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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 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 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  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 pro-
     cesses 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.