Documentation/security/keys-request-key.txt

   1                               ===================
   2                               KEY REQUEST SERVICE
   3                               ===================
   4
   5 The key request service is part of the key retention service (refer to
   6 Documentation/security/keys.txt).  This document explains more fully how
   7 the requesting algorithm works.
   8
   9 The process starts by either the kernel requesting a service by calling
  10 request_key*():
  11
  12         struct key *request_key(const struct key_type *type,
  13                                 const char *description,
  14                                 const char *callout_info);
  15
  16 or:
  17
  18         struct key *request_key_with_auxdata(const struct key_type *type,
  19                                              const char *description,
  20                                              const char *callout_info,
  21                                              size_t callout_len,
  22                                              void *aux);
  23
  24 or:
  25
  26         struct key *request_key_async(const struct key_type *type,
  27                                       const char *description,
  28                                       const char *callout_info,
  29                                       size_t callout_len);
  30
  31 or:
  32
  33         struct key *request_key_async_with_auxdata(const struct key_type *type,
  34                                                    const char *description,
  35                                                    const char *callout_info,
  36                                                    size_t callout_len,
  37                                                    void *aux);
  38
  39 Or by userspace invoking the request_key system call:
  40
  41         key_serial_t request_key(const char *type,
  42                                  const char *description,
  43                                  const char *callout_info,
  44                                  key_serial_t dest_keyring);
  45
  46 The main difference between the access points is that the in-kernel interface
  47 does not need to link the key to a keyring to prevent it from being immediately
  48 destroyed.  The kernel interface returns a pointer directly to the key, and
  49 it's up to the caller to destroy the key.
  50
  51 The request_key*_with_auxdata() calls are like the in-kernel request_key*()
  52 calls, except that they permit auxiliary data to be passed to the upcaller (the
  53 default is NULL).  This is only useful for those key types that define their
  54 own upcall mechanism rather than using /sbin/request-key.
  55
  56 The two async in-kernel calls may return keys that are still in the process of
  57 being constructed.  The two non-async ones will wait for construction to
  58 complete first.
  59
  60 The userspace interface links the key to a keyring associated with the process
  61 to prevent the key from going away, and returns the serial number of the key to
  62 the caller.
  63
  64
  65 The following example assumes that the key types involved don't define their
  66 own upcall mechanisms.  If they do, then those should be substituted for the
  67 forking and execution of /sbin/request-key.
  68
  69
  70 ===========
  71 THE PROCESS
  72 ===========
  73
  74 A request proceeds in the following manner:
  75
  76  (1) Process A calls request_key() [the userspace syscall calls the kernel
  77      interface].
  78
  79  (2) request_key() searches the process's subscribed keyrings to see if there's
  80      a suitable key there.  If there is, it returns the key.  If there isn't,
  81      and callout_info is not set, an error is returned.  Otherwise the process
  82      proceeds to the next step.
  83
  84  (3) request_key() sees that A doesn't have the desired key yet, so it creates
  85      two things:
  86
  87      (a) An uninstantiated key U of requested type and description.
  88
  89      (b) An authorisation key V that refers to key U and notes that process A
  90          is the context in which key U should be instantiated and secured, and
  91          from which associated key requests may be satisfied.
  92
  93  (4) request_key() then forks and executes /sbin/request-key with a new session
  94      keyring that contains a link to auth key V.
  95
  96  (5) /sbin/request-key assumes the authority associated with key U.
  97
  98  (6) /sbin/request-key execs an appropriate program to perform the actual
  99      instantiation.
 100
 101  (7) The program may want to access another key from A's context (say a
 102      Kerberos TGT key).  It just requests the appropriate key, and the keyring
 103      search notes that the session keyring has auth key V in its bottom level.
 104
 105      This will permit it to then search the keyrings of process A with the
 106      UID, GID, groups and security info of process A as if it was process A,
 107      and come up with key W.
 108
 109  (8) The program then does what it must to get the data with which to
 110      instantiate key U, using key W as a reference (perhaps it contacts a
 111      Kerberos server using the TGT) and then instantiates key U.
 112
 113  (9) Upon instantiating key U, auth key V is automatically revoked so that it
 114      may not be used again.
 115
 116 (10) The program then exits 0 and request_key() deletes key V and returns key
 117      U to the caller.
 118
 119 This also extends further.  If key W (step 7 above) didn't exist, key W would
 120 be created uninstantiated, another auth key (X) would be created (as per step
 121 3) and another copy of /sbin/request-key spawned (as per step 4); but the
 122 context specified by auth key X will still be process A, as it was in auth key
 123 V.
 124
 125 This is because process A's keyrings can't simply be attached to
 126 /sbin/request-key at the appropriate places because (a) execve will discard two
 127 of them, and (b) it requires the same UID/GID/Groups all the way through.
 128
 129
 130 ====================================
 131 NEGATIVE INSTANTIATION AND REJECTION
 132 ====================================
 133
 134 Rather than instantiating a key, it is possible for the possessor of an
 135 authorisation key to negatively instantiate a key that's under construction.
 136 This is a short duration placeholder that causes any attempt at re-requesting
 137 the key whilst it exists to fail with error ENOKEY if negated or the specified
 138 error if rejected.
 139
 140 This is provided to prevent excessive repeated spawning of /sbin/request-key
 141 processes for a key that will never be obtainable.
 142
 143 Should the /sbin/request-key process exit anything other than 0 or die on a
 144 signal, the key under construction will be automatically negatively
 145 instantiated for a short amount of time.
 146
 147
 148 ====================
 149 THE SEARCH ALGORITHM
 150 ====================
 151
 152 A search of any particular keyring proceeds in the following fashion:
 153
 154  (1) When the key management code searches for a key (keyring_search_aux) it
 155      firstly calls key_permission(SEARCH) on the keyring it's starting with,
 156      if this denies permission, it doesn't search further.
 157
 158  (2) It considers all the non-keyring keys within that keyring and, if any key
 159      matches the criteria specified, calls key_permission(SEARCH) on it to see
 160      if the key is allowed to be found.  If it is, that key is returned; if
 161      not, the search continues, and the error code is retained if of higher
 162      priority than the one currently set.
 163
 164  (3) It then considers all the keyring-type keys in the keyring it's currently
 165      searching.  It calls key_permission(SEARCH) on each keyring, and if this
 166      grants permission, it recurses, executing steps (2) and (3) on that
 167      keyring.
 168
 169 The process stops immediately a valid key is found with permission granted to
 170 use it.  Any error from a previous match attempt is discarded and the key is
 171 returned.
 172
 173 When search_process_keyrings() is invoked, it performs the following searches
 174 until one succeeds:
 175
 176  (1) If extant, the process's thread keyring is searched.
 177
 178  (2) If extant, the process's process keyring is searched.
 179
 180  (3) The process's session keyring is searched.
 181
 182  (4) If the process has assumed the authority associated with a request_key()
 183      authorisation key then:
 184
 185      (a) If extant, the calling process's thread keyring is searched.
 186
 187      (b) If extant, the calling process's process keyring is searched.
 188
 189      (c) The calling process's session keyring is searched.
 190
 191 The moment one succeeds, all pending errors are discarded and the found key is
 192 returned.
 193
 194 Only if all these fail does the whole thing fail with the highest priority
 195 error.  Note that several errors may have come from LSM.
 196
 197 The error priority is:
 198
 199         EKEYREVOKED > EKEYEXPIRED > ENOKEY
 200
 201 EACCES/EPERM are only returned on a direct search of a specific keyring where
 202 the basal keyring does not grant Search permission.