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Create a new key type using Linux kernel APIs

The Linux key retention service, introduced with the Linux 2.6 kernel, is primarily intended to cache authentication data in the Linux kernel. The service can be used by remote filesystems or other kernel services to manage cryptography, authentication tokens, cross-domain user mappings, and other security concerns. It also enables the Linux kernel to access required keys rapidly, and can be used to delegate key operations such as add, update, and delete to user-space.

This article gives you an overview of the Linux key retention service, defines its terminology, and helps you get started quickly with using Linux keys. You see how to use the Linux key retention service in a kernel module using sample code. The kernel version used in writing this article is 2.6.20.

What is a key?

A key is a unit of cryptographic data, an authentication token, or some similar element represented in the kernel by struct key. struct key is defined in the Linux kernel source under include/linux/key.h.

Listing 1 shows some important fields from struct key. Note that task_struct, user_struct, and signal_struct have been modified to add support for keys.

Listing 1. Important fields from struct key

Attributes of a key

A key has the following attributes:

Serial number: A unique 32-bit non-zero positive number.
Type: The Linux key retention service defines two standard key types: user and keyring. To add a new key type, it must be registered by a kernel service. User-space programs are not allowed to create new key types. Key types are represented in the kernel by structkey_type, which is defined in include/linux/key.h. Some important fields of the key_type structure are shown in Listing 2.
Listing 2. Important fields of key_type

You can also associate a set of operations to a key type. A key_type can define the following operations:
- instantiate creates a new key of a specified type.
- describe prints text describing the key.
- match searches a key based on its description.
- destroy clears all data related to a key.
- request_key searches for a key.
- revoke clears key data and changes the state of the key to REVOKED.
- read reads key data.
- update modifies a key.
Description: A printable string that describes the key. This attribute can also be used to perform search operations.
Access control information: Each key has an owner UID, a GID, and a permissions mask that dictates how it will respond to user-level or kernel-level programs. The permissions mask allocates 8 bits each to the four possible types of key accessor: possessor, user, group, and other. Out of the 8 bits, only 6 bits are defined. The possible permissions are as follows:
- View allows the possessor to view key attributes.
- Read allows the possessor to read the key and lists the key for a keyring.
- Write allows the possessor to modify the payload for a key or keyring and to modify linked keys.
- Search allows the possessor to search keyrings and find keys.
- Link allows the possessor to link a particular key or keyring to a keyring.
- Set Attribute allows the possessor to set the key's UID, GID, and permissions mask.
Expiry Time: Lifetime of a key. Keys can also be permanent.
Payload: The actual security data. Operations defined with struct key_type are used to instantiate the payload with data and also to read back or modify the data. To the kernel, the payload is just a blob of data.
State: A key can be in any of the following states:
- UNINSTANTIATED: The key has been created but is not yet attached to any data.
- INSTANTIATED: The key is instantiated and attached to data; this is a complete state.
- NEGATIVE: A temporary state that denotes that the previous call to user-space failed.
- EXPIRED: Indicates the key has outgrown its pre-defined lifetime.
- REVOKED: The key is moved to this state by a user-space action.
- DEAD: The key_type is unregistered.

Key types

There are two defined key types: keyring and user.

A keyring is a key that contain a set of links to other keys or keyrings. There are six standard keyrings:

Thread-specific
Process-specific
Session-specific
User-specific session
User-default session
Group-specific (not yet implemented)

Only the first three keyrings are automatically searched, in which case they are searched in order. The fourth type, a user-specific session keyring, is not searched directly but it is normally linked to from a session-specific keyring. Login processes such as PAM will bind to the user-default session keyring until another session is created.

User keys are defined to be manipulated by user-space programs.

Three new syscalls

The Linux key retention service provides three new syscalls to manipulate keys in user-space. The first is add_key:

The add_key syscall is used to create keys of type type and length plen. The key description is defined by desc, and its payload is pointed to by payload. The key is linked to the keyring ring. The key type can be user or keyring. Any other key type must be already registered with the kernel via a kernel service. If the key is of type keyring, the payload should be NULL and the plen should be zero.

The next new syscall is request_key:

The request_key syscall searches a process keyring for a key. The basic algorithm used to search for a key is shown in Listing 3.

Listing 3. The request_key algorithm

For detailed information on the workings of the request_key algorithm, refer to Documentation/keys-request-key.txt (see Related topics for a link).

Finally, the syscall keyctl provides a number of functions for managing keys. Various operation can be performed on keys depending on the first argument passed to keyctl. Some of keyctl's operations are listed below:

KEYCTL_DESCRIBE describes a key.
KEYCTL_READ reads payload data from a key.
KEYCTL_UPDATE updates the specified key.
KEYCTL_LINK links a key into a keyring.
KEYCTL_UNLINK unlinks a key or keyring from another keyring.
KEYCTL_JOIN_SESSION_KEYRING replaces a session keyring with a new one.
KEYCTL_REVOKE revokes a key.
KEYCTL_CHOWN changes the ownership of a key.
KEYCTL_SETPERM changes the permissions mask on a key.
KEYCTL_CLEAR clears out a keyring.
KEYCTL_SEARCH searches a keyring tree for a key.
KEYCTL_INSTANTIATE instantiates a partially constructed key.
KEYCTL_NEGATE negatively instantiates a partially constructed key.

For more information on the prototype of keyctl or other possible operations that can be performed by keyctl, refer to the Linux man pages.

Kernel APIs to manage keys

Below is a short listing of the most important Linux kernel APIs for managing keys. For more comprehensive information, download and refer to the Linux key implementation source files (see Download, below).

register_key_type is used to define a new key type.
int register_key_type(struct key_type *type) returns EEXIT if a key type of the same name already exists.
unregister_key_type is used to unregister a key type:
key_put releases a key:
request_key searches for a key matching a given description:
key_alloc allocates a key of a specified type:
key_instantiate_and_link instantiates a key and links it into the target keyring atomically:

Enabling key services

Because the Linux key retention service is still very new, it is turned off by default in the Linux kernel. To enable key services, you must configure the kernel using the CONFIG_KEYS=y option. You can find this option under Security options in the make *config step of kernel compilation.

Listing 4 shows the configuration to enable key services in the Linux kernel.

Listing 4. Enabling key services in the kernel

The source code for keys is organized in the directory linux-2.6.x/security/keys.

Next, you need to download and install the keyutils package. keyutils contains the keyctl command, which you can use to perform a variety of operations on keys. We've already listed some of keyctl's operations. See the Linux man pages to learn more about usage.

Create a new key type

The easiest way to learn about the Linux key retention service is to try it out. The following examples use the Linux key retention service to create a key of a new type. If you haven't already, go ahead and download the sample program now. Do a make to build the binaries of the kernel module and user-level programs. The code has been tested with Linux kernel version 2.6.20.

The sample program has two components: a kernel module and a user-space program. The kernel module registers a new key type. When executed, the user-space program does an ioctl on pre-defined proc-entries, which results in a call to the kernel module. A new key is created as a result of this call. A 'bash' shell is then returned to the user with the new session keyring and the key of a new type linked to the keyring.

Because the user-space program will do an ioctl, the function proc_ioctl() must be registered by the kernel module to handle ioctl requests. All ioctl communication is done using the /proc interface. Listing 5 shows a new key type being declared in the kernel module.

Listing 5. Declaring a new key type

The module then calls register_key_type(), in its init function, to register the new key, which is named mykey. When the kernel module receives an ioctl request, it first, creates a session keyring, by calling key_alloc() to allocate a new key. After a successful call to key_alloc(), we call key_instantiate_and_link() to instantiate the key. After we have created and instantiated the session keyring, we create the key for the user's session. We make the same set of calls to key_alloc(), followed by key_instantiate_and_link(). Upon successful completion of these calls, the user-space session has a new key.

All of these steps are demonstrated in the sample program.

Using the module

Having created a new key type, we next want to try using the kernel module. A basic operation in the module is to view what keyrings a process is subscribed to and what keys and other keyrings those keyrings contain. A call to keyctl show shows the keys in a tree-like structure. Listing 6 shows the state of the keys before running our program.

Listing 6. Viewing process keyrings

Listing 7 shows the output of commands for inserting the module, or unloading the module or the user-level program. These messages go in a syslog file (typically /var/log/messages).

Listing 7. Inserting the kernel module

Next, we execute the user-level program.

Listing 8. Executing the user-level program

And in Listing 9 we actually see the keys.

Listing 9. Status of keys after running the user-level program

Proc files related to keys

Two files are added to /proc to monitor keys: /proc/keys and /proc/key-users. Let's take a closer look at these files.

/proc/keys
If a process wants to know which keys it can view, it can get that information by reading /proc/keys. This file must be enabled when the kernel is configured, because it allows any user to list the keys database.

Listing 10. The /proc/keys file

*Source: linux_kernel_source/security/keys/proc.c:proc_keys_show()

The fields you see in the above file mostly come from struct key, as defined in include/linux/key.h. Possible flag values are shown in Listing 11.

Listing 11. Possible flag values of struct key fields

/proc/key-users
Listing 12 shows the /proc/key-users file.

Listing 12. The /proc/key-users file

The fields shown in Listing 13 correspond to each line sequentially.

Listing 13. Fields of /proc/key-users file

*Source: linux_kernel_source/security/keys/proc.c:proc_key_users_show()

Mostly these are fields of struct key_user defined in security/keys/internal.h.

In conclusion

The Linux key retention service is a new mechanism introduced to hold security-related information for fast access by the Linux kernel. It is still in its early phases and is just beginning to gain wider acceptance. OpenAFS uses the Linux key retention service to implement process authentication group (PAG), and NFSv4 and MIT Kerberos use it also. The Linux key retention service is still under development and may be modified or enhanced in the future.

Downloadable resources

Sample app using the Linux key retention service (key.retention.services.zip | 4KB)

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