SECMODEL(9) BSD Kernel Developer's Manual SECMODEL(9)NAMEsecmodel — security model development guidelines
SYNOPSIS
#include <secmodel/secmodel.h>
int
secmodel_register(secmodel_t *sm, const char *id, const char *name,
prop_dictionary_t behavior, secmodel_eval_t sm_eval,
secmodel_setinfo_t sm_setinfo);
int
secmodel_deregister(secmodel_t sm);
int
secmodel_eval(const char *id, const char *what, void *arg, void *ret);
static int
secmodel_<model>_eval(const char *what, void *arg, void *ret);
DESCRIPTION
NetBSD provides a complete abstraction of the underlying security model
used within the operating system through a set of kauth(9) scopes and
actions. It allows maintaining the traditional security model (based on
a single super-user and above-super-user restrictions known as
securelevel) while decoupling it easily from the system.
It is possible to modify the security model -- either slightly or using
an entirely different model -- by attaching/detaching kauth(9) listeners.
This can be done via the secmodel pluggable framework.
A secmodel is typically implemented as a kernel module(9), and can be
either built-in statically or loaded dynamically at run-time. They base
their decisions on available information, either directly from kernel,
from a userspace daemon or even from a centralized network authorization
server.
DATA TYPES
The secmodel framework offers the following data types:
secmodel_t An opaque type that describes a secmodel.
FUNCTIONS
secmodel_register(sm, id, name, behavior, sm_eval, sm_setinfo)
Register a security model to the secmodel framework and stores
its description inside sm.
sm The secmodel description.
id The unique identifier of the secmodel.
name The descriptive human-readable name of the secmodel.
behavior (optional) a prop_dictionary(3) that declares the
behavior of this security model, like “copy
credentials on fork.”
sm_eval (optional) the secmodel_<model>_eval() callback used
by a secmodel to register an evaluation routine that
can be queried later by another security model.
sm_setinfo (optional) the secmodel_<model>_setinfo() callback
used by a secmodel to register a routine that per‐
mits other security models to alter the secmodel
internals. Currently not implemented.
secmodel_deregister(sm)
Deregister the secmodel described by sm.
secmodel_eval(id, what, arg, ret)
Call the evaluation callback implemented by a security model.
The return value can be either:
- zero (0), when the call succeeded.
- positive, when the error comes directly from the
secmodel framework.
- negative, when the error comes from the evaluation
callback implemented in the targetted security model.
The value is then implementation-defined.
id The unique identifier of the targetted secmodel.
what The query that will be passed down to the targetted
secmodel.
arg The arguments passed to the evaluation routine of the tar‐
getted secmodel.
ret The answer of the evaluation routine.
RETURN VALUES
If successful, functions return 0. Otherwise, the following error values
are returned:
[EEXIST] The secmodel is already registered.
[EFAULT] An invalid address or reference was passed as parameter.
[EINVAL] An invalid value was passed as parameter.
[ENOENT] The targetted secmodel does not exist, or it does not implement
an evaluation callback.
WRITING A SECURITY MODEL
Before writing a security model one should be familiar with the kauth(9)
KPI, its limitations, requirements, and so on. See kauth(9) for details.
A security model is based on the kernel module(9) framework, and can be
built-in statically inside kernel or loaded dynamically at run-time. It
is composed of (code-wise) the following components:
1. module(9) routines, especially a MODULE() declaration and a
secmodel_<model>_modcmd() function used to start (through
MODULE_CMD_INIT) and stop (through MODULE_CMD_FINI) the
secmodel.
2. Entry routines, named secmodel_<model>_init() and
secmodel_<model>_start(), used to initialize and start the
security model, and another function called
secmodel_<model>_stop(), to stop the security model in case
the module is to be unloaded.
3. A sysctl(9) setup routine for the model. This should create
an entry for the model in the sysctl(7) namespace, under the
"security.models.<model>" hierarchy.
All "knobs" for the model should be located under the new
node, as well as a mandatory name variable, indicating a
descriptive human-readable name for the model.
4. A sysctl(9) teardown routine used to destroy the sysctl(7)
tree associated with the model.
5. If the model uses any private data inside credentials, listen‐
ing on the credentials scope, KAUTH_SCOPE_CRED, is required.
6. Optionally, internal data-structures used by the model. These
must all be prefixed with "secmodel_<model>_".
7. A set of listeners, attached to various scopes, used to
enforce the policy the model intends to implement.
8. Finally, a security model should register itself after being
initialized using secmodel_register(), and deregister itself
before being stopped using secmodel_deregister().
EXAMPLES
Below is sample code for a kauth(9) network scope listener for the jenna
security model. It is used to allow users with a user-id below 1000 to
bind to reserved ports (for example, 22/TCP):
int
secmodel_jenna_network_cb(kauth_cred_t cred, kauth_action_t action,
void *cookie, void *arg0, void *arg1, void *arg2, void *arg3)
{
int result;
/* Default defer. */
result = KAUTH_RESULT_DEFER;
switch (action) {
case KAUTH_NETWORK_BIND:
/*
* We only care about bind(2) requests to privileged
* ports.
*/
if ((u_long)arg0 == KAUTH_REQ_NETWORK_BIND_PRIVPORT) {
/*
* If the user-id is below 1000, which may
* indicate a "reserved" user-id, allow the
* request.
*/
if (kauth_cred_geteuid(cred) < 1000)
result = KAUTH_RESULT_ALLOW;
}
break;
}
return (result);
}
There are two main issues, however, with that listener, that you should
be aware of when approaching to write your own security model:
1. kauth(9) uses restrictive decisions: if you attach this lis‐
tener on-top of an existing security model, even if it would
allow the request, it could still be denied.
2. If you attach this listener as the only listener for the net‐
work scope, there are many other requests that will be
deferred and, eventually, denied -- which may not be desired.
That's why before implementing listeners, it should be clear whether they
implement an entirely new from scratch security model, or add on-top of
an existing one.
PROGRAMMING CONSIDERATIONS
There are several things you should remember when writing a security
model:
- Pay attention to the correctness of your secmodel implementa‐
tion of the desired policy. Certain rights can grant more
privileges on the system than others, like allowing calls to
chroot(2) or mounting a file-system.
- All unhandled requests are denied by default.
- Authorization requests can not be issued when the kernel is
holding any locks. This is a requirement from kernel code to
allow designing security models where the request should be
dispatched to userspace or a different host.
- Private listener data -- such as internal data-structures -- is
entirely under the responsibility of the developer. Locking,
synchronization, and garbage collection are all things that
kauth(9) does not take care of for you!
STACKING ON AN EXISTING SECURITY MODEL
One of the shortcomings of kauth(9) is that it does not provide any
stacking mechanism, similar to Linux Security Modules (LSM). This, how‐
ever, is considered a feature in reducing dependency on other people's
code.
To properly "stack" minor adjustments on-top of an existing security
model, one could use one of two approaches:
1. Register an internal scope for the security model to be used as a
fall-back when requests are deferred.
This requires the security model developer to add an internal scope
for every scope the model partly covers, and register the fall-back
listeners to it. In the model's listener(s) for the scope, when a
defer decision is made, the request is passed to be authorized on
the internal scope, effectively using the fall-back security model.
Here is example code that implements the above:
#include <secmodel/bsd44/bsd44.h>
/*
* Internal fall-back scope for the network scope.
*/
#define JENNA_ISCOPE_NETWORK "jenna.iscope.network"
static kauth_scope_t secmodel_jenna_iscope_network;
/*
* Jenna's entry point. Register internal scope for the network scope
* which we partly cover for fall-back authorization.
*/
void
secmodel_jenna_start(void)
{
secmodel_jenna_iscope_network = kauth_register_scope(
JENNA_ISCOPE_NETWORK, NULL, NULL);
kauth_listen_scope(JENNA_ISCOPE_NETWORK,
secmodel_bsd44_suser_network_cb, NULL);
kauth_listen_scope(JENNA_ISCOPE_NETWORK,
secmodel_securelevel_network_cb, NULL);
}
/*
* Jenna sits on top of another model, effectively filtering requests.
* If it has nothing to say, it discards the request. This is a good
* example for fine-tuning a security model for a special need.
*/
int
secmodel_jenna_network_cb(kauth_cred_t cred, kauth_action_t action,
void *cookie, void *arg0, void *arg1, void *arg2, void *arg3)
{
int result;
/* Default defer. */
result = KAUTH_RESULT_DEFER;
switch (action) {
case KAUTH_NETWORK_BIND:
/*
* We only care about bind(2) requests to privileged
* ports.
*/
if ((u_long)arg0 == KAUTH_REQ_NETWORK_BIND_PRIVPORT) {
if (kauth_cred_geteuid(cred) < 1000)
result = KAUTH_RESULT_ALLOW;
}
break;
}
/*
* If we have don't have a decision, fall-back to the bsd44
* security model.
*/
if (result == KAUTH_RESULT_DEFER)
result = kauth_authorize_action(
secmodel_jenna_iscope_network, cred, action,
arg0, arg1, arg2, arg3);
return (result);
}
2. If the above is not desired, or cannot be used for any reason, there
is always the ability to manually call the fall-back routine:
int
secmodel_jenna_network_cb(kauth_cred_t cred, kauth_action_t action,
void *cookie, void *arg0, void *arg1, void *arg2, void *arg3)
{
int result;
/* Default defer. */
result = KAUTH_RESULT_DEFER;
switch (action) {
case KAUTH_NETWORK_BIND:
/*
* We only care about bind(2) requests to privileged
* ports.
*/
if ((u_long)arg0 == KAUTH_REQ_NETWORK_BIND_PRIVPORT) {
if (kauth_cred_geteuid(cred) < 1000)
result = KAUTH_RESULT_ALLOW;
}
break;
}
/*
* If we have don't have a decision, fall-back to the bsd44
* security model's suser behavior.
*/
if (result == KAUTH_RESULT_DEFER)
result = secmodel_bsd44_suser_network_cb(cred, action,
cookie, arg0, arg1, arg2, arg3);
return (result);
}
AVAILABLE SECURITY MODELS
The following is a list of security models available in the default
NetBSD distribution.
secmodel_suser(9)
Implements the super-user (root) security policy.
secmodel_securelevel(9)
Implements the securelevel security model.
secmodel_extensions(9)
Implements extensions to the traditional 4.4BSD security model,
like usermounts.
secmodel_bsd44(9)
Traditional NetBSD security model, derived from 4.4BSD.
secmodel_overlay(9)
Sample overlay security model, sitting on-top of
secmodel_bsd44(9).
CODE REFERENCES
The core of the secmodel implementation is in sys/secmodel/secmodel.c.
The header file <secmodel/secmodel.h> describes the public interface.
To make it easier on developers to write new security models from
scratch, NetBSD maintains an example secmodel under
share/examples/secmodel/.
SEE ALSOkauth(9), module(9), secmodel_bsd44(9), secmodel_extensions(9),
secmodel_overlay(9), secmodel_securelevel(9), secmodel_suser(9)HISTORY
Kernel Authorization was introduced in NetBSD 4.0 as the subsystem
responsible for authorization and credential management. Before its
introduction, there were several ways for providing resource access con‐
trol:
- Checking if the user in question is the super-user via suser().
- Comparing the user-id against hard-coded values, often zero.
- Checking the system securelevel.
The problem with the above is that the interface ("can X do Y?") was
tightly coupled with the implementation ("is X Z?"). kauth(9) allows
separating them, dispatching requests with highly detailed context using
a consistent and clear KPI.
The secmodel framework was extended in NetBSD 6.0 to implement secmodel
registration and evaluation procedure calls.
AUTHORS
Elad Efrat ⟨elad@NetBSD.org⟩
BSD December 4, 2011 BSD
