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NAME | DESCRIPTION | NOTES | SEE ALSO | COLOPHON |
PERSISTENT-KEYRING(7) Linux Programmer's Manual 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.
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.
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 4.12 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/.
Linux 2017-03-13 PERSISTENT-KEYRING(7)
Pages that refer to this page: add_key(2), keyctl(2), request_key(2), keyctl_get_persistent(3), keyrings(7), keyutils(7), process-keyring(7), session-keyring(7), thread-keyring(7), user-keyring(7), user-session-keyring(7)