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- ===================
- KEY REQUEST SERVICE
- ===================
-
-The key request service is part of the key retention service (refer to
-Documentation/security/keys.txt). This document explains more fully how
-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_info);
-
-or:
-
- struct key *request_key_with_auxdata(const struct key_type *type,
- const char *description,
- const char *callout_info,
- size_t callout_len,
- void *aux);
-
-or:
-
- struct key *request_key_async(const struct key_type *type,
- const char *description,
- const char *callout_info,
- size_t callout_len);
-
-or:
-
- struct key *request_key_async_with_auxdata(const struct key_type *type,
- const char *description,
- const char *callout_info,
- size_t callout_len,
- void *aux);
-
-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 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 request_key*_with_auxdata() calls are like the in-kernel request_key*()
-calls, except that they permit auxiliary data to be passed to the upcaller (the
-default is NULL). This is only useful for those key types that define their
-own upcall mechanism rather than using /sbin/request-key.
-
-The two async in-kernel calls may return keys that are still in the process of
-being constructed. The two non-async ones will wait for construction to
-complete first.
-
-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 following example assumes that the key types involved don't define their
-own upcall mechanisms. If they do, then those should be substituted for the
-forking and execution of /sbin/request-key.
-
-
-===========
-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 assumes the authority associated with key U.
-
- (6) /sbin/request-key execs an appropriate program to perform the actual
- instantiation.
-
- (7) 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.
-
- (8) 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.
-
- (9) Upon instantiating key U, auth key V is automatically revoked so that it
- may not be used again.
-
-(10) 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 7 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 AND REJECTION
-====================================
-
-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 if negated or the specified
-error if rejected.
-
-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 assumed the authority associated with a request_key()
- authorisation key 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.