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.