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Cc: Randy Dunlap <randy.dunlap@oracle.com> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
1234 lines
46 KiB
Plaintext
1234 lines
46 KiB
Plaintext
============================
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KERNEL KEY RETENTION SERVICE
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============================
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This service allows cryptographic keys, authentication tokens, cross-domain
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user mappings, and similar to be cached in the kernel for the use of
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filesystems and other kernel services.
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Keyrings are permitted; these are a special type of key that can hold links to
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other keys. Processes each have three standard keyring subscriptions that a
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kernel service can search for relevant keys.
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The key service can be configured on by enabling:
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"Security options"/"Enable access key retention support" (CONFIG_KEYS)
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This document has the following sections:
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- Key overview
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- Key service overview
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- Key access permissions
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- SELinux support
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- New procfs files
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- Userspace system call interface
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- Kernel services
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- Notes on accessing payload contents
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- Defining a key type
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- Request-key callback service
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- Key access filesystem
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============
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KEY OVERVIEW
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============
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In this context, keys represent units of cryptographic data, authentication
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tokens, keyrings, etc.. These are represented in the kernel by struct key.
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Each key has a number of attributes:
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- A serial number.
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- A type.
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- A description (for matching a key in a search).
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- Access control information.
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- An expiry time.
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- A payload.
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- State.
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(*) Each key is issued a serial number of type key_serial_t that is unique for
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the lifetime of that key. All serial numbers are positive non-zero 32-bit
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integers.
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Userspace programs can use a key's serial numbers as a way to gain access
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to it, subject to permission checking.
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(*) Each key is of a defined "type". Types must be registered inside the
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kernel by a kernel service (such as a filesystem) before keys of that type
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can be added or used. Userspace programs cannot define new types directly.
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Key types are represented in the kernel by struct key_type. This defines a
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number of operations that can be performed on a key of that type.
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Should a type be removed from the system, all the keys of that type will
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be invalidated.
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(*) Each key has a description. This should be a printable string. The key
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type provides an operation to perform a match between the description on a
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key and a criterion string.
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(*) Each key has an owner user ID, a group ID and a permissions mask. These
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are used to control what a process may do to a key from userspace, and
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whether a kernel service will be able to find the key.
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(*) Each key can be set to expire at a specific time by the key type's
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instantiation function. Keys can also be immortal.
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(*) Each key can have a payload. This is a quantity of data that represent the
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actual "key". In the case of a keyring, this is a list of keys to which
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the keyring links; in the case of a user-defined key, it's an arbitrary
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blob of data.
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Having a payload is not required; and the payload can, in fact, just be a
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value stored in the struct key itself.
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When a key is instantiated, the key type's instantiation function is
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called with a blob of data, and that then creates the key's payload in
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some way.
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Similarly, when userspace wants to read back the contents of the key, if
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permitted, another key type operation will be called to convert the key's
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attached payload back into a blob of data.
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(*) Each key can be in one of a number of basic states:
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(*) Uninstantiated. The key exists, but does not have any data attached.
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Keys being requested from userspace will be in this state.
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(*) Instantiated. This is the normal state. The key is fully formed, and
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has data attached.
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(*) Negative. This is a relatively short-lived state. The key acts as a
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note saying that a previous call out to userspace failed, and acts as
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a throttle on key lookups. A negative key can be updated to a normal
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state.
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(*) Expired. Keys can have lifetimes set. If their lifetime is exceeded,
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they traverse to this state. An expired key can be updated back to a
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normal state.
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(*) Revoked. A key is put in this state by userspace action. It can't be
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found or operated upon (apart from by unlinking it).
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(*) Dead. The key's type was unregistered, and so the key is now useless.
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====================
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KEY SERVICE OVERVIEW
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====================
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The key service provides a number of features besides keys:
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(*) The key service defines two special key types:
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(+) "keyring"
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Keyrings are special keys that contain a list of other keys. Keyring
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lists can be modified using various system calls. Keyrings should not
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be given a payload when created.
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(+) "user"
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A key of this type has a description and a payload that are arbitrary
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blobs of data. These can be created, updated and read by userspace,
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and aren't intended for use by kernel services.
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(*) Each process subscribes to three keyrings: a thread-specific keyring, a
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process-specific keyring, and a session-specific keyring.
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The thread-specific keyring is discarded from the child when any sort of
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clone, fork, vfork or execve occurs. A new keyring is created only when
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required.
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The process-specific keyring is replaced with an empty one in the child on
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clone, fork, vfork unless CLONE_THREAD is supplied, in which case it is
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shared. execve also discards the process's process keyring and creates a
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new one.
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The session-specific keyring is persistent across clone, fork, vfork and
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execve, even when the latter executes a set-UID or set-GID binary. A
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process can, however, replace its current session keyring with a new one
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by using PR_JOIN_SESSION_KEYRING. It is permitted to request an anonymous
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new one, or to attempt to create or join one of a specific name.
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The ownership of the thread keyring changes when the real UID and GID of
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the thread changes.
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(*) Each user ID resident in the system holds two special keyrings: a user
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specific keyring and a default user session keyring. The default session
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keyring is initialised with a link to the user-specific keyring.
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When a process changes its real UID, if it used to have no session key, it
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will be subscribed to the default session key for the new UID.
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If a process attempts to access its session key when it doesn't have one,
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it will be subscribed to the default for its current UID.
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(*) Each user has two quotas against which the keys they own are tracked. One
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limits the total number of keys and keyrings, the other limits the total
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amount of description and payload space that can be consumed.
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The user can view information on this and other statistics through procfs
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files. The root user may also alter the quota limits through sysctl files
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(see the section "New procfs files").
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Process-specific and thread-specific keyrings are not counted towards a
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user's quota.
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If a system call that modifies a key or keyring in some way would put the
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user over quota, the operation is refused and error EDQUOT is returned.
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(*) There's a system call interface by which userspace programs can create and
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manipulate keys and keyrings.
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(*) There's a kernel interface by which services can register types and search
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for keys.
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(*) There's a way for the a search done from the kernel to call back to
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userspace to request a key that can't be found in a process's keyrings.
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(*) An optional filesystem is available through which the key database can be
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viewed and manipulated.
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======================
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KEY ACCESS PERMISSIONS
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======================
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Keys have an owner user ID, a group access ID, and a permissions mask. The mask
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has up to eight bits each for possessor, user, group and other access. Only
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six of each set of eight bits are defined. These permissions granted are:
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(*) View
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This permits a key or keyring's attributes to be viewed - including key
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type and description.
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(*) Read
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This permits a key's payload to be viewed or a keyring's list of linked
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keys.
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(*) Write
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This permits a key's payload to be instantiated or updated, or it allows a
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link to be added to or removed from a keyring.
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(*) Search
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This permits keyrings to be searched and keys to be found. Searches can
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only recurse into nested keyrings that have search permission set.
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(*) Link
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This permits a key or keyring to be linked to. To create a link from a
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keyring to a key, a process must have Write permission on the keyring and
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Link permission on the key.
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(*) Set Attribute
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This permits a key's UID, GID and permissions mask to be changed.
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For changing the ownership, group ID or permissions mask, being the owner of
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the key or having the sysadmin capability is sufficient.
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===============
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SELINUX SUPPORT
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===============
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The security class "key" has been added to SELinux so that mandatory access
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controls can be applied to keys created within various contexts. This support
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is preliminary, and is likely to change quite significantly in the near future.
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Currently, all of the basic permissions explained above are provided in SELinux
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as well; SELinux is simply invoked after all basic permission checks have been
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performed.
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The value of the file /proc/self/attr/keycreate influences the labeling of
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newly-created keys. If the contents of that file correspond to an SELinux
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security context, then the key will be assigned that context. Otherwise, the
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key will be assigned the current context of the task that invoked the key
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creation request. Tasks must be granted explicit permission to assign a
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particular context to newly-created keys, using the "create" permission in the
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key security class.
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The default keyrings associated with users will be labeled with the default
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context of the user if and only if the login programs have been instrumented to
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properly initialize keycreate during the login process. Otherwise, they will
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be labeled with the context of the login program itself.
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Note, however, that the default keyrings associated with the root user are
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labeled with the default kernel context, since they are created early in the
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boot process, before root has a chance to log in.
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The keyrings associated with new threads are each labeled with the context of
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their associated thread, and both session and process keyrings are handled
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similarly.
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================
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NEW PROCFS FILES
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================
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Two files have been added to procfs by which an administrator can find out
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about the status of the key service:
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(*) /proc/keys
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This lists the keys that are currently viewable by the task reading the
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file, giving information about their type, description and permissions.
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It is not possible to view the payload of the key this way, though some
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information about it may be given.
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The only keys included in the list are those that grant View permission to
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the reading process whether or not it possesses them. Note that LSM
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security checks are still performed, and may further filter out keys that
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the current process is not authorised to view.
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The contents of the file look like this:
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SERIAL FLAGS USAGE EXPY PERM UID GID TYPE DESCRIPTION: SUMMARY
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00000001 I----- 39 perm 1f3f0000 0 0 keyring _uid_ses.0: 1/4
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00000002 I----- 2 perm 1f3f0000 0 0 keyring _uid.0: empty
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00000007 I----- 1 perm 1f3f0000 0 0 keyring _pid.1: empty
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0000018d I----- 1 perm 1f3f0000 0 0 keyring _pid.412: empty
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000004d2 I--Q-- 1 perm 1f3f0000 32 -1 keyring _uid.32: 1/4
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000004d3 I--Q-- 3 perm 1f3f0000 32 -1 keyring _uid_ses.32: empty
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00000892 I--QU- 1 perm 1f000000 0 0 user metal:copper: 0
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00000893 I--Q-N 1 35s 1f3f0000 0 0 user metal:silver: 0
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00000894 I--Q-- 1 10h 003f0000 0 0 user metal:gold: 0
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The flags are:
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I Instantiated
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R Revoked
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D Dead
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Q Contributes to user's quota
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U Under construction by callback to userspace
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N Negative key
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This file must be enabled at kernel configuration time as it allows anyone
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to list the keys database.
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(*) /proc/key-users
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This file lists the tracking data for each user that has at least one key
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on the system. Such data includes quota information and statistics:
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[root@andromeda root]# cat /proc/key-users
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0: 46 45/45 1/100 13/10000
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29: 2 2/2 2/100 40/10000
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32: 2 2/2 2/100 40/10000
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38: 2 2/2 2/100 40/10000
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The format of each line is
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<UID>: User ID to which this applies
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<usage> Structure refcount
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<inst>/<keys> Total number of keys and number instantiated
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<keys>/<max> Key count quota
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<bytes>/<max> Key size quota
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Four new sysctl files have been added also for the purpose of controlling the
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quota limits on keys:
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(*) /proc/sys/kernel/keys/root_maxkeys
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/proc/sys/kernel/keys/root_maxbytes
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These files hold the maximum number of keys that root may have and the
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maximum total number of bytes of data that root may have stored in those
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keys.
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(*) /proc/sys/kernel/keys/maxkeys
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/proc/sys/kernel/keys/maxbytes
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These files hold the maximum number of keys that each non-root user may
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have and the maximum total number of bytes of data that each of those
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users may have stored in their keys.
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Root may alter these by writing each new limit as a decimal number string to
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the appropriate file.
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===============================
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USERSPACE SYSTEM CALL INTERFACE
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===============================
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Userspace can manipulate keys directly through three new syscalls: add_key,
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request_key and keyctl. The latter provides a number of functions for
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manipulating keys.
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When referring to a key directly, userspace programs should use the key's
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serial number (a positive 32-bit integer). However, there are some special
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values available for referring to special keys and keyrings that relate to the
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process making the call:
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CONSTANT VALUE KEY REFERENCED
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============================== ====== ===========================
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KEY_SPEC_THREAD_KEYRING -1 thread-specific keyring
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KEY_SPEC_PROCESS_KEYRING -2 process-specific keyring
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KEY_SPEC_SESSION_KEYRING -3 session-specific keyring
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KEY_SPEC_USER_KEYRING -4 UID-specific keyring
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KEY_SPEC_USER_SESSION_KEYRING -5 UID-session keyring
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KEY_SPEC_GROUP_KEYRING -6 GID-specific keyring
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KEY_SPEC_REQKEY_AUTH_KEY -7 assumed request_key()
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authorisation key
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The main syscalls are:
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(*) Create a new key of given type, description and payload and add it to the
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nominated keyring:
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key_serial_t add_key(const char *type, const char *desc,
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const void *payload, size_t plen,
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key_serial_t keyring);
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If a key of the same type and description as that proposed already exists
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in the keyring, this will try to update it with the given payload, or it
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will return error EEXIST if that function is not supported by the key
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type. The process must also have permission to write to the key to be able
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to update it. The new key will have all user permissions granted and no
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group or third party permissions.
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Otherwise, this will attempt to create a new key of the specified type and
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description, and to instantiate it with the supplied payload and attach it
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to the keyring. In this case, an error will be generated if the process
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does not have permission to write to the keyring.
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The payload is optional, and the pointer can be NULL if not required by
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the type. The payload is plen in size, and plen can be zero for an empty
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payload.
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A new keyring can be generated by setting type "keyring", the keyring name
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as the description (or NULL) and setting the payload to NULL.
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User defined keys can be created by specifying type "user". It is
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recommended that a user defined key's description by prefixed with a type
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ID and a colon, such as "krb5tgt:" for a Kerberos 5 ticket granting
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ticket.
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Any other type must have been registered with the kernel in advance by a
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kernel service such as a filesystem.
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The ID of the new or updated key is returned if successful.
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(*) Search the process's keyrings for a key, potentially calling out to
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userspace to create it.
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key_serial_t request_key(const char *type, const char *description,
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const char *callout_info,
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key_serial_t dest_keyring);
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This function searches all the process's keyrings in the order thread,
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process, session for a matching key. This works very much like
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KEYCTL_SEARCH, including the optional attachment of the discovered key to
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a keyring.
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If a key cannot be found, and if callout_info is not NULL, then
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/sbin/request-key will be invoked in an attempt to obtain a key. The
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callout_info string will be passed as an argument to the program.
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See also Documentation/keys-request-key.txt.
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The keyctl syscall functions are:
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(*) Map a special key ID to a real key ID for this process:
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key_serial_t keyctl(KEYCTL_GET_KEYRING_ID, key_serial_t id,
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int create);
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The special key specified by "id" is looked up (with the key being created
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if necessary) and the ID of the key or keyring thus found is returned if
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it exists.
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If the key does not yet exist, the key will be created if "create" is
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non-zero; and the error ENOKEY will be returned if "create" is zero.
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(*) Replace the session keyring this process subscribes to with a new one:
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key_serial_t keyctl(KEYCTL_JOIN_SESSION_KEYRING, const char *name);
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If name is NULL, an anonymous keyring is created attached to the process
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as its session keyring, displacing the old session keyring.
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If name is not NULL, if a keyring of that name exists, the process
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attempts to attach it as the session keyring, returning an error if that
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is not permitted; otherwise a new keyring of that name is created and
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attached as the session keyring.
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To attach to a named keyring, the keyring must have search permission for
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the process's ownership.
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The ID of the new session keyring is returned if successful.
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(*) Update the specified key:
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long keyctl(KEYCTL_UPDATE, key_serial_t key, const void *payload,
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size_t plen);
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This will try to update the specified key with the given payload, or it
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will return error EOPNOTSUPP if that function is not supported by the key
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type. The process must also have permission to write to the key to be able
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to update it.
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The payload is of length plen, and may be absent or empty as for
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add_key().
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(*) Revoke a key:
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long keyctl(KEYCTL_REVOKE, key_serial_t key);
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This makes a key unavailable for further operations. Further attempts to
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use the key will be met with error EKEYREVOKED, and the key will no longer
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be findable.
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(*) Change the ownership of a key:
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long keyctl(KEYCTL_CHOWN, key_serial_t key, uid_t uid, gid_t gid);
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This function permits a key's owner and group ID to be changed. Either one
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of uid or gid can be set to -1 to suppress that change.
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Only the superuser can change a key's owner to something other than the
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key's current owner. Similarly, only the superuser can change a key's
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group ID to something other than the calling process's group ID or one of
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its group list members.
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(*) Change the permissions mask on a key:
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long keyctl(KEYCTL_SETPERM, key_serial_t key, key_perm_t perm);
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|
|
This function permits the owner of a key or the superuser to change the
|
|
permissions mask on a key.
|
|
|
|
Only bits the available bits are permitted; if any other bits are set,
|
|
error EINVAL will be returned.
|
|
|
|
|
|
(*) Describe a key:
|
|
|
|
long keyctl(KEYCTL_DESCRIBE, key_serial_t key, char *buffer,
|
|
size_t buflen);
|
|
|
|
This function returns a summary of the key's attributes (but not its
|
|
payload data) as a string in the buffer provided.
|
|
|
|
Unless there's an error, it always returns the amount of data it could
|
|
produce, even if that's too big for the buffer, but it won't copy more
|
|
than requested to userspace. If the buffer pointer is NULL then no copy
|
|
will take place.
|
|
|
|
A process must have view permission on the key for this function to be
|
|
successful.
|
|
|
|
If successful, a string is placed in the buffer in the following format:
|
|
|
|
<type>;<uid>;<gid>;<perm>;<description>
|
|
|
|
Where type and description are strings, uid and gid are decimal, and perm
|
|
is hexadecimal. A NUL character is included at the end of the string if
|
|
the buffer is sufficiently big.
|
|
|
|
This can be parsed with
|
|
|
|
sscanf(buffer, "%[^;];%d;%d;%o;%s", type, &uid, &gid, &mode, desc);
|
|
|
|
|
|
(*) Clear out a keyring:
|
|
|
|
long keyctl(KEYCTL_CLEAR, key_serial_t keyring);
|
|
|
|
This function clears the list of keys attached to a keyring. The calling
|
|
process must have write permission on the keyring, and it must be a
|
|
keyring (or else error ENOTDIR will result).
|
|
|
|
|
|
(*) Link a key into a keyring:
|
|
|
|
long keyctl(KEYCTL_LINK, key_serial_t keyring, key_serial_t key);
|
|
|
|
This function creates a link from the keyring to the key. The process must
|
|
have write permission on the keyring and must have link permission on the
|
|
key.
|
|
|
|
Should the keyring not be a keyring, error ENOTDIR will result; and if the
|
|
keyring is full, error ENFILE will result.
|
|
|
|
The link procedure checks the nesting of the keyrings, returning ELOOP if
|
|
it appears too deep or EDEADLK if the link would introduce a cycle.
|
|
|
|
Any links within the keyring to keys that match the new key in terms of
|
|
type and description will be discarded from the keyring as the new one is
|
|
added.
|
|
|
|
|
|
(*) Unlink a key or keyring from another keyring:
|
|
|
|
long keyctl(KEYCTL_UNLINK, key_serial_t keyring, key_serial_t key);
|
|
|
|
This function looks through the keyring for the first link to the
|
|
specified key, and removes it if found. Subsequent links to that key are
|
|
ignored. The process must have write permission on the keyring.
|
|
|
|
If the keyring is not a keyring, error ENOTDIR will result; and if the key
|
|
is not present, error ENOENT will be the result.
|
|
|
|
|
|
(*) Search a keyring tree for a key:
|
|
|
|
key_serial_t keyctl(KEYCTL_SEARCH, key_serial_t keyring,
|
|
const char *type, const char *description,
|
|
key_serial_t dest_keyring);
|
|
|
|
This searches the keyring tree headed by the specified keyring until a key
|
|
is found that matches the type and description criteria. Each keyring is
|
|
checked for keys before recursion into its children occurs.
|
|
|
|
The process must have search permission on the top level keyring, or else
|
|
error EACCES will result. Only keyrings that the process has search
|
|
permission on will be recursed into, and only keys and keyrings for which
|
|
a process has search permission can be matched. If the specified keyring
|
|
is not a keyring, ENOTDIR will result.
|
|
|
|
If the search succeeds, the function will attempt to link the found key
|
|
into the destination keyring if one is supplied (non-zero ID). All the
|
|
constraints applicable to KEYCTL_LINK apply in this case too.
|
|
|
|
Error ENOKEY, EKEYREVOKED or EKEYEXPIRED will be returned if the search
|
|
fails. On success, the resulting key ID will be returned.
|
|
|
|
|
|
(*) Read the payload data from a key:
|
|
|
|
long keyctl(KEYCTL_READ, key_serial_t keyring, char *buffer,
|
|
size_t buflen);
|
|
|
|
This function attempts to read the payload data from the specified key
|
|
into the buffer. The process must have read permission on the key to
|
|
succeed.
|
|
|
|
The returned data will be processed for presentation by the key type. For
|
|
instance, a keyring will return an array of key_serial_t entries
|
|
representing the IDs of all the keys to which it is subscribed. The user
|
|
defined key type will return its data as is. If a key type does not
|
|
implement this function, error EOPNOTSUPP will result.
|
|
|
|
As much of the data as can be fitted into the buffer will be copied to
|
|
userspace if the buffer pointer is not NULL.
|
|
|
|
On a successful return, the function will always return the amount of data
|
|
available rather than the amount copied.
|
|
|
|
|
|
(*) Instantiate a partially constructed key.
|
|
|
|
long keyctl(KEYCTL_INSTANTIATE, key_serial_t key,
|
|
const void *payload, size_t plen,
|
|
key_serial_t keyring);
|
|
|
|
If the kernel calls back to userspace to complete the instantiation of a
|
|
key, userspace should use this call to supply data for the key before the
|
|
invoked process returns, or else the key will be marked negative
|
|
automatically.
|
|
|
|
The process must have write access on the key to be able to instantiate
|
|
it, and the key must be uninstantiated.
|
|
|
|
If a keyring is specified (non-zero), the key will also be linked into
|
|
that keyring, however all the constraints applying in KEYCTL_LINK apply in
|
|
this case too.
|
|
|
|
The payload and plen arguments describe the payload data as for add_key().
|
|
|
|
|
|
(*) Negatively instantiate a partially constructed key.
|
|
|
|
long keyctl(KEYCTL_NEGATE, key_serial_t key,
|
|
unsigned timeout, key_serial_t keyring);
|
|
|
|
If the kernel calls back to userspace to complete the instantiation of a
|
|
key, userspace should use this call mark the key as negative before the
|
|
invoked process returns if it is unable to fulfil the request.
|
|
|
|
The process must have write access on the key to be able to instantiate
|
|
it, and the key must be uninstantiated.
|
|
|
|
If a keyring is specified (non-zero), the key will also be linked into
|
|
that keyring, however all the constraints applying in KEYCTL_LINK apply in
|
|
this case too.
|
|
|
|
|
|
(*) Set the default request-key destination keyring.
|
|
|
|
long keyctl(KEYCTL_SET_REQKEY_KEYRING, int reqkey_defl);
|
|
|
|
This sets the default keyring to which implicitly requested keys will be
|
|
attached for this thread. reqkey_defl should be one of these constants:
|
|
|
|
CONSTANT VALUE NEW DEFAULT KEYRING
|
|
====================================== ====== =======================
|
|
KEY_REQKEY_DEFL_NO_CHANGE -1 No change
|
|
KEY_REQKEY_DEFL_DEFAULT 0 Default[1]
|
|
KEY_REQKEY_DEFL_THREAD_KEYRING 1 Thread keyring
|
|
KEY_REQKEY_DEFL_PROCESS_KEYRING 2 Process keyring
|
|
KEY_REQKEY_DEFL_SESSION_KEYRING 3 Session keyring
|
|
KEY_REQKEY_DEFL_USER_KEYRING 4 User keyring
|
|
KEY_REQKEY_DEFL_USER_SESSION_KEYRING 5 User session keyring
|
|
KEY_REQKEY_DEFL_GROUP_KEYRING 6 Group keyring
|
|
|
|
The old default will be returned if successful and error EINVAL will be
|
|
returned if reqkey_defl is not one of the above values.
|
|
|
|
The default keyring can be overridden by the keyring indicated to the
|
|
request_key() system call.
|
|
|
|
Note that this setting is inherited across fork/exec.
|
|
|
|
[1] The default is: the thread keyring if there is one, otherwise
|
|
the process keyring if there is one, otherwise the session keyring if
|
|
there is one, otherwise the user default session keyring.
|
|
|
|
|
|
(*) Set the timeout on a key.
|
|
|
|
long keyctl(KEYCTL_SET_TIMEOUT, key_serial_t key, unsigned timeout);
|
|
|
|
This sets or clears the timeout on a key. The timeout can be 0 to clear
|
|
the timeout or a number of seconds to set the expiry time that far into
|
|
the future.
|
|
|
|
The process must have attribute modification access on a key to set its
|
|
timeout. Timeouts may not be set with this function on negative, revoked
|
|
or expired keys.
|
|
|
|
|
|
(*) Assume the authority granted to instantiate a key
|
|
|
|
long keyctl(KEYCTL_ASSUME_AUTHORITY, key_serial_t key);
|
|
|
|
This assumes or divests the authority required to instantiate the
|
|
specified key. Authority can only be assumed if the thread has the
|
|
authorisation key associated with the specified key in its keyrings
|
|
somewhere.
|
|
|
|
Once authority is assumed, searches for keys will also search the
|
|
requester's keyrings using the requester's security label, UID, GID and
|
|
groups.
|
|
|
|
If the requested authority is unavailable, error EPERM will be returned,
|
|
likewise if the authority has been revoked because the target key is
|
|
already instantiated.
|
|
|
|
If the specified key is 0, then any assumed authority will be divested.
|
|
|
|
The assumed authoritative key is inherited across fork and exec.
|
|
|
|
|
|
(*) Get the LSM security context attached to a key.
|
|
|
|
long keyctl(KEYCTL_GET_SECURITY, key_serial_t key, char *buffer,
|
|
size_t buflen)
|
|
|
|
This function returns a string that represents the LSM security context
|
|
attached to a key in the buffer provided.
|
|
|
|
Unless there's an error, it always returns the amount of data it could
|
|
produce, even if that's too big for the buffer, but it won't copy more
|
|
than requested to userspace. If the buffer pointer is NULL then no copy
|
|
will take place.
|
|
|
|
A NUL character is included at the end of the string if the buffer is
|
|
sufficiently big. This is included in the returned count. If no LSM is
|
|
in force then an empty string will be returned.
|
|
|
|
A process must have view permission on the key for this function to be
|
|
successful.
|
|
|
|
|
|
===============
|
|
KERNEL SERVICES
|
|
===============
|
|
|
|
The kernel services for key management are fairly simple to deal with. They can
|
|
be broken down into two areas: keys and key types.
|
|
|
|
Dealing with keys is fairly straightforward. Firstly, the kernel service
|
|
registers its type, then it searches for a key of that type. It should retain
|
|
the key as long as it has need of it, and then it should release it. For a
|
|
filesystem or device file, a search would probably be performed during the open
|
|
call, and the key released upon close. How to deal with conflicting keys due to
|
|
two different users opening the same file is left to the filesystem author to
|
|
solve.
|
|
|
|
To access the key manager, the following header must be #included:
|
|
|
|
<linux/key.h>
|
|
|
|
Specific key types should have a header file under include/keys/ that should be
|
|
used to access that type. For keys of type "user", for example, that would be:
|
|
|
|
<keys/user-type.h>
|
|
|
|
Note that there are two different types of pointers to keys that may be
|
|
encountered:
|
|
|
|
(*) struct key *
|
|
|
|
This simply points to the key structure itself. Key structures will be at
|
|
least four-byte aligned.
|
|
|
|
(*) key_ref_t
|
|
|
|
This is equivalent to a struct key *, but the least significant bit is set
|
|
if the caller "possesses" the key. By "possession" it is meant that the
|
|
calling processes has a searchable link to the key from one of its
|
|
keyrings. There are three functions for dealing with these:
|
|
|
|
key_ref_t make_key_ref(const struct key *key,
|
|
unsigned long possession);
|
|
|
|
struct key *key_ref_to_ptr(const key_ref_t key_ref);
|
|
|
|
unsigned long is_key_possessed(const key_ref_t key_ref);
|
|
|
|
The first function constructs a key reference from a key pointer and
|
|
possession information (which must be 0 or 1 and not any other value).
|
|
|
|
The second function retrieves the key pointer from a reference and the
|
|
third retrieves the possession flag.
|
|
|
|
When accessing a key's payload contents, certain precautions must be taken to
|
|
prevent access vs modification races. See the section "Notes on accessing
|
|
payload contents" for more information.
|
|
|
|
(*) To search for a key, call:
|
|
|
|
struct key *request_key(const struct key_type *type,
|
|
const char *description,
|
|
const char *callout_info);
|
|
|
|
This is used to request a key or keyring with a description that matches
|
|
the description specified according to the key type's match function. This
|
|
permits approximate matching to occur. If callout_string is not NULL, then
|
|
/sbin/request-key will be invoked in an attempt to obtain the key from
|
|
userspace. In that case, callout_string will be passed as an argument to
|
|
the program.
|
|
|
|
Should the function fail error ENOKEY, EKEYEXPIRED or EKEYREVOKED will be
|
|
returned.
|
|
|
|
If successful, the key will have been attached to the default keyring for
|
|
implicitly obtained request-key keys, as set by KEYCTL_SET_REQKEY_KEYRING.
|
|
|
|
See also Documentation/keys-request-key.txt.
|
|
|
|
|
|
(*) To search for a key, passing auxiliary data to the upcaller, call:
|
|
|
|
struct key *request_key_with_auxdata(const struct key_type *type,
|
|
const char *description,
|
|
const void *callout_info,
|
|
size_t callout_len,
|
|
void *aux);
|
|
|
|
This is identical to request_key(), except that the auxiliary data is
|
|
passed to the key_type->request_key() op if it exists, and the callout_info
|
|
is a blob of length callout_len, if given (the length may be 0).
|
|
|
|
|
|
(*) A key can be requested asynchronously by calling one of:
|
|
|
|
struct key *request_key_async(const struct key_type *type,
|
|
const char *description,
|
|
const void *callout_info,
|
|
size_t callout_len);
|
|
|
|
or:
|
|
|
|
struct key *request_key_async_with_auxdata(const struct key_type *type,
|
|
const char *description,
|
|
const char *callout_info,
|
|
size_t callout_len,
|
|
void *aux);
|
|
|
|
which are asynchronous equivalents of request_key() and
|
|
request_key_with_auxdata() respectively.
|
|
|
|
These two functions return with the key potentially still under
|
|
construction. To wait for construction completion, the following should be
|
|
called:
|
|
|
|
int wait_for_key_construction(struct key *key, bool intr);
|
|
|
|
The function will wait for the key to finish being constructed and then
|
|
invokes key_validate() to return an appropriate value to indicate the state
|
|
of the key (0 indicates the key is usable).
|
|
|
|
If intr is true, then the wait can be interrupted by a signal, in which
|
|
case error ERESTARTSYS will be returned.
|
|
|
|
|
|
(*) When it is no longer required, the key should be released using:
|
|
|
|
void key_put(struct key *key);
|
|
|
|
Or:
|
|
|
|
void key_ref_put(key_ref_t key_ref);
|
|
|
|
These can be called from interrupt context. If CONFIG_KEYS is not set then
|
|
the argument will not be parsed.
|
|
|
|
|
|
(*) Extra references can be made to a key by calling the following function:
|
|
|
|
struct key *key_get(struct key *key);
|
|
|
|
These need to be disposed of by calling key_put() when they've been
|
|
finished with. The key pointer passed in will be returned. If the pointer
|
|
is NULL or CONFIG_KEYS is not set then the key will not be dereferenced and
|
|
no increment will take place.
|
|
|
|
|
|
(*) A key's serial number can be obtained by calling:
|
|
|
|
key_serial_t key_serial(struct key *key);
|
|
|
|
If key is NULL or if CONFIG_KEYS is not set then 0 will be returned (in the
|
|
latter case without parsing the argument).
|
|
|
|
|
|
(*) If a keyring was found in the search, this can be further searched by:
|
|
|
|
key_ref_t keyring_search(key_ref_t keyring_ref,
|
|
const struct key_type *type,
|
|
const char *description)
|
|
|
|
This searches the keyring tree specified for a matching key. Error ENOKEY
|
|
is returned upon failure (use IS_ERR/PTR_ERR to determine). If successful,
|
|
the returned key will need to be released.
|
|
|
|
The possession attribute from the keyring reference is used to control
|
|
access through the permissions mask and is propagated to the returned key
|
|
reference pointer if successful.
|
|
|
|
|
|
(*) To check the validity of a key, this function can be called:
|
|
|
|
int validate_key(struct key *key);
|
|
|
|
This checks that the key in question hasn't expired or and hasn't been
|
|
revoked. Should the key be invalid, error EKEYEXPIRED or EKEYREVOKED will
|
|
be returned. If the key is NULL or if CONFIG_KEYS is not set then 0 will be
|
|
returned (in the latter case without parsing the argument).
|
|
|
|
|
|
(*) To register a key type, the following function should be called:
|
|
|
|
int register_key_type(struct key_type *type);
|
|
|
|
This will return error EEXIST if a type of the same name is already
|
|
present.
|
|
|
|
|
|
(*) To unregister a key type, call:
|
|
|
|
void unregister_key_type(struct key_type *type);
|
|
|
|
|
|
Under some circumstances, it may be desirable to deal with a bundle of keys.
|
|
The facility provides access to the keyring type for managing such a bundle:
|
|
|
|
struct key_type key_type_keyring;
|
|
|
|
This can be used with a function such as request_key() to find a specific
|
|
keyring in a process's keyrings. A keyring thus found can then be searched
|
|
with keyring_search(). Note that it is not possible to use request_key() to
|
|
search a specific keyring, so using keyrings in this way is of limited utility.
|
|
|
|
|
|
===================================
|
|
NOTES ON ACCESSING PAYLOAD CONTENTS
|
|
===================================
|
|
|
|
The simplest payload is just a number in key->payload.value. In this case,
|
|
there's no need to indulge in RCU or locking when accessing the payload.
|
|
|
|
More complex payload contents must be allocated and a pointer to them set in
|
|
key->payload.data. One of the following ways must be selected to access the
|
|
data:
|
|
|
|
(1) Unmodifiable key type.
|
|
|
|
If the key type does not have a modify method, then the key's payload can
|
|
be accessed without any form of locking, provided that it's known to be
|
|
instantiated (uninstantiated keys cannot be "found").
|
|
|
|
(2) The key's semaphore.
|
|
|
|
The semaphore could be used to govern access to the payload and to control
|
|
the payload pointer. It must be write-locked for modifications and would
|
|
have to be read-locked for general access. The disadvantage of doing this
|
|
is that the accessor may be required to sleep.
|
|
|
|
(3) RCU.
|
|
|
|
RCU must be used when the semaphore isn't already held; if the semaphore
|
|
is held then the contents can't change under you unexpectedly as the
|
|
semaphore must still be used to serialise modifications to the key. The
|
|
key management code takes care of this for the key type.
|
|
|
|
However, this means using:
|
|
|
|
rcu_read_lock() ... rcu_dereference() ... rcu_read_unlock()
|
|
|
|
to read the pointer, and:
|
|
|
|
rcu_dereference() ... rcu_assign_pointer() ... call_rcu()
|
|
|
|
to set the pointer and dispose of the old contents after a grace period.
|
|
Note that only the key type should ever modify a key's payload.
|
|
|
|
Furthermore, an RCU controlled payload must hold a struct rcu_head for the
|
|
use of call_rcu() and, if the payload is of variable size, the length of
|
|
the payload. key->datalen cannot be relied upon to be consistent with the
|
|
payload just dereferenced if the key's semaphore is not held.
|
|
|
|
|
|
===================
|
|
DEFINING A KEY TYPE
|
|
===================
|
|
|
|
A kernel service may want to define its own key type. For instance, an AFS
|
|
filesystem might want to define a Kerberos 5 ticket key type. To do this, it
|
|
author fills in a key_type struct and registers it with the system.
|
|
|
|
Source files that implement key types should include the following header file:
|
|
|
|
<linux/key-type.h>
|
|
|
|
The structure has a number of fields, some of which are mandatory:
|
|
|
|
(*) const char *name
|
|
|
|
The name of the key type. This is used to translate a key type name
|
|
supplied by userspace into a pointer to the structure.
|
|
|
|
|
|
(*) size_t def_datalen
|
|
|
|
This is optional - it supplies the default payload data length as
|
|
contributed to the quota. If the key type's payload is always or almost
|
|
always the same size, then this is a more efficient way to do things.
|
|
|
|
The data length (and quota) on a particular key can always be changed
|
|
during instantiation or update by calling:
|
|
|
|
int key_payload_reserve(struct key *key, size_t datalen);
|
|
|
|
With the revised data length. Error EDQUOT will be returned if this is not
|
|
viable.
|
|
|
|
|
|
(*) int (*instantiate)(struct key *key, const void *data, size_t datalen);
|
|
|
|
This method is called to attach a payload to a key during construction.
|
|
The payload attached need not bear any relation to the data passed to this
|
|
function.
|
|
|
|
If the amount of data attached to the key differs from the size in
|
|
keytype->def_datalen, then key_payload_reserve() should be called.
|
|
|
|
This method does not have to lock the key in order to attach a payload.
|
|
The fact that KEY_FLAG_INSTANTIATED is not set in key->flags prevents
|
|
anything else from gaining access to the key.
|
|
|
|
It is safe to sleep in this method.
|
|
|
|
|
|
(*) int (*update)(struct key *key, const void *data, size_t datalen);
|
|
|
|
If this type of key can be updated, then this method should be provided.
|
|
It is called to update a key's payload from the blob of data provided.
|
|
|
|
key_payload_reserve() should be called if the data length might change
|
|
before any changes are actually made. Note that if this succeeds, the type
|
|
is committed to changing the key because it's already been altered, so all
|
|
memory allocation must be done first.
|
|
|
|
The key will have its semaphore write-locked before this method is called,
|
|
but this only deters other writers; any changes to the key's payload must
|
|
be made under RCU conditions, and call_rcu() must be used to dispose of
|
|
the old payload.
|
|
|
|
key_payload_reserve() should be called before the changes are made, but
|
|
after all allocations and other potentially failing function calls are
|
|
made.
|
|
|
|
It is safe to sleep in this method.
|
|
|
|
|
|
(*) int (*match)(const struct key *key, const void *desc);
|
|
|
|
This method is called to match a key against a description. It should
|
|
return non-zero if the two match, zero if they don't.
|
|
|
|
This method should not need to lock the key in any way. The type and
|
|
description can be considered invariant, and the payload should not be
|
|
accessed (the key may not yet be instantiated).
|
|
|
|
It is not safe to sleep in this method; the caller may hold spinlocks.
|
|
|
|
|
|
(*) void (*revoke)(struct key *key);
|
|
|
|
This method is optional. It is called to discard part of the payload
|
|
data upon a key being revoked. The caller will have the key semaphore
|
|
write-locked.
|
|
|
|
It is safe to sleep in this method, though care should be taken to avoid
|
|
a deadlock against the key semaphore.
|
|
|
|
|
|
(*) void (*destroy)(struct key *key);
|
|
|
|
This method is optional. It is called to discard the payload data on a key
|
|
when it is being destroyed.
|
|
|
|
This method does not need to lock the key to access the payload; it can
|
|
consider the key as being inaccessible at this time. Note that the key's
|
|
type may have been changed before this function is called.
|
|
|
|
It is not safe to sleep in this method; the caller may hold spinlocks.
|
|
|
|
|
|
(*) void (*describe)(const struct key *key, struct seq_file *p);
|
|
|
|
This method is optional. It is called during /proc/keys reading to
|
|
summarise a key's description and payload in text form.
|
|
|
|
This method will be called with the RCU read lock held. rcu_dereference()
|
|
should be used to read the payload pointer if the payload is to be
|
|
accessed. key->datalen cannot be trusted to stay consistent with the
|
|
contents of the payload.
|
|
|
|
The description will not change, though the key's state may.
|
|
|
|
It is not safe to sleep in this method; the RCU read lock is held by the
|
|
caller.
|
|
|
|
|
|
(*) long (*read)(const struct key *key, char __user *buffer, size_t buflen);
|
|
|
|
This method is optional. It is called by KEYCTL_READ to translate the
|
|
key's payload into something a blob of data for userspace to deal with.
|
|
Ideally, the blob should be in the same format as that passed in to the
|
|
instantiate and update methods.
|
|
|
|
If successful, the blob size that could be produced should be returned
|
|
rather than the size copied.
|
|
|
|
This method will be called with the key's semaphore read-locked. This will
|
|
prevent the key's payload changing. It is not necessary to use RCU locking
|
|
when accessing the key's payload. It is safe to sleep in this method, such
|
|
as might happen when the userspace buffer is accessed.
|
|
|
|
|
|
(*) int (*request_key)(struct key_construction *cons, const char *op,
|
|
void *aux);
|
|
|
|
This method is optional. If provided, request_key() and friends will
|
|
invoke this function rather than upcalling to /sbin/request-key to operate
|
|
upon a key of this type.
|
|
|
|
The aux parameter is as passed to request_key_async_with_auxdata() and
|
|
similar or is NULL otherwise. Also passed are the construction record for
|
|
the key to be operated upon and the operation type (currently only
|
|
"create").
|
|
|
|
This method is permitted to return before the upcall is complete, but the
|
|
following function must be called under all circumstances to complete the
|
|
instantiation process, whether or not it succeeds, whether or not there's
|
|
an error:
|
|
|
|
void complete_request_key(struct key_construction *cons, int error);
|
|
|
|
The error parameter should be 0 on success, -ve on error. The
|
|
construction record is destroyed by this action and the authorisation key
|
|
will be revoked. If an error is indicated, the key under construction
|
|
will be negatively instantiated if it wasn't already instantiated.
|
|
|
|
If this method returns an error, that error will be returned to the
|
|
caller of request_key*(). complete_request_key() must be called prior to
|
|
returning.
|
|
|
|
The key under construction and the authorisation key can be found in the
|
|
key_construction struct pointed to by cons:
|
|
|
|
(*) struct key *key;
|
|
|
|
The key under construction.
|
|
|
|
(*) struct key *authkey;
|
|
|
|
The authorisation key.
|
|
|
|
|
|
============================
|
|
REQUEST-KEY CALLBACK SERVICE
|
|
============================
|
|
|
|
To create a new key, the kernel will attempt to execute the following command
|
|
line:
|
|
|
|
/sbin/request-key create <key> <uid> <gid> \
|
|
<threadring> <processring> <sessionring> <callout_info>
|
|
|
|
<key> is the key being constructed, and the three keyrings are the process
|
|
keyrings from the process that caused the search to be issued. These are
|
|
included for two reasons:
|
|
|
|
(1) There may be an authentication token in one of the keyrings that is
|
|
required to obtain the key, eg: a Kerberos Ticket-Granting Ticket.
|
|
|
|
(2) The new key should probably be cached in one of these rings.
|
|
|
|
This program should set it UID and GID to those specified before attempting to
|
|
access any more keys. It may then look around for a user specific process to
|
|
hand the request off to (perhaps a path held in placed in another key by, for
|
|
example, the KDE desktop manager).
|
|
|
|
The program (or whatever it calls) should finish construction of the key by
|
|
calling KEYCTL_INSTANTIATE, which also permits it to cache the key in one of
|
|
the keyrings (probably the session ring) before returning. Alternatively, the
|
|
key can be marked as negative with KEYCTL_NEGATE; this also permits the key to
|
|
be cached in one of the keyrings.
|
|
|
|
If it returns with the key remaining in the unconstructed state, the key will
|
|
be marked as being negative, it will be added to the session keyring, and an
|
|
error will be returned to the key requestor.
|
|
|
|
Supplementary information may be provided from whoever or whatever invoked this
|
|
service. This will be passed as the <callout_info> parameter. If no such
|
|
information was made available, then "-" will be passed as this parameter
|
|
instead.
|
|
|
|
|
|
Similarly, the kernel may attempt to update an expired or a soon to expire key
|
|
by executing:
|
|
|
|
/sbin/request-key update <key> <uid> <gid> \
|
|
<threadring> <processring> <sessionring>
|
|
|
|
In this case, the program isn't required to actually attach the key to a ring;
|
|
the rings are provided for reference.
|