chroot

Lecture

Example: Filesystem Isolation

chroot() changes the root directory for a process and its children processes. In other word, it changes the meaning of /.

For example, chroot("/tmp/jail") sets the root directory to be /tmp/jail. To get access the other directories, we must escape from this sandbox.

For this chapter, we will use busybox for sandboxing purposes. Busybox packs many shell utilizes into a tiny executable file. Sometimes it is the entire user sapce on embedded devices.

Effects of chroot

chroot("/tmp/jail") has two effects on the current process:

  1. Change the meaning of / to /tmp/jail.

  2. Change the meaning of /tmp/jail/.. to /tmp/jail.

In other word, it really makes /tmp/jail a jail that prevents the process from going out.

However, chroot("/tmp/jail") does NOT:

  • Close resources that reside outside of the jail.

  • chdir("/") into the jail.

  • Do anything else!

Pitfall 1: Previously Open Resources

Neither of the effects of chroot() do anything to previously-open resources. How is this useful?

Similar to open() and execve(), Linux has openat() and execveat():

int open(char *pathname, int flags);
int openat(int dirfd, char *pathname, int flags);
int execve(char *pathname, char **argv, char **envp);
int execveat(int dirfd, char *pathname, char **argv, char **envp, int flags);

int dirfd can be a file descriptor representing any open()ed directory, or the special value AT_FDCWD (note: chroot() does not change the current working directory)!

Pitfall 2: Forgetfulness

The Linux kernel does not keep track of previous chroots for a process. That is, we are able to use chroot again inside a jail. We are going to see how to use this idea to escape from the sandbox in babyjail.

Is chroot Safe?

No. chroot is a historical way of sandboxing. It is not a good choice for modern production environment. However, it is still being used in some scenarios, so it is worthy to learn it.

Generally, a user with an effective ID of 0 (i.e., a process run as root or SUIDed to root) can always break out of a chroot, unless the chroot syscall is blocked!

A replacement for chroot is seccomp and we will be covering it in the next section.

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