--- /dev/null
+ ===================
+ 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.
/sbin/request-key will be invoked in an attempt to obtain a key. The
callout_info string will be passed as an argument to the program.
+ See also Documentation/keys-request-key.txt.
+
The keyctl syscall functions are:
(*) Read the payload data from a key:
- key_serial_t keyctl(KEYCTL_READ, key_serial_t keyring, char *buffer,
- size_t buflen);
+ long keyctl(KEYCTL_READ, key_serial_t keyring, char *buffer,
+ size_t buflen);
This function attempts to read the payload data from the specified key
into the buffer. The process must have read permission on the key to
(*) Instantiate a partially constructed key.
- key_serial_t keyctl(KEYCTL_INSTANTIATE, key_serial_t key,
- const void *payload, size_t plen,
- key_serial_t keyring);
+ long keyctl(KEYCTL_INSTANTIATE, key_serial_t key,
+ const void *payload, size_t plen,
+ key_serial_t keyring);
If the kernel calls back to userspace to complete the instantiation of a
key, userspace should use this call to supply data for the key before the
(*) Negatively instantiate a partially constructed key.
- key_serial_t keyctl(KEYCTL_NEGATE, key_serial_t key,
- unsigned timeout, key_serial_t keyring);
+ long keyctl(KEYCTL_NEGATE, key_serial_t key,
+ unsigned timeout, key_serial_t keyring);
If the kernel calls back to userspace to complete the instantiation of a
key, userspace should use this call mark the key as negative before the
If successful, the key will have been attached to the default keyring for
implicitly obtained request-key keys, as set by KEYCTL_SET_REQKEY_KEYRING.
+ See also Documentation/keys-request-key.txt.
+
(*) When it is no longer required, the key should be released using:
projects / GitHub / LineageOS / android_kernel_motorola_exynos9610.git / commitdiff