Re: [RCF] [PATCH] unprivileged mount/umount

Ram
2005-05-13T01:19:54Z

On Thu, 2005-05-12 at 11:51, Miklos Szeredi wrote:
> > > I'm not sure passing directory file descriptors is the right semantic
> > > we want - but at least it provides a point of explicit control (in
> > > much the same way as a bind). Are you sure the clone + open("/") +
> > > pass-to-parent scenario you allows the parent to traverse the child's
> > > private name space through that fd?
> >
> > Pretty sure.
>
> Yup. Attached a little program that can be used to try this out. It
> creates a new namespace in the child, does a bind mount (so the
> namespaces can be differentiated), then sends the file descriptor of
> "/" to the parent. The parent does fchdir(fd), then starts a shell.

> So the result is that CWD is under the child namespace, while root is
> under the initial namespace.
>

r u sure, this program works? Sorry if I am saying something dumb here.
Correct me. When a file descriptor is sent from one process to other,
arn't they referring to different files in each of the processes.
fd=5 may be pointing to file 'xyz' in parent process,
where as fd=5 will be pointing to 'abc' in the child process.

This program did not work for me, and I was wondering if adding
CLONE_FILES in clone() would help. Because that would make sure
that both
the processes share the same file descriptor. It did not work too.

What am I understanding wrong?

In any case my opinion is if this program works than the hole should
be closed instead of exploting it to access different namespace. I
know Jamie is going to pounce at me. ;)

RP

> I also tried bind mounting from the child's namespace to the parent's,
> and that works too. But the new mount's mnt_namespace is copied from
> the old, which makes the mount un-removable. This is most likely not
> intentional, IOW a bug.
>
> Miklos
>
> === newns.c =========================================================
> #define _GNU_SOURCE
>
> #include <stdio.h>
> #include <unistd.h>
> #include <stdlib.h>
> #include <signal.h>
> #include <sched.h>
> #include <errno.h>
> #include <fcntl.h>
> #include <unistd.h>
> #include <sys/un.h>
> #include <sys/socket.h>
>
> static int socks[2];
>
> static int send_fd(int sock_fd, int fd)
> {
> int retval;
> struct msghdr msg;
> struct cmsghdr *p_cmsg;
> struct iovec vec;
> char cmsgbuf[CMSG_SPACE(sizeof(fd))];
> int *p_fds;
> char sendchar = 0;
>
> msg.msg_control = cmsgbuf;
> msg.msg_controllen = sizeof(cmsgbuf);
> p_cmsg = CMSG_FIRSTHDR(&msg);
> p_cmsg->cmsg_level = SOL_SOCKET;
> p_cmsg->cmsg_type = SCM_RIGHTS;
> p_cmsg->cmsg_len = CMSG_LEN(sizeof(fd));
> p_fds = (int *) CMSG_DATA(p_cmsg);
> *p_fds = fd;
> msg.msg_controllen = p_cmsg->cmsg_len;
> msg.msg_name = NULL;
> msg.msg_namelen = 0;
> msg.msg_iov = &vec;
> msg.msg_iovlen = 1;
> msg.msg_flags = 0;
> /* "To pass file descriptors or credentials you need to send/read at
> * least one byte" (man 7 unix) */
> vec.iov_base = &sendchar;
> vec.iov_len = sizeof(sendchar);
> while ((retval = sendmsg(sock_fd, &msg, 0)) == -1 && errno == EINTR);
> if (retval != 1) {
> perror("sending file descriptor");
> return -1;
> }
> return 0;
> }
>
> static int receive_fd(int fd)
> {
> struct msghdr msg;
> struct iovec iov;
> char buf[1];
> int rv;
> int connfd = -1;
> char ccmsg[CMSG_SPACE(sizeof(connfd))];
> struct cmsghdr *cmsg;
>
> iov.iov_base = buf;
> iov.iov_len = 1;
>
> msg.msg_name = 0;
> msg.msg_namelen = 0;
> msg.msg_iov = &iov;
> msg.msg_iovlen = 1;
> /* old BSD implementations should use msg_accrights instead of
> * msg_control; the interface is different. */
> msg.msg_control = ccmsg;
> msg.msg_controllen = sizeof(ccmsg);
>
> while(((rv = recvmsg(fd, &msg, 0)) == -1) && errno == EINTR);
> if (rv == -1) {
> perror("recvmsg");
> return -1;
> }
> if(!rv) {
> /* EOF */
> return -1;
> }
>
> cmsg = CMSG_FIRSTHDR(&msg);
> if (!cmsg->cmsg_type == SCM_RIGHTS) {
> fprintf(stderr, "got control message of unknown type %d\n",
> cmsg->cmsg_type);
> return -1;
> }
> return *(int*)CMSG_DATA(cmsg);
> }
>
> int childfn(void *p)
> {
> int fd;
>
> (void) p;
> mkdir("/tmp/clonetest", 755);
> mkdir("/tmp/clonetest/dir1", 755);
> mkdir("/tmp/clonetest/dir1/subdir1", 755);
> mkdir("/tmp/clonetest/mnt", 755);
> system("mount --bind /tmp/clonetest/dir1 /tmp/clonetest/mnt");
> fd = open("/", O_RDONLY | O_DIRECTORY);
> send_fd(socks[0], fd);
> sleep(1000);
> return 1;
> }
>
> int main()
> {
> char buf[10000];
> pid_t pid;
> int res;
> int childfd;
>
> res = socketpair(AF_UNIX, SOCK_STREAM, 0, socks);
> if (res == -1) {
> perror("socketpair");
> return 1;
> }
>
> pid = clone(childfn, buf+5000, CLONE_NEWNS | SIGCHLD, NULL);
> if ((int) pid == -1) {
> perror("clone");
> exit(1);
> }
>
> childfd = receive_fd(socks[1]);
> res = fchdir(childfd);
> if (res == -1) {
> perror("fchdir");
> return 1;
> }
> execl("/bin/bash", "/bin/bash", NULL);
>
> return 0;
> }