persistent-keyring(7) Miscellaneous Information 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 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 per-
sistent keyring'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 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 trig-
gers, at which point it is garbage collected. This allows the persis-
tent keyring to carry keys beyond the life of the kernel's record of
the corresponding UID (the destruction of which results in the destruc-
tion of the user-keyring(7) and the user-session-keyring(7)). The per-
sistent 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 them-
selves 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 man-pages 6.7 2023-10-31 persistent-keyring(7)
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