Chapter 4. The Jail Subsystem

On most UNIX® systems, root has omnipotent power. This promotes insecurity. If an attacker gained root on a system, he would have every function at his fingertips. In FreeBSD there are sysctls which dilute the power of root, in order to minimize the damage caused by an attacker. Specifically, one of these functions is called secure levels. Similarly, another function which is present from FreeBSD 4.0 and onward, is a utility called jail(8). Jail chroots an environment and sets certain restrictions on processes which are forked within the jail. For example, a jailed process cannot affect processes outside the jail, utilize certain system calls, or inflict any damage on the host environment.

Jail is becoming the new security model. People are running potentially vulnerable servers such as Apache, BIND, and sendmail within jails, so that if an attacker gains root within the jail, it is only an annoyance, and not a devastation. This article mainly focuses on the internals (source code) of jail. For information on how to set up a jail see the handbook entry on jails.

4.1. Architecture

Jail consists of two realms: the userland program, jail(8), and the code implemented within the kernel: the jail(2) system call and associated restrictions. I will be discussing the userland program and then how jail is implemented within the kernel.

4.1.1. Userland Code

The source for the userland jail is located in /usr/src/usr.sbin/jail, consisting of one file, jail.c. The program takes these arguments: the path of the jail, hostname, IP address, and the command to be executed.

4.1.1.1. Data Structures

In jail.c, the first thing I would note is the declaration of an important structure struct jail j; which was included from /usr/include/sys/jail.h.

The definition of the jail structure is:

/usr/include/sys/jail.h:

struct jail {
        u_int32_t       version;
        char            *path;
        char            *hostname;
        u_int32_t       ip_number;
};

As you can see, there is an entry for each of the arguments passed to the jail(8) program, and indeed, they are set during its execution.

/usr/src/usr.sbin/jail/jail.c
char path[PATH_MAX];
...
if (realpath(argv[0], path) == NULL)
    err(1, "realpath: %s", argv[0]);
if (chdir(path) != 0)
    err(1, "chdir: %s", path);
memset(&j, 0, sizeof(j));
j.version = 0;
j.path = path;
j.hostname = argv[1];

4.1.1.2. Networking

One of the arguments passed to the jail(8) program is an IP address with which the jail can be accessed over the network. jail(8) translates the IP address given into host byte order and then stores it in j (the jail structure).

/usr/src/usr.sbin/jail/jail.c:
struct in_addr in;
...
if (inet_aton(argv[2], &in) == 0)
    errx(1, "Could not make sense of ip-number: %s", argv[2]);
j.ip_number = ntohl(in.s_addr);

The inet_aton(3) function "interprets the specified character string as an Internet address, placing the address into the structure provided." The ip_number member in the jail structure is set only when the IP address placed onto the in structure by inet_aton(3) is translated into host byte order by ntohl(3).

4.1.1.3. Jailing the Process

Finally, the userland program jails the process. Jail now becomes an imprisoned process itself and then executes the command given using execv(3).

/usr/src/usr.sbin/jail/jail.c
i = jail(&j);
...
if (execv(argv[3], argv + 3) != 0)
    err(1, "execv: %s", argv[3]);

As you can see, the jail() function is called, and its argument is the jail structure which has been filled with the arguments given to the program. Finally, the program you specify is executed. I will now discuss how jail is implemented within the kernel.

4.1.2. Kernel Space

We will now be looking at the file /usr/src/sys/kern/kern_jail.c. This is the file where the jail(2) system call, appropriate sysctls, and networking functions are defined.

4.1.2.1. Sysctls

In kern_jail.c, the following sysctls are defined:

/usr/src/sys/kern/kern_jail.c:
int     jail_set_hostname_allowed = 1;
SYSCTL_INT(_security_jail, OID_AUTO, set_hostname_allowed, CTLFLAG_RW,
    &jail_set_hostname_allowed, 0,
    "Processes in jail can set their hostnames");

int     jail_socket_unixiproute_only = 1;
SYSCTL_INT(_security_jail, OID_AUTO, socket_unixiproute_only, CTLFLAG_RW,
    &jail_socket_unixiproute_only, 0,
    "Processes in jail are limited to creating UNIX/IPv4/route sockets only");

int     jail_sysvipc_allowed = 0;
SYSCTL_INT(_security_jail, OID_AUTO, sysvipc_allowed, CTLFLAG_RW,
    &jail_sysvipc_allowed, 0,
    "Processes in jail can use System V IPC primitives");

static int jail_enforce_statfs = 2;
SYSCTL_INT(_security_jail, OID_AUTO, enforce_statfs, CTLFLAG_RW,
    &jail_enforce_statfs, 0,
    "Processes in jail cannot see all mounted file systems");

int    jail_allow_raw_sockets = 0;
SYSCTL_INT(_security_jail, OID_AUTO, allow_raw_sockets, CTLFLAG_RW,
    &jail_allow_raw_sockets, 0,
    "Prison root can create raw sockets");

int    jail_chflags_allowed = 0;
SYSCTL_INT(_security_jail, OID_AUTO, chflags_allowed, CTLFLAG_RW,
    &jail_chflags_allowed, 0,
    "Processes in jail can alter system file flags");

int     jail_mount_allowed = 0;
SYSCTL_INT(_security_jail, OID_AUTO, mount_allowed, CTLFLAG_RW,
    &jail_mount_allowed, 0,
    "Processes in jail can mount/unmount jail-friendly file systems");

Each of these sysctls can be accessed by the user through the sysctl(8) program. Throughout the kernel, these specific sysctls are recognized by their name. For example, the name of the first sysctl is security.jail.set_hostname_allowed.

4.1.2.2. jail(2) System Call

Like all system calls, the jail(2) system call takes two arguments, struct thread *td and struct jail_args *uap. td is a pointer to the thread structure which describes the calling thread. In this context, uap is a pointer to the structure in which a pointer to the jail structure passed by the userland jail.c is contained. When I described the userland program before, you saw that the jail(2) system call was given a jail structure as its own argument.

/usr/src/sys/kern/kern_jail.c:
/*
 * struct jail_args {
 *  struct jail *jail;
 * };
 */
int
jail(struct thread *td, struct jail_args *uap)

Therefore, uap→jail can be used to access the jail structure which was passed to the system call. Next, the system call copies the jail structure into kernel space using the copyin(9) function. copyin(9) takes three arguments: the address of the data which is to be copied into kernel space, uap→jail, where to store it, j and the size of the storage. The jail structure pointed by uap→jail is copied into kernel space and is stored in another jail structure, j.

/usr/src/sys/kern/kern_jail.c:
error = copyin(uap->jail, &j, sizeof(j));

There is another important structure defined in jail.h. It is the prison structure. The prison structure is used exclusively within kernel space. Here is the definition of the prison structure.

/usr/include/sys/jail.h:
struct prison {
        LIST_ENTRY(prison) pr_list;                     /* (a) all prisons */
        int              pr_id;                         /* (c) prison id */
        int              pr_ref;                        /* (p) refcount */
        char             pr_path[MAXPATHLEN];           /* (c) chroot path */
        struct vnode    *pr_root;                       /* (c) vnode to rdir */
        char             pr_host[MAXHOSTNAMELEN];       /* (p) jail hostname */
        u_int32_t        pr_ip;                         /* (c) ip addr host */
        void            *pr_linux;                      /* (p) linux abi */
        int              pr_securelevel;                /* (p) securelevel */
        struct task      pr_task;                       /* (d) destroy task */
        struct mtx       pr_mtx;
      void            **pr_slots;                     /* (p) additional data */
};

The jail(2) system call then allocates memory for a prison structure and copies data between the jail and prison structure.

/usr/src/sys/kern/kern_jail.c:
MALLOC(pr, struct prison *, sizeof(*pr), M_PRISON, M_WAITOK | M_ZERO);
...
error = copyinstr(j.path, &pr->pr_path, sizeof(pr->pr_path), 0);
if (error)
    goto e_killmtx;
...
error = copyinstr(j.hostname, &pr->pr_host, sizeof(pr->pr_host), 0);
if (error)
     goto e_dropvnref;
pr->pr_ip = j.ip_number;

Next, we will discuss another important system call jail_attach(2), which implements the function to put a process into the jail.

/usr/src/sys/kern/kern_jail.c:
/*
 * struct jail_attach_args {
 *      int jid;
 * };
 */
int
jail_attach(struct thread *td, struct jail_attach_args *uap)

This system call makes the changes that can distinguish a jailed process from those unjailed ones. To understand what jail_attach(2) does for us, certain background information is needed.

On FreeBSD, each kernel visible thread is identified by its thread structure, while the processes are described by their proc structures. You can find the definitions of the thread and proc structure in /usr/include/sys/proc.h. For example, the td argument in any system call is actually a pointer to the calling thread’s thread structure, as stated before. The td_proc member in the thread structure pointed by td is a pointer to the proc structure which represents the process that contains the thread represented by td. The proc structure contains members which can describe the owner’s identity(p_ucred), the process resource limits(p_limit), and so on. In the ucred structure pointed by p_ucred member in the proc structure, there is a pointer to the prison structure(cr_prison).

/usr/include/sys/proc.h:
struct thread {
    ...
    struct proc *td_proc;
    ...
};
struct proc {
    ...
    struct ucred *p_ucred;
    ...
};
/usr/include/sys/ucred.h
struct ucred {
    ...
    struct prison *cr_prison;
    ...
};

In kern_jail.c, the function jail() then calls function jail_attach() with a given jid. And jail_attach() calls function change_root() to change the root directory of the calling process. The jail_attach() then creates a new ucred structure, and attaches the newly created ucred structure to the calling process after it has successfully attached the prison structure to the ucred structure. From then on, the calling process is recognized as jailed. When the kernel routine jailed() is called in the kernel with the newly created ucred structure as its argument, it returns 1 to tell that the credential is connected with a jail. The public ancestor process of all the process forked within the jail, is the process which runs jail(8), as it calls the jail(2) system call. When a program is executed through execve(2), it inherits the jailed property of its parent’s ucred structure, therefore it has a jailed ucred structure.

/usr/src/sys/kern/kern_jail.c
int
jail(struct thread *td, struct jail_args *uap)
{
...
    struct jail_attach_args jaa;
...
    error = jail_attach(td, &jaa);
    if (error)
        goto e_dropprref;
...
}

int
jail_attach(struct thread *td, struct jail_attach_args *uap)
{
    struct proc *p;
    struct ucred *newcred, *oldcred;
    struct prison *pr;
...
    p = td->td_proc;
...
    pr = prison_find(uap->jid);
...
    change_root(pr->pr_root, td);
...
    newcred->cr_prison = pr;
    p->p_ucred = newcred;
...
}

When a process is forked from its parent process, the fork(2) system call uses crhold() to maintain the credential for the newly forked process. It inherently keep the newly forked child’s credential consistent with its parent, so the child process is also jailed.

/usr/src/sys/kern/kern_fork.c:
p2->p_ucred = crhold(td->td_ucred);
...
td2->td_ucred = crhold(p2->p_ucred);

4.2. Restrictions

Throughout the kernel there are access restrictions relating to jailed processes. Usually, these restrictions only check whether the process is jailed, and if so, returns an error. For example:

if (jailed(td->td_ucred))
    return (EPERM);

4.2.1. SysV IPC

System V IPC is based on messages. Processes can send each other these messages which tell them how to act. The functions which deal with messages are: msgctl(3), msgget(3), msgsnd(3) and msgrcv(3). Earlier, I mentioned that there were certain sysctls you could turn on or off in order to affect the behavior of jail. One of these sysctls was security.jail.sysvipc_allowed. By default, this sysctl is set to 0. If it were set to 1, it would defeat the whole purpose of having a jail; privileged users from the jail would be able to affect processes outside the jailed environment. The difference between a message and a signal is that the message only consists of the signal number.

/usr/src/sys/kern/sysv_msg.c:

  • msgget(key, msgflg): msgget returns (and possibly creates) a message descriptor that designates a message queue for use in other functions.

  • msgctl(msgid, cmd, buf): Using this function, a process can query the status of a message descriptor.

  • msgsnd(msgid, msgp, msgsz, msgflg): msgsnd sends a message to a process.

  • msgrcv(msgid, msgp, msgsz, msgtyp, msgflg): a process receives messages using this function

In each of the system calls corresponding to these functions, there is this conditional:

/usr/src/sys/kern/sysv_msg.c:
if (!jail_sysvipc_allowed && jailed(td->td_ucred))
    return (ENOSYS);

Semaphore system calls allow processes to synchronize execution by doing a set of operations atomically on a set of semaphores. Basically semaphores provide another way for processes lock resources. However, process waiting on a semaphore, that is being used, will sleep until the resources are relinquished. The following semaphore system calls are blocked inside a jail: semget(2), semctl(2) and semop(2).

/usr/src/sys/kern/sysv_sem.c:

  • semctl(semid, semnum, cmd, …​): semctl does the specified cmd on the semaphore queue indicated by semid.

  • semget(key, nsems, flag): semget creates an array of semaphores, corresponding to key.

    key and flag take on the same meaning as they do in msgget.

  • semop(semid, array, nops): semop performs a group of operations indicated by array, to the set of semaphores identified by semid.

System V IPC allows for processes to share memory. Processes can communicate directly with each other by sharing parts of their virtual address space and then reading and writing data stored in the shared memory. These system calls are blocked within a jailed environment: shmdt(2), shmat(2), shmctl(2) and shmget(2).

/usr/src/sys/kern/sysv_shm.c:

  • shmctl(shmid, cmd, buf): shmctl does various control operations on the shared memory region identified by shmid.

  • shmget(key, size, flag): shmget accesses or creates a shared memory region of size bytes.

  • shmat(shmid, addr, flag): shmat attaches a shared memory region identified by shmid to the address space of a process.

  • shmdt(addr): shmdt detaches the shared memory region previously attached at addr.

4.2.2. Sockets

Jail treats the socket(2) system call and related lower-level socket functions in a special manner. In order to determine whether a certain socket is allowed to be created, it first checks to see if the sysctl security.jail.socket_unixiproute_only is set. If set, sockets are only allowed to be created if the family specified is either PF_LOCAL, PF_INET or PF_ROUTE. Otherwise, it returns an error.

/usr/src/sys/kern/uipc_socket.c:
int
socreate(int dom, struct socket **aso, int type, int proto,
    struct ucred *cred, struct thread *td)
{
    struct protosw *prp;
...
    if (jailed(cred) && jail_socket_unixiproute_only &&
        prp->pr_domain->dom_family != PF_LOCAL &&
        prp->pr_domain->dom_family != PF_INET &&
        prp->pr_domain->dom_family != PF_ROUTE) {
        return (EPROTONOSUPPORT);
    }
...
}

4.2.3. Berkeley Packet Filter

The Berkeley Packet Filter provides a raw interface to data link layers in a protocol independent fashion. BPF is now controlled by the devfs(8) whether it can be used in a jailed environment.

4.2.4. Protocols

There are certain protocols which are very common, such as TCP, UDP, IP and ICMP. IP and ICMP are on the same level: the network layer 2. There are certain precautions which are taken in order to prevent a jailed process from binding a protocol to a certain address only if the nam parameter is set. nam is a pointer to a sockaddr structure, which describes the address on which to bind the service. A more exact definition is that sockaddr "may be used as a template for referring to the identifying tag and length of each address". In the function in_pcbbind_setup(), sin is a pointer to a sockaddr_in structure, which contains the port, address, length and domain family of the socket which is to be bound. Basically, this disallows any processes from jail to be able to specify the address that does not belong to the jail in which the calling process exists.

/usr/src/sys/netinet/in_pcb.c:
int
in_pcbbind_setup(struct inpcb *inp, struct sockaddr *nam, in_addr_t *laddrp,
    u_short *lportp, struct ucred *cred)
{
    ...
    struct sockaddr_in *sin;
    ...
    if (nam) {
        sin = (struct sockaddr_in *)nam;
        ...
        if (sin->sin_addr.s_addr != INADDR_ANY)
            if (prison_ip(cred, 0, &sin->sin_addr.s_addr))
                return(EINVAL);
        ...
        if (lport) {
            ...
            if (prison && prison_ip(cred, 0, &sin->sin_addr.s_addr))
                return (EADDRNOTAVAIL);
            ...
        }
    }
    if (lport == 0) {
        ...
        if (laddr.s_addr != INADDR_ANY)
            if (prison_ip(cred, 0, &laddr.s_addr))
                return (EINVAL);
        ...
    }
...
    if (prison_ip(cred, 0, &laddr.s_addr))
        return (EINVAL);
...
}

You might be wondering what function prison_ip() does. prison_ip() is given three arguments, a pointer to the credential(represented by cred), any flags, and an IP address. It returns 1 if the IP address does NOT belong to the jail or 0 otherwise. As you can see from the code, if it is indeed an IP address not belonging to the jail, the protocol is not allowed to bind to that address.

/usr/src/sys/kern/kern_jail.c:
int
prison_ip(struct ucred *cred, int flag, u_int32_t *ip)
{
    u_int32_t tmp;

    if (!jailed(cred))
        return (0);
    if (flag)
        tmp = *ip;
    else
        tmp = ntohl(*ip);
    if (tmp == INADDR_ANY) {
        if (flag)
            *ip = cred->cr_prison->pr_ip;
        else
            *ip = htonl(cred->cr_prison->pr_ip);
        return (0);
    }
    if (tmp == INADDR_LOOPBACK) {
        if (flag)
            *ip = cred->cr_prison->pr_ip;
        else
            *ip = htonl(cred->cr_prison->pr_ip);
        return (0);
    }
    if (cred->cr_prison->pr_ip != tmp)
        return (1);
    return (0);
}

4.2.5. Filesystem

Even root users within the jail are not allowed to unset or modify any file flags, such as immutable, append-only, and undeleteable flags, if the securelevel is greater than 0.

/usr/src/sys/ufs/ufs/ufs_vnops.c:
static int
ufs_setattr(ap)
    ...
{
    ...
        if (!priv_check_cred(cred, PRIV_VFS_SYSFLAGS, 0)) {
            if (ip->i_flags
                & (SF_NOUNLINK | SF_IMMUTABLE | SF_APPEND)) {
                    error = securelevel_gt(cred, 0);
                    if (error)
                        return (error);
            }
            ...
        }
}
/usr/src/sys/kern/kern_priv.c
int
priv_check_cred(struct ucred *cred, int priv, int flags)
{
    ...
    error = prison_priv_check(cred, priv);
    if (error)
        return (error);
    ...
}
/usr/src/sys/kern/kern_jail.c
int
prison_priv_check(struct ucred *cred, int priv)
{
    ...
    switch (priv) {
    ...
    case PRIV_VFS_SYSFLAGS:
        if (jail_chflags_allowed)
            return (0);
        else
            return (EPERM);
    ...
    }
    ...
}

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