Ubuntu Manpage: keyrings - In-kernel key management and retention facility

Provided by: keyutils_1.5.9-9.2ubuntu2.1_amd64

NAME

       keyrings - In-kernel key management and retention facility

OVERVIEW

       The  keyrings  facility  is  primarily a way for drivers to retain or cache security data, authentication
       keys, encryption keys and other data in the kernel.

       System call interfaces are provided so that userspace programs can manage those objects and also use  the
       facility for their own purposes.

       A  library  and  some  userspace utilities are provided to allow access to the facility.  See keyutils(7)
       manual page for more information.

       This document contains the following sections:

              - Keys.
              - Key types.
              - Keyrings.
              - Anchoring keys.
              - Possession.
              - Access rights.
              - Searching for keys.
              - On-demand key creation.
              - Users.

KEYS

The facility provides the concept of a 'key', where all of the above examples are encapsulated within
objects of this type.

A 'key' contains the following elements:

Serial number
This is a unique integer handle by which a key is referred to in system call arguments.

Type This defines what sort of data can be held in the key, how the proposed content of the key will be
parsed and how the payload will be used.

There are a number of general purpose types available, plus some specialist types defined by
specific drivers.

Description
This is a printable string that is used as the search term for the key (in conjunction with the
type) as well as a display name. The description may be partially matched or exactly matched.

Payload
This is the actual content of a key. This is usually set when a key is created, but it is
possible for the kernel to upcall to userspace to finish the instantiation of a key if that key
wasn't already known to the kernel when it was requested.

A key's payload can be read and updated if the key type supports it and if suitable permission is
granted to the caller.

Access rights
Each key has an owning user ID, an owning group and a security label - much as files do. They
also have a set of permissions, though there are more than for a normal UNIX file, and there is an
additional category beyond the usual user, group and other (see below).

Note that keys are quota controlled since they represent unswappable kernel memory and the owning
user ID specifies whose quota is to be debited.

Expiration time
Each key can have an expiration time set. When that time is reached, the key is marked as being
expired and accesses to it fail with EKEYEXPIRED. If not deleted, updated or replaced, after a
set amount of time, expired keys are automatically removed along with all links to them and ENOKEY
will be reported.

Reference count
Each key has a reference count. Keys are referenced by keyrings, by current active users and by a
process's credentials. When the reference count reaches zero, the key is scheduled for garbage
collection.

See the keyctl_describe(3) manual page for more information.

KEY TYPES

       The facility provides several basic types of key:

       keyring
              Keys of this type are special.  The payload consists of a set of links to other keys, analogous to
              a  directory  holding links to files.  The main purpose of a keyring is to prevent other keys from
              being garbage collected because nothing refers to them.

       user   This is a general purpose key type.  It may be instantiated with an arbitrary blob of data  of  up
              to  about  32KB.   It  is  kept  entirely  within  kernel  memory.   It may be read and updated by
              userspace.

       big_key
              This is similar to user but it may hold data up to 1MB in size.  The data may  be  stored  in  the
              swap space rather than in kernel memory if the size exceeds the overhead of doing so (a tmpfs file
              is used - which requires filesystem structures to be allocated in the kernel).

       logon  This is similar to user but the contents may not be read by userspace.

       There  are  more  specialised  key  types  available  also, but they're not discussed here as they're not
       intended for normal userspace use.

KEYRINGS

       As previously mentioned, keyrings are a special type of key that contain links to other keys  (which  may
       include  other  keyrings).   Keys  may  be linked to by multiple keyrings.  Keyrings may be considered as
       analogous to UNIX directories where each directory contains a set of hard links to files.

       Several of the syscall functions available may only be applied to keyrings:

       Adding A key may be added to a keyring by system calls that create keys.  This prevents the new key  from
              being immediately deleted when the system call driver releases its last reference to the key.

       Linking
              A  link  may be added to a keyring pointing to a key that is already known, provided this does not
              create a self-referential cycle.

       Unlinking
              A link may be removed from a keyring.  When the last link to a key is removed, that  key  will  be
              scheduled for deletion by the garbage collector.

       Clearing
              All the links may be removed from a keyring.

       Searching
              A  keyring may be considered the root of a tree or subtree in which keyrings form the branches and
              non-keyrings the leaves.  This tree may be searched for a leaf  matching  a  particular  type  and
              description.

       See  the  keyctl_clear(3),  keyctl_link(3),  keyctl_search(3)  and keyctl_unlink(3) manual pages for more
       information.

ANCHORING KEYS

To prevent a key from being prematurely garbage collected, it must anchored to keep its reference count
elevated when it is not in active use by the kernel.

Keyrings are used to anchor other keys - each link is a reference on a key - but whilst keyrings are
available to link to keys, keyrings themselves are just keys and are also subject to the same anchoring
necessity.

The kernel makes available a number of anchor keyrings. Note that some of these keyrings will only be
created when first accessed.

Process keyrings
Process credentials themselves reference keyrings with specific semantics. These keyrings are
pinned as long as the set of credentials exists - which is usually as long as the process does.

There are three keyrings with different inheritance/sharing rules: The session keyring (inherited
and shared by all child processes), the process keyring (shared by all threads in a process) and
the thread keyring (specific to a particular thread).

User keyrings
Each UID known to the kernel has a record that contains two keyrings: The user keyring and the
user session keyring. These exist for as long as the UID record in the kernel exists. A link to
the user keyring is placed in a new session keyring by pam_keyinit when a new login session is
initiated.

Persistent keyrings
There is a persistent keyring available to each UID known to the system. It may persist beyond
the life of the UID record previously mentioned, but has an expiration time set such that it is
automatically cleaned up after a set time. This, for example, permits cron scripts to use
credentials left when the user logs out.

Note that the expiration time is reset every time the persistent key is requested.

Special keyrings
There are special keyrings owned by the kernel that can anchor keys for special purposes. An
example of this is the system keyring used for holding encryption keys for module signature
verification.

These are usually closed to direct alteration by userspace.

See the thread-keyring(7), process-keyring(7), session-keyring(7), user-keyring(7), user-session-
keyring(7), and persistent-keyring(7) manual pages for more information.

POSSESSION

       The  concept of 'possession' is important to understanding the keyrings security model.  Whether a thread
       possesses a key is determined by the following rules:

       (1)    Any key or keyring that does not grant Search permission to the  caller  is  ignored  in  all  the
              following rules.

       (2)    A  thread possesses its session, process and thread keyrings directly because those are pointed to
              by its credentials.

       (3)    If a keyring is possessed, then any key it links to is also possessed.

       (4)    If any key a keyring links to is itself a keyring, then rule (3) applies recursively.

       (5)    If a process is upcalled from the kernel  to  instantiate  a  key,  then  it  also  possess's  the
              requester's keyrings as in rule (1) as if it were the requester.

       Note  that  possession is not a fundamental property of a key, but must rather be calculated each time it
       is needed.

       Possession is designed to allow setuid programs run from, say, a user's shell to access the user's  keys.
       It also allows the prevention of access to keys just on the basis of UID and GID matches.

       When  it creates the session keyring, the pam_keyinit module adds a link to the user keyring, thus making
       the user keyring and anything it contains possessed by default.

ACCESS RIGHTS

Each key has the following security-related attributes:

- The owning user ID
- The ID of a group that is permitted to access the key
- A security label
- A permissions mask

The permissions mask is used to govern the following rights:

View If set, the attributes of a key may be read. This includes the type, description and access
rights (excluding the security label).

Read If set, the payload of a key may be read and a list of the serial numbers to which a keyring has
links may be read.

Write If set, the payload of a key may be updated, links may be added to or removed from a keyring, a
keyring may be cleared completely and a key may be revoked.

Search If set, keyrings and subkeyrings may be searched and keys and keyrings may be found by that
search.

Link If set, an additional link may be made to a key from a keyring. The initial link to a key when it
is created doesn't require this permit.

Setattr
If set, the ownership details on a key and its security label may be changed, its expiration time
may be set and it may be revoked.

The permissions mask contains four sets of rights. The first three sets are mutually exclusive. One and
only one will be in force at any one time. In order of descending priority:

User Used if the key's user ID matches the caller's fsuid.

Group Used if the user ID didn't match and the key's group ID matches the caller's fsgid or one of the
caller's supplementary group list.

Other Used if neither the key's user ID nor group ID matched.

The fourth set of rights is:

Possessor
Used if a key is determined to be possessed by the caller.

The complete set of rights for a key is the set union of whichever of the first three sets is selected
plus the fourth if the key is possessed.

If any right is granted to a thread for a key, then that thread will see the key listed in /proc/keys.
If no rights at all are granted, then that thread can't even tell that the key exists.

In addition to access rights, any active Linux Security Module may prevent access to a key if its policy
so dictates. A key may be given a security label or other attribute by the LSM which can be retrieved.

See the keyctl_chown(3), keyctl_describe(3), keyctl_get_security(3), keyctl_setperm(3) and selinux(8)
manual pages for more information.

SEARCHING FOR KEYS

       One of the key features of this facility is the ability  to  find  a  key  that  it  is  retaining.   The
       request_key()  system  call is the primary point of access for userspace to find a key to use (the kernel
       has something similar available).

       The search algorithm works as follows:

       (1)    The three process keyrings are searched in the following order: the thread keyring if  it  exists,
              the  process  keyring  if  it  exists and then either the session keyring if it exists or the user
              session keyring if that exists.

       (2)    If the caller was a process that was invoked  by  the  request_key()  upcall  mechanism  then  the
              keyrings of the original caller of that request_key() will be searched as well.

       (3)    Each  keyring  is  searched  first  for a match, then the keyrings referred to by that keyring are
              searched.

       (4)    If a matching key is found that is valid, then the search terminates and that key is returned.

       (5)    If a matching key is found that has an error state attached, that error state  is  noted  and  the
              search continues.

       (6)    If  valid  matching  key  is found, then the first noted error state is returned or else ENOKEY is
              returned.

       It is also possible to search a specific keyring, in which case only steps (3) to (6) apply.

       See the request_key(2) and keyctl_search(3) manual pages for more information.

ON-DEMAND KEY CREATION

       If a key cannot be found, the request_key() system call will, if given a callout_info argument, create  a
       new  key  and  then upcall to userspace to instantiate the key.  This allows keys to be created on an as-
       needed basis.

       Typically, this will involve the kernel forking and exec'ing request-key program, which will then execute
       the appopriate handler based on its configuration.

       The handler is passed a special authorisation key that allows it and only it to instantiate the new  key.
       This  is  also  used  to  permit searches performed by the handler program to also search the requester's
       keyrings.

       See   the   keyctl_assume_authority(3),   keyctl_instantiate(3),   keyctl_negate(3),    keyctl_reject(3),
       request_key(2), request-key(8) and request-key.conf(5) manual pages for more information.

USERS

The facility has a number of users and usages, but is not limited to those that already exist.

In-kernel users of this facility include:

Network filesystems - DNS
The kernel uses the upcall mechanism provided by the keys to upcall to userspace to do DNS lookups
and then to cache the results.

AF_RXRPC and kAFS - Authentication
The AF_RXRPC network protocol and the in-kernel AFS filesystem store the ticket needed to do
secured or encrypted traffic in keys. These are then looked up by network operations on AF_RXRPC
and filesystem operations on kAFS.

NFS - User ID mapping
The NFS filesystem uses keys to store foreign user ID to local user ID mapping.

CIFS - Password
The CIFS filesystem uses keys to store passwords for accessing remote shares.

Module verification
The kernel build process can be made to cryptographically sign modules. That signature is then
checked when a module is loaded.

Userspace users of this facility include:

Kerberos key storage
The MIT Kerberos 5 facility (libkrb5) can use keys to store authentication tokens which can be
made to be automatically cleaned up a set time after the user last uses them, but until then
permits them to hang around after the user has logged out so that cron scripts can use them.