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.TH "PERSISTENT-KEYRING" 7 2017-03-13 Linux "Linux Programmer's Manual"

.SH NAME

persistent-keyring \- per-user persistent keyring

.SH DESCRIPTION

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

.I _persistent.<UID>

where

.I <UID>

is the user ID of the corresponding user.

.PP

The persistent keyring may not be accessed directly,

even by processes with the appropriate UID.

.\" FIXME The meaning of the preceding sentence isn't clear. What is meant?

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

.BR keyctl_get_persistent (3)

function.

.PP

If a persistent keyring does not exist when it is accessed by the

.BR keyctl_get_persistent (3)

operation, it will be automatically created.

.PP

Each time the

.BR keyctl_get_persistent (3)

operation is performed,

the persistent key's expiration timer is reset to the value in:

.PP

    /proc/sys/kernel/keys/persistent_keyring_expiry

.PP

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

.BR keyctl_get_persistent (3).

.PP

The persistent keyring is not directly searched by

.BR request_key (2);

it is searched only if it is linked into one of the keyrings

that is searched by

.BR request_key (2).

.PP

The persistent keyring is independent of

.BR clone (2),

.BR fork (2),

.BR vfork (2),

.BR execve (2),

and

.BR _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

.BR user-keyring (7)

and the

.BR 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

.BR cron (8).

.PP

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.

.\"

.SS Special operations

The

.I keyutils

library provides the

.BR keyctl_get_persistent (3)

function for manipulating persistent keyrings.

(This function is an interface to the

.BR keyctl (2)

.B 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

.BR CAP_SETUID

capability, the persistent keyring corresponding to some other UID

in the same user namespace.

.SH NOTES

Each user namespace owns a keyring called

.IR .persistent_register

that contains links to all of the persistent keys in that namespace.

(The

.IR .persistent_register

keyring can be seen when reading the contents of the

.IR /proc/keys

file for the UID 0 in the namespace.)

The

.BR keyctl_get_persistent (3)

operation looks for a key with a name of the form

.IR _persistent.<UID>

in that keyring,

creates the key if it does not exist, and links it into the keyring.

.SH SEE ALSO

.ad l

.nh

.BR keyctl (1),

.BR keyctl (3),

.BR keyctl_get_persistent (3),

.BR keyrings (7),

.BR process\-keyring (7),

.BR session\-keyring (7),

.BR thread\-keyring (7),

.BR user\-keyring (7),

.BR user\-session\-keyring (7)

.SH COLOPHON

This page is part of release 4.15 of the Linux

.I 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/.