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PERSISTENT-KEYRING(7) Linux Programmer's Manual PERSISTENT-KEYRING(7)
NAME
persistent-keyring - per-user persistent keyring
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 beā
tween 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 correspondā
ing 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) funcā
tion.
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 reā
quest_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.
Special operations
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.
NOTES
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.
SEE ALSO
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)
Linux 2020-08-13 PERSISTENT-KEYRING(7)