keyring(7) - Linux manual page (original) (raw)

persistent-keyring(7) — Linux manual page

persist...keyring(7) Miscellaneous Information Manual_persist...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)](../man3/keyctl%5Fget%5Fpersistent.3.html) function.

   If a persistent keyring does not exist when it is accessed by the
   [keyctl_get_persistent(3)](../man3/keyctl%5Fget%5Fpersistent.3.html) operation, it will be automatically
   created.

   Each time the [keyctl_get_persistent(3)](../man3/keyctl%5Fget%5Fpersistent.3.html) operation is performed, the
   persistent 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)](../man3/keyctl%5Fget%5Fpersistent.3.html).

   The persistent keyring is not directly searched by [request_key(2)](../man2/request%5Fkey.2.html);
   it is searched only if it is linked into one of the keyrings that
   is searched by [request_key(2)](../man2/request%5Fkey.2.html).

   The persistent keyring is independent of [clone(2)](../man2/clone.2.html), [fork(2)](../man2/fork.2.html),
   [vfork(2)](../man2/vfork.2.html), [execve(2)](../man2/execve.2.html), and [_exit(2)](../man2/%5Fexit.2.html).  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)](../man7/user-keyring.7.html) and the
   [user-session-keyring(7)](../man7/user-session-keyring.7.html)).  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)](../man8/cron.8.html).

   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 _.persistentregister_
   that contains links to all of the persistent keys in that
   namespace.  (The _.persistentregister_ 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)](../man3/keyctl%5Fget%5Fpersistent.3.html) 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.

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Linux man-pages 6.10 2024-05-02 persist...keyring(7)