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b5f545c880
Make it possible for a running process (such as gssapid) to be able to
instantiate a key, as was requested by Trond Myklebust for NFS4.
The patch makes the following changes:
(1) A new, optional key type method has been added. This permits a key type
to intercept requests at the point /sbin/request-key is about to be
spawned and do something else with them - passing them over the
rpc_pipefs files or netlink sockets for instance.
The uninstantiated key, the authorisation key and the intended operation
name are passed to the method.
(2) The callout_info is no longer passed as an argument to /sbin/request-key
to prevent unauthorised viewing of this data using ps or by looking in
/proc/pid/cmdline.
This means that the old /sbin/request-key program will not work with the
patched kernel as it will expect to see an extra argument that is no
longer there.
A revised keyutils package will be made available tomorrow.
(3) The callout_info is now attached to the authorisation key. Reading this
key will retrieve the information.
(4) A new field has been added to the task_struct. This holds the
authorisation key currently active for a thread. Searches now look here
for the caller's set of keys rather than looking for an auth key in the
lowest level of the session keyring.
This permits a thread to be servicing multiple requests at once and to
switch between them. Note that this is per-thread, not per-process, and
so is usable in multithreaded programs.
The setting of this field is inherited across fork and exec.
(5) A new keyctl function (KEYCTL_ASSUME_AUTHORITY) has been added that
permits a thread to assume the authority to deal with an uninstantiated
key. Assumption is only permitted if the authorisation key associated
with the uninstantiated key is somewhere in the thread's keyrings.
This function can also clear the assumption.
(6) A new magic key specifier has been added to refer to the currently
assumed authorisation key (KEY_SPEC_REQKEY_AUTH_KEY).
(7) Instantiation will only proceed if the appropriate authorisation key is
assumed first. The assumed authorisation key is discarded if
instantiation is successful.
(8) key_validate() is moved from the file of request_key functions to the
file of permissions functions.
(9) The documentation is updated.
From: <Valdis.Kletnieks@vt.edu>
Build fix.
Signed-off-by: David Howells <dhowells@redhat.com>
Cc: Trond Myklebust <trond.myklebust@fys.uio.no>
Cc: Alexander Zangerl <az@bond.edu.au>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>
164 lines
6.2 KiB
Plaintext
164 lines
6.2 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 that the
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requesting 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 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 two access points is that the in-kernel
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interface does not need to link the key to a keyring to prevent it from being
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immediately destroyed. The kernel interface returns a pointer directly to the
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key, and it's up to the caller to destroy the key.
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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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===========
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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, and
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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 be
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created uninstantiated, another auth key (X) would be created (as per step 3)
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and another copy of /sbin/request-key spawned (as per step 4); but the context
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specified by auth key X will still be process A, as it was in auth key 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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