13db38ed7SKees Cook=================== 23db38ed7SKees CookKey Request Service 33db38ed7SKees Cook=================== 43db38ed7SKees Cook 53db38ed7SKees CookThe key request service is part of the key retention service (refer to 6c7f66400STom SaegerDocumentation/security/keys/core.rst). This document explains more fully how 73db38ed7SKees Cookthe requesting algorithm works. 83db38ed7SKees Cook 93db38ed7SKees CookThe process starts by either the kernel requesting a service by calling 103db38ed7SKees Cook``request_key*()``:: 113db38ed7SKees Cook 123db38ed7SKees Cook struct key *request_key(const struct key_type *type, 133db38ed7SKees Cook const char *description, 143db38ed7SKees Cook const char *callout_info); 153db38ed7SKees Cook 163db38ed7SKees Cookor:: 173db38ed7SKees Cook 183db38ed7SKees Cook struct key *request_key_with_auxdata(const struct key_type *type, 193db38ed7SKees Cook const char *description, 203db38ed7SKees Cook const char *callout_info, 213db38ed7SKees Cook size_t callout_len, 223db38ed7SKees Cook void *aux); 233db38ed7SKees Cook 243db38ed7SKees Cookor:: 253db38ed7SKees Cook 263db38ed7SKees Cook struct key *request_key_async(const struct key_type *type, 273db38ed7SKees Cook const char *description, 283db38ed7SKees Cook const char *callout_info, 293db38ed7SKees Cook size_t callout_len); 303db38ed7SKees Cook 313db38ed7SKees Cookor:: 323db38ed7SKees Cook 333db38ed7SKees Cook struct key *request_key_async_with_auxdata(const struct key_type *type, 343db38ed7SKees Cook const char *description, 353db38ed7SKees Cook const char *callout_info, 363db38ed7SKees Cook size_t callout_len, 373db38ed7SKees Cook void *aux); 383db38ed7SKees Cook 393db38ed7SKees CookOr by userspace invoking the request_key system call:: 403db38ed7SKees Cook 413db38ed7SKees Cook key_serial_t request_key(const char *type, 423db38ed7SKees Cook const char *description, 433db38ed7SKees Cook const char *callout_info, 443db38ed7SKees Cook key_serial_t dest_keyring); 453db38ed7SKees Cook 463db38ed7SKees CookThe main difference between the access points is that the in-kernel interface 473db38ed7SKees Cookdoes not need to link the key to a keyring to prevent it from being immediately 483db38ed7SKees Cookdestroyed. The kernel interface returns a pointer directly to the key, and 493db38ed7SKees Cookit's up to the caller to destroy the key. 503db38ed7SKees Cook 513db38ed7SKees CookThe request_key*_with_auxdata() calls are like the in-kernel request_key*() 523db38ed7SKees Cookcalls, except that they permit auxiliary data to be passed to the upcaller (the 533db38ed7SKees Cookdefault is NULL). This is only useful for those key types that define their 543db38ed7SKees Cookown upcall mechanism rather than using /sbin/request-key. 553db38ed7SKees Cook 563db38ed7SKees CookThe two async in-kernel calls may return keys that are still in the process of 573db38ed7SKees Cookbeing constructed. The two non-async ones will wait for construction to 583db38ed7SKees Cookcomplete first. 593db38ed7SKees Cook 603db38ed7SKees CookThe userspace interface links the key to a keyring associated with the process 613db38ed7SKees Cookto prevent the key from going away, and returns the serial number of the key to 623db38ed7SKees Cookthe caller. 633db38ed7SKees Cook 643db38ed7SKees Cook 653db38ed7SKees CookThe following example assumes that the key types involved don't define their 663db38ed7SKees Cookown upcall mechanisms. If they do, then those should be substituted for the 673db38ed7SKees Cookforking and execution of /sbin/request-key. 683db38ed7SKees Cook 693db38ed7SKees Cook 703db38ed7SKees CookThe Process 713db38ed7SKees Cook=========== 723db38ed7SKees Cook 733db38ed7SKees CookA request proceeds in the following manner: 743db38ed7SKees Cook 753db38ed7SKees Cook 1) Process A calls request_key() [the userspace syscall calls the kernel 763db38ed7SKees Cook interface]. 773db38ed7SKees Cook 783db38ed7SKees Cook 2) request_key() searches the process's subscribed keyrings to see if there's 793db38ed7SKees Cook a suitable key there. If there is, it returns the key. If there isn't, 803db38ed7SKees Cook and callout_info is not set, an error is returned. Otherwise the process 813db38ed7SKees Cook proceeds to the next step. 823db38ed7SKees Cook 833db38ed7SKees Cook 3) request_key() sees that A doesn't have the desired key yet, so it creates 843db38ed7SKees Cook two things: 853db38ed7SKees Cook 863db38ed7SKees Cook a) An uninstantiated key U of requested type and description. 873db38ed7SKees Cook 883db38ed7SKees Cook b) An authorisation key V that refers to key U and notes that process A 893db38ed7SKees Cook is the context in which key U should be instantiated and secured, and 903db38ed7SKees Cook from which associated key requests may be satisfied. 913db38ed7SKees Cook 923db38ed7SKees Cook 4) request_key() then forks and executes /sbin/request-key with a new session 933db38ed7SKees Cook keyring that contains a link to auth key V. 943db38ed7SKees Cook 953db38ed7SKees Cook 5) /sbin/request-key assumes the authority associated with key U. 963db38ed7SKees Cook 973db38ed7SKees Cook 6) /sbin/request-key execs an appropriate program to perform the actual 983db38ed7SKees Cook instantiation. 993db38ed7SKees Cook 1003db38ed7SKees Cook 7) The program may want to access another key from A's context (say a 1013db38ed7SKees Cook Kerberos TGT key). It just requests the appropriate key, and the keyring 1023db38ed7SKees Cook search notes that the session keyring has auth key V in its bottom level. 1033db38ed7SKees Cook 1043db38ed7SKees Cook This will permit it to then search the keyrings of process A with the 1053db38ed7SKees Cook UID, GID, groups and security info of process A as if it was process A, 1063db38ed7SKees Cook and come up with key W. 1073db38ed7SKees Cook 1085ea787a7SJonathan Corbet 8) The program then does what it must to get the data with which to 1093db38ed7SKees Cook instantiate key U, using key W as a reference (perhaps it contacts a 1103db38ed7SKees Cook Kerberos server using the TGT) and then instantiates key U. 1113db38ed7SKees Cook 1123db38ed7SKees Cook 9) Upon instantiating key U, auth key V is automatically revoked so that it 1133db38ed7SKees Cook may not be used again. 1143db38ed7SKees Cook 1153db38ed7SKees Cook 10) The program then exits 0 and request_key() deletes key V and returns key 1163db38ed7SKees Cook U to the caller. 1173db38ed7SKees Cook 1183db38ed7SKees CookThis also extends further. If key W (step 7 above) didn't exist, key W would 1193db38ed7SKees Cookbe created uninstantiated, another auth key (X) would be created (as per step 1203db38ed7SKees Cook3) and another copy of /sbin/request-key spawned (as per step 4); but the 1213db38ed7SKees Cookcontext specified by auth key X will still be process A, as it was in auth key 1223db38ed7SKees CookV. 1233db38ed7SKees Cook 1243db38ed7SKees CookThis is because process A's keyrings can't simply be attached to 1253db38ed7SKees Cook/sbin/request-key at the appropriate places because (a) execve will discard two 1263db38ed7SKees Cookof them, and (b) it requires the same UID/GID/Groups all the way through. 1273db38ed7SKees Cook 1283db38ed7SKees Cook 1293db38ed7SKees CookNegative Instantiation And Rejection 1303db38ed7SKees Cook==================================== 1313db38ed7SKees Cook 1323db38ed7SKees CookRather than instantiating a key, it is possible for the possessor of an 1333db38ed7SKees Cookauthorisation key to negatively instantiate a key that's under construction. 1343db38ed7SKees CookThis is a short duration placeholder that causes any attempt at re-requesting 135806654a9SWill Deaconthe key while it exists to fail with error ENOKEY if negated or the specified 1363db38ed7SKees Cookerror if rejected. 1373db38ed7SKees Cook 1383db38ed7SKees CookThis is provided to prevent excessive repeated spawning of /sbin/request-key 1393db38ed7SKees Cookprocesses for a key that will never be obtainable. 1403db38ed7SKees Cook 1413db38ed7SKees CookShould the /sbin/request-key process exit anything other than 0 or die on a 1423db38ed7SKees Cooksignal, the key under construction will be automatically negatively 1433db38ed7SKees Cookinstantiated for a short amount of time. 1443db38ed7SKees Cook 1453db38ed7SKees Cook 1463db38ed7SKees CookThe Search Algorithm 1473db38ed7SKees Cook==================== 1483db38ed7SKees Cook 1493db38ed7SKees CookA search of any particular keyring proceeds in the following fashion: 1503db38ed7SKees Cook 151e59428f7SDavid Howells 1) When the key management code searches for a key (keyring_search_rcu) it 1523db38ed7SKees Cook firstly calls key_permission(SEARCH) on the keyring it's starting with, 1533db38ed7SKees Cook if this denies permission, it doesn't search further. 1543db38ed7SKees Cook 1553db38ed7SKees Cook 2) It considers all the non-keyring keys within that keyring and, if any key 1563db38ed7SKees Cook matches the criteria specified, calls key_permission(SEARCH) on it to see 1573db38ed7SKees Cook if the key is allowed to be found. If it is, that key is returned; if 1583db38ed7SKees Cook not, the search continues, and the error code is retained if of higher 1593db38ed7SKees Cook priority than the one currently set. 1603db38ed7SKees Cook 1613db38ed7SKees Cook 3) It then considers all the keyring-type keys in the keyring it's currently 1623db38ed7SKees Cook searching. It calls key_permission(SEARCH) on each keyring, and if this 1633db38ed7SKees Cook grants permission, it recurses, executing steps (2) and (3) on that 1643db38ed7SKees Cook keyring. 1653db38ed7SKees Cook 1663db38ed7SKees CookThe process stops immediately a valid key is found with permission granted to 1673db38ed7SKees Cookuse it. Any error from a previous match attempt is discarded and the key is 1683db38ed7SKees Cookreturned. 1693db38ed7SKees Cook 1703db38ed7SKees CookWhen search_process_keyrings() is invoked, it performs the following searches 1713db38ed7SKees Cookuntil one succeeds: 1723db38ed7SKees Cook 1733db38ed7SKees Cook 1) If extant, the process's thread keyring is searched. 1743db38ed7SKees Cook 1753db38ed7SKees Cook 2) If extant, the process's process keyring is searched. 1763db38ed7SKees Cook 1773db38ed7SKees Cook 3) The process's session keyring is searched. 1783db38ed7SKees Cook 1793db38ed7SKees Cook 4) If the process has assumed the authority associated with a request_key() 1803db38ed7SKees Cook authorisation key then: 1813db38ed7SKees Cook 1823db38ed7SKees Cook a) If extant, the calling process's thread keyring is searched. 1833db38ed7SKees Cook 1843db38ed7SKees Cook b) If extant, the calling process's process keyring is searched. 1853db38ed7SKees Cook 1863db38ed7SKees Cook c) The calling process's session keyring is searched. 1873db38ed7SKees Cook 1883db38ed7SKees CookThe moment one succeeds, all pending errors are discarded and the found key is 1893db38ed7SKees Cookreturned. 1903db38ed7SKees Cook 1913db38ed7SKees CookOnly if all these fail does the whole thing fail with the highest priority 1923db38ed7SKees Cookerror. Note that several errors may have come from LSM. 1933db38ed7SKees Cook 1943db38ed7SKees CookThe error priority is:: 1953db38ed7SKees Cook 1963db38ed7SKees Cook EKEYREVOKED > EKEYEXPIRED > ENOKEY 1973db38ed7SKees Cook 1983db38ed7SKees CookEACCES/EPERM are only returned on a direct search of a specific keyring where 1993db38ed7SKees Cookthe basal keyring does not grant Search permission. 200