man7/persistent-keyring.7

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  11 .\"
  12 .TH "PERSISTENT-KEYRING" 7 2017-03-13 Linux "Linux Programmer's Manual"
  13 .SH NAME
  14 persistent-keyring \- per-user persistent keyring
  15 .SH DESCRIPTION
  16 The persistent keyring is a keyring used to anchor keys on behalf of a user.
  17 Each UID the kernel deals with has its own persistent keyring that
  18 is shared between all threads owned by that UID.
  19 The persistent keyring has a name (description) of the form
  20 .I _persistent.<UID>
  21 where
  22 .I <UID>
  23 is the user ID of the corresponding user.
  24 .PP
  25 The persistent keyring may not be accessed directly,
  26 even by processes with the appropriate UID.
  27 .\" FIXME The meaning of the preceding sentence isn't clear. What is meant?
  28 Instead, it must first be linked to one of a process's keyrings,
  29 before that keyring can access the persistent keyring
  30 by virtue of its possessor permits.
  31 This linking is done with the
  32 .BR keyctl_get_persistent (3)
  33 function.
  34 .PP
  35 If a persistent keyring does not exist when it is accessed by the
  36 .BR keyctl_get_persistent (3)
  37 operation, it will be automatically created.
  38 .PP
  39 Each time the
  40 .BR keyctl_get_persistent (3)
  41 operation is performed,
  42 the persistent key's expiration timer is reset to the value in:
  43 .PP
  44     /proc/sys/kernel/keys/persistent_keyring_expiry
  45 .PP
  46 Should the timeout be reached,
  47 the persistent keyring will be removed and
  48 everything it pins can then be garbage collected.
  49 The key will then be re-created on a subsequent call to
  50 .BR keyctl_get_persistent (3).
  51 .PP
  52 The persistent keyring is not directly searched by
  53 .BR request_key (2);
  54 it is searched only if it is linked into one of the keyrings
  55 that is searched by
  56 .BR request_key (2).
  57 .PP
  58 The persistent keyring is independent of
  59 .BR clone (2),
  60 .BR fork (2),
  61 .BR vfork (2),
  62 .BR execve (2),
  63 and
  64 .BR _exit (2).
  65 It persists until its expiration timer triggers,
  66 at which point it is garbage collected.
  67 This allows the persistent keyring to carry keys beyond the life of
  68 the kernel's record of the corresponding UID
  69 (the destruction of which results in the destruction of the
  70 .BR user-keyring (7)
  71 and the
  72 .BR user-session-keyring (7)).
  73 The persistent keyring can thus be used to
  74 hold authentication tokens for processes that run without user interaction,
  75 such as programs started by
  76 .BR cron (8).
  77 .PP
  78 The persistent keyring is used to store UID-specific objects that
  79 themselves have limited lifetimes (e.g., kerberos tokens).
  80 If those tokens cease to be used
  81 (i.e., the persistent keyring is not accessed),
  82 then the timeout of the persistent keyring ensures that
  83 the corresponding objects are automatically discarded.
  84 .\"
  85 .SS Special operations
  86 The
  87 .I keyutils
  88 library provides the
  89 .BR keyctl_get_persistent (3)
  90 function for manipulating persistent keyrings.
  91 (This function is an interface to the
  92 .BR keyctl (2)
  93 .B KEYCTL_GET_PERSISTENT
  94 operation.)
  95 This operation allows the calling thread to get the persistent keyring
  96 corresponding to its own UID or, if the thread has the
  97 .BR CAP_SETUID
  98 capability, the persistent keyring corresponding to some other UID
  99 in the same user namespace.
 100 .SH NOTES
 101 Each user namespace owns a keyring called
 102 .IR .persistent_register
 103 that contains links to all of the persistent keys in that namespace.
 104 (The
 105 .IR .persistent_register
 106 keyring can be seen when reading the contents of the
 107 .IR /proc/keys
 108 file for the UID 0 in the namespace.)
 109 The
 110 .BR keyctl_get_persistent (3)
 111 operation looks for a key with a name of the form
 112 .IR _persistent.<UID>
 113 in that keyring,
 114 creates the key if it does not exist, and links it into the keyring.
 115 .SH SEE ALSO
 116 .ad l
 117 .nh
 118 .BR keyctl (1),
 119 .BR keyctl (3),
 120 .BR keyctl_get_persistent (3),
 121 .BR keyrings (7),
 122 .BR process\-keyring (7),
 123 .BR session\-keyring (7),
 124 .BR thread\-keyring (7),
 125 .BR user\-keyring (7),
 126 .BR user\-session\-keyring (7)