Documentation/keys-request-key.txt - linux - Git at Google

 			      ===================
 			      KEY REQUEST SERVICE
 			      ===================

 The key request service is part of the key retention service (refer to
 Documentation/keys.txt). This document explains more fully how that the
 requesting algorithm works.

 The process starts by either the kernel requesting a service by calling
 request_key():

 	struct key *request_key(const struct key_type *type,
 				const char *description,
 				const char *callout_string);

 Or by userspace invoking the request_key system call:

 	key_serial_t request_key(const char *type,
 				 const char *description,
 				 const char *callout_info,
 				 key_serial_t dest_keyring);

 The main difference between the two access points is that the in-kernel
 interface does not need to link the key to a keyring to prevent it from being
 immediately destroyed. The kernel interface returns a pointer directly to the
 key, and it's up to the caller to destroy the key.

 The userspace interface links the key to a keyring associated with the process
 to prevent the key from going away, and returns the serial number of the key to
 the caller.


 ===========
 THE PROCESS
 ===========

 A request proceeds in the following manner:

  (1) Process A calls request_key() [the userspace syscall calls the kernel
      interface].

  (2) request_key() searches the process's subscribed keyrings to see if there's
      a suitable key there. If there is, it returns the key. If there isn't, and
      callout_info is not set, an error is returned. Otherwise the process
      proceeds to the next step.

  (3) request_key() sees that A doesn't have the desired key yet, so it creates
      two things:

      (a) An uninstantiated key U of requested type and description.

      (b) An authorisation key V that refers to key U and notes that process A
      	 is the context in which key U should be instantiated and secured, and
      	 from which associated key requests may be satisfied.

  (4) request_key() then forks and executes /sbin/request-key with a new session
      keyring that contains a link to auth key V.

  (5) /sbin/request-key execs an appropriate program to perform the actual
      instantiation.

  (6) The program may want to access another key from A's context (say a
      Kerberos TGT key). It just requests the appropriate key, and the keyring
      search notes that the session keyring has auth key V in its bottom level.

      This will permit it to then search the keyrings of process A with the
      UID, GID, groups and security info of process A as if it was process A,
      and come up with key W.

  (7) The program then does what it must to get the data with which to
      instantiate key U, using key W as a reference (perhaps it contacts a
      Kerberos server using the TGT) and then instantiates key U.

  (8) Upon instantiating key U, auth key V is automatically revoked so that it
      may not be used again.

  (9) The program then exits 0 and request_key() deletes key V and returns key
      U to the caller.

 This also extends further. If key W (step 5 above) didn't exist, key W would be
 created uninstantiated, another auth key (X) would be created [as per step 3]
 and another copy of /sbin/request-key spawned [as per step 4]; but the context
 specified by auth key X will still be process A, as it was in auth key V.

 This is because process A's keyrings can't simply be attached to
 /sbin/request-key at the appropriate places because (a) execve will discard two
 of them, and (b) it requires the same UID/GID/Groups all the way through.


 ======================
 NEGATIVE INSTANTIATION
 ======================

 Rather than instantiating a key, it is possible for the possessor of an
 authorisation key to negatively instantiate a key that's under construction.
 This is a short duration placeholder that causes any attempt at re-requesting
 the key whilst it exists to fail with error ENOKEY.

 This is provided to prevent excessive repeated spawning of /sbin/request-key
 processes for a key that will never be obtainable.

 Should the /sbin/request-key process exit anything other than 0 or die on a
 signal, the key under construction will be automatically negatively
 instantiated for a short amount of time.


 ====================
 THE SEARCH ALGORITHM
 ====================

 A search of any particular keyring proceeds in the following fashion:

  (1) When the key management code searches for a key (keyring_search_aux) it
      firstly calls key_permission(SEARCH) on the keyring it's starting with,
      if this denies permission, it doesn't search further.

  (2) It considers all the non-keyring keys within that keyring and, if any key
      matches the criteria specified, calls key_permission(SEARCH) on it to see
      if the key is allowed to be found. If it is, that key is returned; if
      not, the search continues, and the error code is retained if of higher
      priority than the one currently set.

  (3) It then considers all the keyring-type keys in the keyring it's currently
      searching. It calls key_permission(SEARCH) on each keyring, and if this
      grants permission, it recurses, executing steps (2) and (3) on that
      keyring.

 The process stops immediately a valid key is found with permission granted to
 use it. Any error from a previous match attempt is discarded and the key is
 returned.

 When search_process_keyrings() is invoked, it performs the following searches
 until one succeeds:

  (1) If extant, the process's thread keyring is searched.

  (2) If extant, the process's process keyring is searched.

  (3) The process's session keyring is searched.

  (4) If the process has a request_key() authorisation key in its session
      keyring then:

      (a) If extant, the calling process's thread keyring is searched.

      (b) If extant, the calling process's process keyring is searched.

      (c) The calling process's session keyring is searched.

 The moment one succeeds, all pending errors are discarded and the found key is
 returned.

 Only if all these fail does the whole thing fail with the highest priority
 error. Note that several errors may have come from LSM.

 The error priority is:

 	EKEYREVOKED > EKEYEXPIRED > ENOKEY

 EACCES/EPERM are only returned on a direct search of a specific keyring where
 the basal keyring does not grant Search permission.
	===================
	KEY REQUEST SERVICE
	===================

	The key request service is part of the key retention service (refer to
	Documentation/keys.txt). This document explains more fully how that the
	requesting algorithm works.

	The process starts by either the kernel requesting a service by calling
	request_key():

	struct key request_key(const struct key_type type,
	const char *description,
	const char *callout_string);

	Or by userspace invoking the request_key system call:

	key_serial_t request_key(const char *type,
	const char *description,
	const char *callout_info,
	key_serial_t dest_keyring);

	The main difference between the two access points is that the in-kernel
	interface does not need to link the key to a keyring to prevent it from being
	immediately destroyed. The kernel interface returns a pointer directly to the
	key, and it's up to the caller to destroy the key.

	The userspace interface links the key to a keyring associated with the process
	to prevent the key from going away, and returns the serial number of the key to
	the caller.


	===========
	THE PROCESS
	===========

	A request proceeds in the following manner:

	(1) Process A calls request_key() [the userspace syscall calls the kernel
	interface].

	(2) request_key() searches the process's subscribed keyrings to see if there's
	a suitable key there. If there is, it returns the key. If there isn't, and
	callout_info is not set, an error is returned. Otherwise the process
	proceeds to the next step.

	(3) request_key() sees that A doesn't have the desired key yet, so it creates
	two things:

	(a) An uninstantiated key U of requested type and description.

	(b) An authorisation key V that refers to key U and notes that process A
	is the context in which key U should be instantiated and secured, and
	from which associated key requests may be satisfied.

	(4) request_key() then forks and executes /sbin/request-key with a new session
	keyring that contains a link to auth key V.

	(5) /sbin/request-key execs an appropriate program to perform the actual
	instantiation.

	(6) The program may want to access another key from A's context (say a
	Kerberos TGT key). It just requests the appropriate key, and the keyring
	search notes that the session keyring has auth key V in its bottom level.

	This will permit it to then search the keyrings of process A with the
	UID, GID, groups and security info of process A as if it was process A,
	and come up with key W.

	(7) The program then does what it must to get the data with which to
	instantiate key U, using key W as a reference (perhaps it contacts a
	Kerberos server using the TGT) and then instantiates key U.

	(8) Upon instantiating key U, auth key V is automatically revoked so that it
	may not be used again.

	(9) The program then exits 0 and request_key() deletes key V and returns key
	U to the caller.

	This also extends further. If key W (step 5 above) didn't exist, key W would be
	created uninstantiated, another auth key (X) would be created [as per step 3]
	and another copy of /sbin/request-key spawned [as per step 4]; but the context
	specified by auth key X will still be process A, as it was in auth key V.

	This is because process A's keyrings can't simply be attached to
	/sbin/request-key at the appropriate places because (a) execve will discard two
	of them, and (b) it requires the same UID/GID/Groups all the way through.


	======================
	NEGATIVE INSTANTIATION
	======================

	Rather than instantiating a key, it is possible for the possessor of an
	authorisation key to negatively instantiate a key that's under construction.
	This is a short duration placeholder that causes any attempt at re-requesting
	the key whilst it exists to fail with error ENOKEY.

	This is provided to prevent excessive repeated spawning of /sbin/request-key
	processes for a key that will never be obtainable.

	Should the /sbin/request-key process exit anything other than 0 or die on a
	signal, the key under construction will be automatically negatively
	instantiated for a short amount of time.


	====================
	THE SEARCH ALGORITHM
	====================

	A search of any particular keyring proceeds in the following fashion:

	(1) When the key management code searches for a key (keyring_search_aux) it
	firstly calls key_permission(SEARCH) on the keyring it's starting with,
	if this denies permission, it doesn't search further.

	(2) It considers all the non-keyring keys within that keyring and, if any key
	matches the criteria specified, calls key_permission(SEARCH) on it to see
	if the key is allowed to be found. If it is, that key is returned; if
	not, the search continues, and the error code is retained if of higher
	priority than the one currently set.

	(3) It then considers all the keyring-type keys in the keyring it's currently
	searching. It calls key_permission(SEARCH) on each keyring, and if this
	grants permission, it recurses, executing steps (2) and (3) on that
	keyring.

	The process stops immediately a valid key is found with permission granted to
	use it. Any error from a previous match attempt is discarded and the key is
	returned.

	When search_process_keyrings() is invoked, it performs the following searches
	until one succeeds:

	(1) If extant, the process's thread keyring is searched.

	(2) If extant, the process's process keyring is searched.

	(3) The process's session keyring is searched.

	(4) If the process has a request_key() authorisation key in its session
	keyring then:

	(a) If extant, the calling process's thread keyring is searched.

	(b) If extant, the calling process's process keyring is searched.

	(c) The calling process's session keyring is searched.

	The moment one succeeds, all pending errors are discarded and the found key is
	returned.

	Only if all these fail does the whole thing fail with the highest priority
	error. Note that several errors may have come from LSM.

	The error priority is:

	EKEYREVOKED > EKEYEXPIRED > ENOKEY

	EACCES/EPERM are only returned on a direct search of a specific keyring where
	the basal keyring does not grant Search permission.