persistent-keyring - per-user persistent keyring
The persistent keyring is a keyring used to anchor keys on behalf of
a user. Each UID the kernel deals with has its own persistent keyring
that is shared between all threads owned by that UID. The persistent
keyring has a name (description) of the form
_persistent.<UID> where <UID> is the user
ID of the corresponding user.
The persistent keyring may not be accessed directly, even by processes with the appropriate UID. Instead, it must first be linked to one of a process's keyrings, before that keyring can access the persistent keyring by virtue of its possessor permits. This linking is done with the keyctl_get_persistent(3) function.
If a persistent keyring does not exist when it is accessed by the keyctl_get_persistent(3) operation, it will be automatically created.
Each time the keyctl_get_persistent(3) operation is performed, the persistent keyring's expiration timer is reset to the value in:
/proc/sys/kernel/keys/persistent_keyring_expiryShould the timeout be reached, the persistent keyring will be removed and everything it pins can then be garbage collected. The keyring will then be re-created on a subsequent call to keyctl_get_persistent(3).
The persistent keyring is not directly searched by request_key(2); it is searched only if it is linked into one of the keyrings that is searched by request_key(2).
The persistent keyring is independent of clone(2), fork(2), vfork(2), execve(2), and _exit(2). It persists until its expiration timer triggers, at which point it is garbage collected. This allows the persistent keyring to carry keys beyond the life of the kernel's record of the corresponding UID (the destruction of which results in the destruction of the user-keyring(7) and the user-session-keyring(7)). The persistent keyring can thus be used to hold authentication tokens for processes that run without user interaction, such as programs started by cron(8).
The persistent keyring is used to store UID-specific objects that themselves have limited lifetimes (e.g., kerberos tokens). If those tokens cease to be used (i.e., the persistent keyring is not accessed), then the timeout of the persistent keyring ensures that the corresponding objects are automatically discarded.
The keyutils library provides the
keyctl_get_persistent(3) function for manipulating
persistent keyrings. (This function is an interface to the
keyctl(2) KEYCTL_GET_PERSISTENT
operation.) This operation allows the calling thread to get the
persistent keyring corresponding to its own UID or, if the thread has
the CAP_SETUID capability, the persistent keyring
corresponding to some other UID in the same user namespace.
Each user namespace owns a keyring called
.persistent_register that contains links to all of the
persistent keys in that namespace. (The .persistent_register
keyring can be seen when reading the contents of the /proc/keys
file for the UID 0 in the namespace.) The
keyctl_get_persistent(3) operation looks for a key with
a name of the form _persistent.UID in that keyring, creates the
key if it does not exist, and links it into the keyring.
keyctl(1), keyctl(3), keyctl_get_persistent(3), keyrings(7), process-keyring(7), session-keyring(7), thread-keyring(7), user-keyring(7), user-session-keyring(7)