Manpages - persistent-keyring.7

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 key’s expiration timer is reset to the value in:

/proc/sys/kernel/keys/persistent_keyring_expiry

Should the timeout be reached, the persistent keyring will be removed and everything it pins can then be garbage collected. The key 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.

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)

This page is part of release 5.13 of the Linux man-pages project. A description of the project, information about reporting bugs, and the latest version of this page, can be found at https://www.kernel.org/doc/man-pages/.