1 =================== 1 ===================
2 Key Request Service 2 Key Request Service
3 =================== 3 ===================
4 4
5 The key request service is part of the key ret 5 The key request service is part of the key retention service (refer to
6 Documentation/security/keys/core.rst). This d 6 Documentation/security/keys/core.rst). This document explains more fully how
7 the requesting algorithm works. 7 the requesting algorithm works.
8 8
9 The process starts by either the kernel reques 9 The process starts by either the kernel requesting a service by calling
10 ``request_key*()``:: 10 ``request_key*()``::
11 11
12 struct key *request_key(const struct k 12 struct key *request_key(const struct key_type *type,
13 const char *de 13 const char *description,
14 const char *ca 14 const char *callout_info);
15 15
16 or:: 16 or::
17 17
18 struct key *request_key_tag(const stru <<
19 const char <<
20 const stru <<
21 const char <<
22 <<
23 or:: <<
24 <<
25 struct key *request_key_with_auxdata(c 18 struct key *request_key_with_auxdata(const struct key_type *type,
26 c 19 const char *description,
27 c <<
28 c 20 const char *callout_info,
29 s 21 size_t callout_len,
30 v 22 void *aux);
31 23
32 or:: 24 or::
33 25
34 struct key *request_key_rcu(const stru !! 26 struct key *request_key_async(const struct key_type *type,
35 const char !! 27 const char *description,
36 const stru !! 28 const char *callout_info,
>> 29 size_t callout_len);
>> 30
>> 31 or::
>> 32
>> 33 struct key *request_key_async_with_auxdata(const struct key_type *type,
>> 34 const char *description,
>> 35 const char *callout_info,
>> 36 size_t callout_len,
>> 37 void *aux);
37 38
38 Or by userspace invoking the request_key syste 39 Or by userspace invoking the request_key system call::
39 40
40 key_serial_t request_key(const char *t 41 key_serial_t request_key(const char *type,
41 const char *d 42 const char *description,
42 const char *c 43 const char *callout_info,
43 key_serial_t 44 key_serial_t dest_keyring);
44 45
45 The main difference between the access points 46 The main difference between the access points is that the in-kernel interface
46 does not need to link the key to a keyring to 47 does not need to link the key to a keyring to prevent it from being immediately
47 destroyed. The kernel interface returns a poi 48 destroyed. The kernel interface returns a pointer directly to the key, and
48 it's up to the caller to destroy the key. 49 it's up to the caller to destroy the key.
49 50
50 The request_key_tag() call is like the in-kern !! 51 The request_key*_with_auxdata() calls are like the in-kernel request_key*()
51 also takes a domain tag that allows keys to be !! 52 calls, except that they permit auxiliary data to be passed to the upcaller (the
52 killed off as a group. !! 53 default is NULL). This is only useful for those key types that define their
53 !! 54 own upcall mechanism rather than using /sbin/request-key.
54 The request_key_with_auxdata() calls is like t !! 55
55 that they permit auxiliary data to be passed t !! 56 The two async in-kernel calls may return keys that are still in the process of
56 NULL). This is only useful for those key type !! 57 being constructed. The two non-async ones will wait for construction to
57 mechanism rather than using /sbin/request-key. !! 58 complete first.
58 <<
59 The request_key_rcu() call is like the request <<
60 doesn't check for keys that are under construc <<
61 construct missing keys. <<
62 59
63 The userspace interface links the key to a key 60 The userspace interface links the key to a keyring associated with the process
64 to prevent the key from going away, and return 61 to prevent the key from going away, and returns the serial number of the key to
65 the caller. 62 the caller.
66 63
67 64
68 The following example assumes that the key typ 65 The following example assumes that the key types involved don't define their
69 own upcall mechanisms. If they do, then those 66 own upcall mechanisms. If they do, then those should be substituted for the
70 forking and execution of /sbin/request-key. 67 forking and execution of /sbin/request-key.
71 68
72 69
73 The Process 70 The Process
74 =========== 71 ===========
75 72
76 A request proceeds in the following manner: 73 A request proceeds in the following manner:
77 74
78 1) Process A calls request_key() [the usersp 75 1) Process A calls request_key() [the userspace syscall calls the kernel
79 interface]. 76 interface].
80 77
81 2) request_key() searches the process's subs 78 2) request_key() searches the process's subscribed keyrings to see if there's
82 a suitable key there. If there is, it re 79 a suitable key there. If there is, it returns the key. If there isn't,
83 and callout_info is not set, an error is 80 and callout_info is not set, an error is returned. Otherwise the process
84 proceeds to the next step. 81 proceeds to the next step.
85 82
86 3) request_key() sees that A doesn't have th 83 3) request_key() sees that A doesn't have the desired key yet, so it creates
87 two things: 84 two things:
88 85
89 a) An uninstantiated key U of requested 86 a) An uninstantiated key U of requested type and description.
90 87
91 b) An authorisation key V that refers to 88 b) An authorisation key V that refers to key U and notes that process A
92 is the context in which key U should 89 is the context in which key U should be instantiated and secured, and
93 from which associated key requests ma 90 from which associated key requests may be satisfied.
94 91
95 4) request_key() then forks and executes /sb 92 4) request_key() then forks and executes /sbin/request-key with a new session
96 keyring that contains a link to auth key 93 keyring that contains a link to auth key V.
97 94
98 5) /sbin/request-key assumes the authority a 95 5) /sbin/request-key assumes the authority associated with key U.
99 96
100 6) /sbin/request-key execs an appropriate pr 97 6) /sbin/request-key execs an appropriate program to perform the actual
101 instantiation. 98 instantiation.
102 99
103 7) The program may want to access another ke 100 7) The program may want to access another key from A's context (say a
104 Kerberos TGT key). It just requests the 101 Kerberos TGT key). It just requests the appropriate key, and the keyring
105 search notes that the session keyring has 102 search notes that the session keyring has auth key V in its bottom level.
106 103
107 This will permit it to then search the ke 104 This will permit it to then search the keyrings of process A with the
108 UID, GID, groups and security info of pro 105 UID, GID, groups and security info of process A as if it was process A,
109 and come up with key W. 106 and come up with key W.
110 107
111 8) The program then does what it must to get 108 8) The program then does what it must to get the data with which to
112 instantiate key U, using key W as a refer 109 instantiate key U, using key W as a reference (perhaps it contacts a
113 Kerberos server using the TGT) and then i 110 Kerberos server using the TGT) and then instantiates key U.
114 111
115 9) Upon instantiating key U, auth key V is a 112 9) Upon instantiating key U, auth key V is automatically revoked so that it
116 may not be used again. 113 may not be used again.
117 114
118 10) The program then exits 0 and request_key 115 10) The program then exits 0 and request_key() deletes key V and returns key
119 U to the caller. 116 U to the caller.
120 117
121 This also extends further. If key W (step 7 a 118 This also extends further. If key W (step 7 above) didn't exist, key W would
122 be created uninstantiated, another auth key (X 119 be created uninstantiated, another auth key (X) would be created (as per step
123 3) and another copy of /sbin/request-key spawn 120 3) and another copy of /sbin/request-key spawned (as per step 4); but the
124 context specified by auth key X will still be 121 context specified by auth key X will still be process A, as it was in auth key
125 V. 122 V.
126 123
127 This is because process A's keyrings can't sim 124 This is because process A's keyrings can't simply be attached to
128 /sbin/request-key at the appropriate places be 125 /sbin/request-key at the appropriate places because (a) execve will discard two
129 of them, and (b) it requires the same UID/GID/ 126 of them, and (b) it requires the same UID/GID/Groups all the way through.
130 127
131 128
132 Negative Instantiation And Rejection 129 Negative Instantiation And Rejection
133 ==================================== 130 ====================================
134 131
135 Rather than instantiating a key, it is possibl 132 Rather than instantiating a key, it is possible for the possessor of an
136 authorisation key to negatively instantiate a 133 authorisation key to negatively instantiate a key that's under construction.
137 This is a short duration placeholder that caus 134 This is a short duration placeholder that causes any attempt at re-requesting
138 the key while it exists to fail with error ENO 135 the key while it exists to fail with error ENOKEY if negated or the specified
139 error if rejected. 136 error if rejected.
140 137
141 This is provided to prevent excessive repeated 138 This is provided to prevent excessive repeated spawning of /sbin/request-key
142 processes for a key that will never be obtaina 139 processes for a key that will never be obtainable.
143 140
144 Should the /sbin/request-key process exit anyt 141 Should the /sbin/request-key process exit anything other than 0 or die on a
145 signal, the key under construction will be aut 142 signal, the key under construction will be automatically negatively
146 instantiated for a short amount of time. 143 instantiated for a short amount of time.
147 144
148 145
149 The Search Algorithm 146 The Search Algorithm
150 ==================== 147 ====================
151 148
152 A search of any particular keyring proceeds in 149 A search of any particular keyring proceeds in the following fashion:
153 150
154 1) When the key management code searches for !! 151 1) When the key management code searches for a key (keyring_search_aux) it
155 firstly calls key_permission(SEARCH) on t 152 firstly calls key_permission(SEARCH) on the keyring it's starting with,
156 if this denies permission, it doesn't sea 153 if this denies permission, it doesn't search further.
157 154
158 2) It considers all the non-keyring keys wit 155 2) It considers all the non-keyring keys within that keyring and, if any key
159 matches the criteria specified, calls key 156 matches the criteria specified, calls key_permission(SEARCH) on it to see
160 if the key is allowed to be found. If it 157 if the key is allowed to be found. If it is, that key is returned; if
161 not, the search continues, and the error 158 not, the search continues, and the error code is retained if of higher
162 priority than the one currently set. 159 priority than the one currently set.
163 160
164 3) It then considers all the keyring-type ke 161 3) It then considers all the keyring-type keys in the keyring it's currently
165 searching. It calls key_permission(SEARC 162 searching. It calls key_permission(SEARCH) on each keyring, and if this
166 grants permission, it recurses, executing 163 grants permission, it recurses, executing steps (2) and (3) on that
167 keyring. 164 keyring.
168 165
169 The process stops immediately a valid key is f 166 The process stops immediately a valid key is found with permission granted to
170 use it. Any error from a previous match attem 167 use it. Any error from a previous match attempt is discarded and the key is
171 returned. 168 returned.
172 169
173 When request_key() is invoked, if CONFIG_KEYS_ <<
174 one-key cache is first checked for a match. <<
175 <<
176 When search_process_keyrings() is invoked, it 170 When search_process_keyrings() is invoked, it performs the following searches
177 until one succeeds: 171 until one succeeds:
178 172
179 1) If extant, the process's thread keyring i 173 1) If extant, the process's thread keyring is searched.
180 174
181 2) If extant, the process's process keyring 175 2) If extant, the process's process keyring is searched.
182 176
183 3) The process's session keyring is searched 177 3) The process's session keyring is searched.
184 178
185 4) If the process has assumed the authority 179 4) If the process has assumed the authority associated with a request_key()
186 authorisation key then: 180 authorisation key then:
187 181
188 a) If extant, the calling process's thre 182 a) If extant, the calling process's thread keyring is searched.
189 183
190 b) If extant, the calling process's proc 184 b) If extant, the calling process's process keyring is searched.
191 185
192 c) The calling process's session keyring 186 c) The calling process's session keyring is searched.
193 187
194 The moment one succeeds, all pending errors ar 188 The moment one succeeds, all pending errors are discarded and the found key is
195 returned. If CONFIG_KEYS_REQUEST_CACHE=y, the !! 189 returned.
196 per-task cache, displacing the previous key. <<
197 just prior to resumption of userspace. <<
198 190
199 Only if all these fail does the whole thing fa 191 Only if all these fail does the whole thing fail with the highest priority
200 error. Note that several errors may have come 192 error. Note that several errors may have come from LSM.
201 193
202 The error priority is:: 194 The error priority is::
203 195
204 EKEYREVOKED > EKEYEXPIRED > ENOKEY 196 EKEYREVOKED > EKEYEXPIRED > ENOKEY
205 197
206 EACCES/EPERM are only returned on a direct sea 198 EACCES/EPERM are only returned on a direct search of a specific keyring where
207 the basal keyring does not grant Search permis 199 the basal keyring does not grant Search permission.
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