199 lines
7.3 KiB
Plaintext
199 lines
7.3 KiB
Plaintext
===================
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KEY REQUEST SERVICE
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===================
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The key request service is part of the key retention service (refer to
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Documentation/keys.txt). This document explains more fully how the requesting
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algorithm works.
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The process starts by either the kernel requesting a service by calling
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request_key*():
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struct key *request_key(const struct key_type *type,
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const char *description,
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const char *callout_string);
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or:
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struct key *request_key_with_auxdata(const struct key_type *type,
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const char *description,
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const char *callout_string,
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void *aux);
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or:
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struct key *request_key_async(const struct key_type *type,
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const char *description,
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const char *callout_string);
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or:
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struct key *request_key_async_with_auxdata(const struct key_type *type,
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const char *description,
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const char *callout_string,
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void *aux);
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Or by userspace invoking the request_key system call:
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key_serial_t request_key(const char *type,
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const char *description,
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const char *callout_info,
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key_serial_t dest_keyring);
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The main difference between the access points is that the in-kernel interface
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does not need to link the key to a keyring to prevent it from being immediately
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destroyed. The kernel interface returns a pointer directly to the key, and
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it's up to the caller to destroy the key.
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The request_key*_with_auxdata() calls are like the in-kernel request_key*()
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calls, except that they permit auxiliary data to be passed to the upcaller (the
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default is NULL). This is only useful for those key types that define their
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own upcall mechanism rather than using /sbin/request-key.
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The two async in-kernel calls may return keys that are still in the process of
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being constructed. The two non-async ones will wait for construction to
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complete first.
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The userspace interface links the key to a keyring associated with the process
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to prevent the key from going away, and returns the serial number of the key to
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the caller.
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The following example assumes that the key types involved don't define their
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own upcall mechanisms. If they do, then those should be substituted for the
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forking and execution of /sbin/request-key.
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===========
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THE PROCESS
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===========
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A request proceeds in the following manner:
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(1) Process A calls request_key() [the userspace syscall calls the kernel
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interface].
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(2) request_key() searches the process's subscribed keyrings to see if there's
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a suitable key there. If there is, it returns the key. If there isn't,
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and callout_info is not set, an error is returned. Otherwise the process
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proceeds to the next step.
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(3) request_key() sees that A doesn't have the desired key yet, so it creates
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two things:
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(a) An uninstantiated key U of requested type and description.
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(b) An authorisation key V that refers to key U and notes that process A
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is the context in which key U should be instantiated and secured, and
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from which associated key requests may be satisfied.
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(4) request_key() then forks and executes /sbin/request-key with a new session
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keyring that contains a link to auth key V.
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(5) /sbin/request-key assumes the authority associated with key U.
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(6) /sbin/request-key execs an appropriate program to perform the actual
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instantiation.
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(7) The program may want to access another key from A's context (say a
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Kerberos TGT key). It just requests the appropriate key, and the keyring
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search notes that the session keyring has auth key V in its bottom level.
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This will permit it to then search the keyrings of process A with the
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UID, GID, groups and security info of process A as if it was process A,
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and come up with key W.
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(8) The program then does what it must to get the data with which to
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instantiate key U, using key W as a reference (perhaps it contacts a
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Kerberos server using the TGT) and then instantiates key U.
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(9) Upon instantiating key U, auth key V is automatically revoked so that it
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may not be used again.
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(10) The program then exits 0 and request_key() deletes key V and returns key
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U to the caller.
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This also extends further. If key W (step 7 above) didn't exist, key W would
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be created uninstantiated, another auth key (X) would be created (as per step
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3) and another copy of /sbin/request-key spawned (as per step 4); but the
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context specified by auth key X will still be process A, as it was in auth key
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V.
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This is because process A's keyrings can't simply be attached to
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/sbin/request-key at the appropriate places because (a) execve will discard two
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of them, and (b) it requires the same UID/GID/Groups all the way through.
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======================
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NEGATIVE INSTANTIATION
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======================
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Rather than instantiating a key, it is possible for the possessor of an
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authorisation key to negatively instantiate a key that's under construction.
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This is a short duration placeholder that causes any attempt at re-requesting
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the key whilst it exists to fail with error ENOKEY.
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This is provided to prevent excessive repeated spawning of /sbin/request-key
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processes for a key that will never be obtainable.
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Should the /sbin/request-key process exit anything other than 0 or die on a
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signal, the key under construction will be automatically negatively
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instantiated for a short amount of time.
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====================
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THE SEARCH ALGORITHM
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====================
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A search of any particular keyring proceeds in the following fashion:
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(1) When the key management code searches for a key (keyring_search_aux) it
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firstly calls key_permission(SEARCH) on the keyring it's starting with,
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if this denies permission, it doesn't search further.
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(2) It considers all the non-keyring keys within that keyring and, if any key
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matches the criteria specified, calls key_permission(SEARCH) on it to see
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if the key is allowed to be found. If it is, that key is returned; if
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not, the search continues, and the error code is retained if of higher
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priority than the one currently set.
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(3) It then considers all the keyring-type keys in the keyring it's currently
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searching. It calls key_permission(SEARCH) on each keyring, and if this
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grants permission, it recurses, executing steps (2) and (3) on that
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keyring.
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The process stops immediately a valid key is found with permission granted to
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use it. Any error from a previous match attempt is discarded and the key is
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returned.
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When search_process_keyrings() is invoked, it performs the following searches
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until one succeeds:
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(1) If extant, the process's thread keyring is searched.
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(2) If extant, the process's process keyring is searched.
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(3) The process's session keyring is searched.
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(4) If the process has assumed the authority associated with a request_key()
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authorisation key then:
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(a) If extant, the calling process's thread keyring is searched.
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(b) If extant, the calling process's process keyring is searched.
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(c) The calling process's session keyring is searched.
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The moment one succeeds, all pending errors are discarded and the found key is
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returned.
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Only if all these fail does the whole thing fail with the highest priority
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error. Note that several errors may have come from LSM.
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The error priority is:
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EKEYREVOKED > EKEYEXPIRED > ENOKEY
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EACCES/EPERM are only returned on a direct search of a specific keyring where
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the basal keyring does not grant Search permission.
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