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.
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.
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.
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];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).
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.
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.
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.
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.hstruct 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);All FreeBSD documents are available for download at https://download.freebsd.org/ftp/doc/
Questions that are not answered by the
documentation may be
sent to <freebsd-questions@FreeBSD.org>.
Send questions about this document to <freebsd-doc@FreeBSD.org>.