persistent-keyring(7) — Linux manual page

NAME | DESCRIPTION | NOTES | SEE ALSO | COLOPHON

PERSISTENT-KEYRING(7)   Linux Programmer's Manual  PERSISTENT-KEYRING(7)

NAME         top

       persistent-keyring - per-user persistent keyring

DESCRIPTION         top

       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.

NOTES         top

       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         top

       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)

COLOPHON         top

       This page is part of release 5.10 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                          2020-08-13          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)