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persistent-keyring.7: Various clean-ups and additions
Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>
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Michael Kerrisk
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@ -16,49 +16,97 @@ persistent-keyring \- per-user persistent keyring
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The persistent keyring is a keyring used to anchor keys on behalf of a user.
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Each UID the kernel deals with has its own persistent keyring that
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is shared between all threads owned by that UID.
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.P
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The persistent keyring is created on demand when a thread requests it.
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The keyring's expiration timer is reset every time it is accessed
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to the value in:
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.IP
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/proc/sys/kernel/keys/persistent_keyring_expiry
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.P
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The persistent keyring is not searched by
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.BR request_key (2)
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unless it is
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referred to by a keyring that is.
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.P
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The persistent keyring may not be accessed directly, even by processes with
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the appropriate UID.
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Instead it must be linked to one of a process's keyrings
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first before that keyring can access it by virtue of its possessor permits.
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This is done with
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The persistent keyring may not be accessed directly,
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even by processes with the appropriate UID.
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.\" FIXME The meaning of the preceding sentence isn't clear. What is meant?
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Instead, it must first be linked to one of a process's keyrings,
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before that keyring can access the persistent keyring
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by virtue of its possessor permits.
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This linking is done with the
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.BR keyctl_get_persistent (3)
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function.
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If a persistent keyring does not exist when it is accessed by the
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.BR keyctl_get_persistent (3)
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operation, it will be automatically created.
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Each time the
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.BR keyctl_get_persistent (3)
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operation is performed,
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the persistent key's expiration timer is reset to the value in:
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/proc/sys/kernel/keys/persistent_keyring_expiry
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Should the timeout be reached,
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the persistent keyring will be removed and
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everything it pins can then be garbage collected.
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The key will then be re-created on a subsequent call to
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.BR keyctl_get_persistent (3).
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.P
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Persistent keyrings are independent of
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The persistent keyring is not directly searched by
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.BR request_key (2);
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it is searched only if it is linked into one of the keyrings
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that is searched by
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.BR request_key (2).
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The persistent keyring is independent of
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.BR clone (2),
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.BR fork (2),
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.BR vfork (2),
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.BR execve (2),
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and
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.BR _exit (2).
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They persist until their expiration timers trigger - at which point
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they are garbage collected.
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This allows them to carry keys beyond the life of
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the kernel's record of the corresponding UID (the destruction of which results
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in the destruction of the user and user session keyrings).
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.P
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If a persistent keyring does not exist when it is accessed, it will be
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created.
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It persists until its expiration timer triggers,
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at which point it is garbage collected.
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This allows the persistent keyring to carry keys beyond the life of
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the kernel's record of the corresponding UID
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(the destruction of which results in the destruction of the
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.BR user-keyring (7)
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and the
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.BR user-session-keyring (7)).
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The persistent keyring can thus be used to
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hold authentication tokens for processes that run without user interaction,
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such as programs started by
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.BR cron (8).
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The persistent keyring is used to store UID-specific objects that
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themselves have limited lifetimes (e.g., kerberos tokens).
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If those tokens cease to be used
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(i.e., the persistent keyring is not accessed),
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then the timeout of the persistent keyring ensures that
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the corresponding objects are automatically discarded.
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.\"
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.SS Special operations
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The keyutils library provides a special operation for manipulating persistent
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keyrings:
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The
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.I keyutils
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library provides the
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.BR keyctl_get_persistent (3)
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This operation allows the caller to get the persistent keyring corresponding
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to their own UID or, if they have
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.BR CAP_SETUID ,
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the persistent keyring
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corresponding to some other UID in the same user namespace.
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function for manipulating persistent keyrings.
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(This function is an interface to the
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.BR keyctl (2)
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.B KEYCTL_GET_PERSISTENT
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operation.)
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This operation allows the calling thread to get the persistent keyring
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corresponding to its own UID or, if the thread has the
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.BR CAP_SETUID
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capability, the persistent keyring corresponding to some other UID
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in the same user namespace.
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.SH NOTES
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Each user namespace owns a keyring called
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.IR .persistent_register
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that contains links to all of the persistent keys in that namespace.
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(The
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.IR .persistent_register
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keyring can be seen when reading the contents of the
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.IR /proc/keys
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file for the UID 0 in the namespace.)
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The
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.BR keyctl_get_persistent (3)
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operation looks for a key with a name of the form
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.IR _persistent.<UID>
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in that keyring,
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creates the key if it does not exist, and links it into the keyring.
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.SH SEE ALSO
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.ad l
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.nh
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