual (inner) /host.
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if (nested_host) {
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fail_if(g_mkdir("/real-host", 0755));
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fail_if(mount("/host/host", "/real-host", NULL, MS_BIND | MS_REC, NULL));
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// For some reason umount("/host") returns EBUSY even immediately after
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// pivot_root. We detach it at least to keep `/proc/mounts` from blowing
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// up in nested cases.
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fail_if(umount2("/host", MNT_DETACH));
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fail_if(mount("/real-host", "/host", NULL, MS_MOVE, NULL));
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fail_if(rmdir("/real-host"));
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}
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fail_if(chdir("/"));
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fail_if(execvp(*argv, argv));
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}
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else {
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int status;
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fail_if(waitpid(cpid, &status, 0) != cpid);
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fail_if(rmdir(prefix));
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if (WIFEXITED(status))
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return WEXITSTATUS(status);
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else if (WIFSIGNALED(status))
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kill(getpid(), WTERMSIG(status));
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return 1;
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}
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}
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