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- #include <signal.h>
- #include <stdio.h>
- #include <stdlib.h>
- #include <unistd.h>
- #include <errno.h>
- #include <fcntl.h>
- #include <string.h>
- #include <stddef.h>
- #include <sys/sysmacros.h>
- #include <sys/types.h>
- #include <sys/wait.h>
- #include <sys/socket.h>
- #include <sys/stat.h>
- #include <sys/mman.h>
- #include <sys/syscall.h>
- #include <sys/user.h>
- #include <sys/ioctl.h>
- #include <sys/ptrace.h>
- #include <sys/mount.h>
- #include <linux/limits.h>
- #include <linux/filter.h>
- #include <linux/seccomp.h>
- #define ARRAY_SIZE(x) (sizeof(x) / sizeof(*(x)))
- static int seccomp(unsigned int op, unsigned int flags, void *args)
- {
- errno = 0;
- return syscall(__NR_seccomp, op, flags, args);
- }
- static int send_fd(int sock, int fd)
- {
- struct msghdr msg = {};
- struct cmsghdr *cmsg;
- char buf[CMSG_SPACE(sizeof(int))] = {0}, c = 'c';
- struct iovec io = {
- .iov_base = &c,
- .iov_len = 1,
- };
- msg.msg_iov = &io;
- msg.msg_iovlen = 1;
- msg.msg_control = buf;
- msg.msg_controllen = sizeof(buf);
- cmsg = CMSG_FIRSTHDR(&msg);
- cmsg->cmsg_level = SOL_SOCKET;
- cmsg->cmsg_type = SCM_RIGHTS;
- cmsg->cmsg_len = CMSG_LEN(sizeof(int));
- *((int *)CMSG_DATA(cmsg)) = fd;
- msg.msg_controllen = cmsg->cmsg_len;
- if (sendmsg(sock, &msg, 0) < 0) {
- perror("sendmsg");
- return -1;
- }
- return 0;
- }
- static int recv_fd(int sock)
- {
- struct msghdr msg = {};
- struct cmsghdr *cmsg;
- char buf[CMSG_SPACE(sizeof(int))] = {0}, c = 'c';
- struct iovec io = {
- .iov_base = &c,
- .iov_len = 1,
- };
- msg.msg_iov = &io;
- msg.msg_iovlen = 1;
- msg.msg_control = buf;
- msg.msg_controllen = sizeof(buf);
- if (recvmsg(sock, &msg, 0) < 0) {
- perror("recvmsg");
- return -1;
- }
- cmsg = CMSG_FIRSTHDR(&msg);
- return *((int *)CMSG_DATA(cmsg));
- }
- static int user_trap_syscall(int nr, unsigned int flags)
- {
- struct sock_filter filter[] = {
- BPF_STMT(BPF_LD+BPF_W+BPF_ABS,
- offsetof(struct seccomp_data, nr)),
- BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, nr, 0, 1),
- BPF_STMT(BPF_RET+BPF_K, SECCOMP_RET_USER_NOTIF),
- BPF_STMT(BPF_RET+BPF_K, SECCOMP_RET_ALLOW),
- };
- struct sock_fprog prog = {
- .len = (unsigned short)ARRAY_SIZE(filter),
- .filter = filter,
- };
- return seccomp(SECCOMP_SET_MODE_FILTER, flags, &prog);
- }
- static int handle_req(struct seccomp_notif *req,
- struct seccomp_notif_resp *resp, int listener)
- {
- char path[PATH_MAX], source[PATH_MAX], target[PATH_MAX];
- int ret = -1, mem;
- resp->id = req->id;
- resp->error = -EPERM;
- resp->val = 0;
- if (req->data.nr != __NR_mount) {
- fprintf(stderr, "huh? trapped something besides mount? %d\n", req->data.nr);
- return -1;
- }
- /* Only allow bind mounts. */
- if (!(req->data.args[3] & MS_BIND))
- return 0;
- /*
- * Ok, let's read the task's memory to see where they wanted their
- * mount to go.
- */
- snprintf(path, sizeof(path), "/proc/%d/mem", req->pid);
- mem = open(path, O_RDONLY);
- if (mem < 0) {
- perror("open mem");
- return -1;
- }
- /*
- * Now we avoid a TOCTOU: we referred to a pid by its pid, but since
- * the pid that made the syscall may have died, we need to confirm that
- * the pid is still valid after we open its /proc/pid/mem file. We can
- * ask the listener fd this as follows.
- *
- * Note that this check should occur *after* any task-specific
- * resources are opened, to make sure that the task has not died and
- * we're not wrongly reading someone else's state in order to make
- * decisions.
- */
- if (ioctl(listener, SECCOMP_IOCTL_NOTIF_ID_VALID, &req->id) < 0) {
- fprintf(stderr, "task died before we could map its memory\n");
- goto out;
- }
- /*
- * Phew, we've got the right /proc/pid/mem. Now we can read it. Note
- * that to avoid another TOCTOU, we should read all of the pointer args
- * before we decide to allow the syscall.
- */
- if (lseek(mem, req->data.args[0], SEEK_SET) < 0) {
- perror("seek");
- goto out;
- }
- ret = read(mem, source, sizeof(source));
- if (ret < 0) {
- perror("read");
- goto out;
- }
- if (lseek(mem, req->data.args[1], SEEK_SET) < 0) {
- perror("seek");
- goto out;
- }
- ret = read(mem, target, sizeof(target));
- if (ret < 0) {
- perror("read");
- goto out;
- }
- /*
- * Our policy is to only allow bind mounts inside /tmp. This isn't very
- * interesting, because we could do unprivlieged bind mounts with user
- * namespaces already, but you get the idea.
- */
- if (!strncmp(source, "/tmp/", 5) && !strncmp(target, "/tmp/", 5)) {
- if (mount(source, target, NULL, req->data.args[3], NULL) < 0) {
- ret = -1;
- perror("actual mount");
- goto out;
- }
- resp->error = 0;
- }
- /* Even if we didn't allow it because of policy, generating the
- * response was be a success, because we want to tell the worker EPERM.
- */
- ret = 0;
- out:
- close(mem);
- return ret;
- }
- int main(void)
- {
- int sk_pair[2], ret = 1, status, listener;
- pid_t worker = 0 , tracer = 0;
- if (socketpair(PF_LOCAL, SOCK_SEQPACKET, 0, sk_pair) < 0) {
- perror("socketpair");
- return 1;
- }
- worker = fork();
- if (worker < 0) {
- perror("fork");
- goto close_pair;
- }
- if (worker == 0) {
- listener = user_trap_syscall(__NR_mount,
- SECCOMP_FILTER_FLAG_NEW_LISTENER);
- if (listener < 0) {
- perror("seccomp");
- exit(1);
- }
- /*
- * Drop privileges. We definitely can't mount as uid 1000.
- */
- if (setuid(1000) < 0) {
- perror("setuid");
- exit(1);
- }
- /*
- * Send the listener to the parent; also serves as
- * synchronization.
- */
- if (send_fd(sk_pair[1], listener) < 0)
- exit(1);
- close(listener);
- if (mkdir("/tmp/foo", 0755) < 0) {
- perror("mkdir");
- exit(1);
- }
- /*
- * Try a bad mount just for grins.
- */
- if (mount("/dev/sda", "/tmp/foo", NULL, 0, NULL) != -1) {
- fprintf(stderr, "huh? mounted /dev/sda?\n");
- exit(1);
- }
- if (errno != EPERM) {
- perror("bad error from mount");
- exit(1);
- }
- /*
- * Ok, we expect this one to succeed.
- */
- if (mount("/tmp/foo", "/tmp/foo", NULL, MS_BIND, NULL) < 0) {
- perror("mount");
- exit(1);
- }
- exit(0);
- }
- /*
- * Get the listener from the child.
- */
- listener = recv_fd(sk_pair[0]);
- if (listener < 0)
- goto out_kill;
- /*
- * Fork a task to handle the requests. This isn't strictly necessary,
- * but it makes the particular writing of this sample easier, since we
- * can just wait ofr the tracee to exit and kill the tracer.
- */
- tracer = fork();
- if (tracer < 0) {
- perror("fork");
- goto out_kill;
- }
- if (tracer == 0) {
- struct seccomp_notif *req;
- struct seccomp_notif_resp *resp;
- struct seccomp_notif_sizes sizes;
- if (seccomp(SECCOMP_GET_NOTIF_SIZES, 0, &sizes) < 0) {
- perror("seccomp(GET_NOTIF_SIZES)");
- goto out_close;
- }
- req = malloc(sizes.seccomp_notif);
- if (!req)
- goto out_close;
- resp = malloc(sizes.seccomp_notif_resp);
- if (!resp)
- goto out_req;
- memset(resp, 0, sizes.seccomp_notif_resp);
- while (1) {
- memset(req, 0, sizes.seccomp_notif);
- if (ioctl(listener, SECCOMP_IOCTL_NOTIF_RECV, req)) {
- perror("ioctl recv");
- goto out_resp;
- }
- if (handle_req(req, resp, listener) < 0)
- goto out_resp;
- /*
- * ENOENT here means that the task may have gotten a
- * signal and restarted the syscall. It's up to the
- * handler to decide what to do in this case, but for
- * the sample code, we just ignore it. Probably
- * something better should happen, like undoing the
- * mount, or keeping track of the args to make sure we
- * don't do it again.
- */
- if (ioctl(listener, SECCOMP_IOCTL_NOTIF_SEND, resp) < 0 &&
- errno != ENOENT) {
- perror("ioctl send");
- goto out_resp;
- }
- }
- out_resp:
- free(resp);
- out_req:
- free(req);
- out_close:
- close(listener);
- exit(1);
- }
- close(listener);
- if (waitpid(worker, &status, 0) != worker) {
- perror("waitpid");
- goto out_kill;
- }
- if (umount2("/tmp/foo", MNT_DETACH) < 0 && errno != EINVAL) {
- perror("umount2");
- goto out_kill;
- }
- if (remove("/tmp/foo") < 0 && errno != ENOENT) {
- perror("remove");
- exit(1);
- }
- if (!WIFEXITED(status) || WEXITSTATUS(status)) {
- fprintf(stderr, "worker exited nonzero\n");
- goto out_kill;
- }
- ret = 0;
- out_kill:
- if (tracer > 0)
- kill(tracer, SIGKILL);
- if (worker > 0)
- kill(worker, SIGKILL);
- close_pair:
- close(sk_pair[0]);
- close(sk_pair[1]);
- return ret;
- }
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