1 Kernel module signing facility 1 Kernel module signing facility
2 ------------------------------ 2 ------------------------------
3 3
4 .. CONTENTS 4 .. CONTENTS
5 .. 5 ..
6 .. - Overview. 6 .. - Overview.
7 .. - Configuring module signing. 7 .. - Configuring module signing.
8 .. - Generating signing keys. 8 .. - Generating signing keys.
9 .. - Public keys in the kernel. 9 .. - Public keys in the kernel.
10 .. - Manually signing modules. 10 .. - Manually signing modules.
11 .. - Signed modules and stripping. 11 .. - Signed modules and stripping.
12 .. - Loading signed modules. 12 .. - Loading signed modules.
13 .. - Non-valid signatures and unsigned modules 13 .. - Non-valid signatures and unsigned modules.
14 .. - Administering/protecting the private key. 14 .. - Administering/protecting the private key.
15 15
16 16
17 ======== 17 ========
18 Overview 18 Overview
19 ======== 19 ========
20 20
21 The kernel module signing facility cryptograph 21 The kernel module signing facility cryptographically signs modules during
22 installation and then checks the signature upo 22 installation and then checks the signature upon loading the module. This
23 allows increased kernel security by disallowin 23 allows increased kernel security by disallowing the loading of unsigned modules
24 or modules signed with an invalid key. Module 24 or modules signed with an invalid key. Module signing increases security by
25 making it harder to load a malicious module in 25 making it harder to load a malicious module into the kernel. The module
26 signature checking is done by the kernel so th 26 signature checking is done by the kernel so that it is not necessary to have
27 trusted userspace bits. 27 trusted userspace bits.
28 28
29 This facility uses X.509 ITU-T standard certif 29 This facility uses X.509 ITU-T standard certificates to encode the public keys
30 involved. The signatures are not themselves e 30 involved. The signatures are not themselves encoded in any industrial standard
31 type. The built-in facility currently only su !! 31 type. The facility currently only supports the RSA public key encryption
32 public key signing standard (though it is plug !! 32 standard (though it is pluggable and permits others to be used). The possible
33 used). The possible hash algorithms that can !! 33 hash algorithms that can be used are SHA-1, SHA-224, SHA-256, SHA-384, and
34 sizes 256, 384, and 512 (the algorithm is sele !! 34 SHA-512 (the algorithm is selected by data in the signature).
35 35
36 36
37 ========================== 37 ==========================
38 Configuring module signing 38 Configuring module signing
39 ========================== 39 ==========================
40 40
41 The module signing facility is enabled by goin 41 The module signing facility is enabled by going to the
42 :menuselection:`Enable Loadable Module Support 42 :menuselection:`Enable Loadable Module Support` section of
43 the kernel configuration and turning on:: 43 the kernel configuration and turning on::
44 44
45 CONFIG_MODULE_SIG "Module signat 45 CONFIG_MODULE_SIG "Module signature verification"
46 46
47 This has a number of options available: 47 This has a number of options available:
48 48
49 (1) :menuselection:`Require modules to be val 49 (1) :menuselection:`Require modules to be validly signed`
50 (``CONFIG_MODULE_SIG_FORCE``) 50 (``CONFIG_MODULE_SIG_FORCE``)
51 51
52 This specifies how the kernel should deal 52 This specifies how the kernel should deal with a module that has a
53 signature for which the key is not known 53 signature for which the key is not known or a module that is unsigned.
54 54
55 If this is off (ie. "permissive"), then m 55 If this is off (ie. "permissive"), then modules for which the key is not
56 available and modules that are unsigned a 56 available and modules that are unsigned are permitted, but the kernel will
57 be marked as being tainted, and the conce 57 be marked as being tainted, and the concerned modules will be marked as
58 tainted, shown with the character 'E'. 58 tainted, shown with the character 'E'.
59 59
60 If this is on (ie. "restrictive"), only m 60 If this is on (ie. "restrictive"), only modules that have a valid
61 signature that can be verified by a publi 61 signature that can be verified by a public key in the kernel's possession
62 will be loaded. All other modules will g 62 will be loaded. All other modules will generate an error.
63 63
64 Irrespective of the setting here, if the 64 Irrespective of the setting here, if the module has a signature block that
65 cannot be parsed, it will be rejected out 65 cannot be parsed, it will be rejected out of hand.
66 66
67 67
68 (2) :menuselection:`Automatically sign all mo 68 (2) :menuselection:`Automatically sign all modules`
69 (``CONFIG_MODULE_SIG_ALL``) 69 (``CONFIG_MODULE_SIG_ALL``)
70 70
71 If this is on then modules will be automa 71 If this is on then modules will be automatically signed during the
72 modules_install phase of a build. If thi 72 modules_install phase of a build. If this is off, then the modules must
73 be signed manually using:: 73 be signed manually using::
74 74
75 scripts/sign-file 75 scripts/sign-file
76 76
77 77
78 (3) :menuselection:`Which hash algorithm shou 78 (3) :menuselection:`Which hash algorithm should modules be signed with?`
79 79
80 This presents a choice of which hash algo 80 This presents a choice of which hash algorithm the installation phase will
81 sign the modules with: 81 sign the modules with:
82 82
83 =============================== ====== 83 =============================== ==========================================
>> 84 ``CONFIG_MODULE_SIG_SHA1`` :menuselection:`Sign modules with SHA-1`
>> 85 ``CONFIG_MODULE_SIG_SHA224`` :menuselection:`Sign modules with SHA-224`
84 ``CONFIG_MODULE_SIG_SHA256`` :menus 86 ``CONFIG_MODULE_SIG_SHA256`` :menuselection:`Sign modules with SHA-256`
85 ``CONFIG_MODULE_SIG_SHA384`` :menus 87 ``CONFIG_MODULE_SIG_SHA384`` :menuselection:`Sign modules with SHA-384`
86 ``CONFIG_MODULE_SIG_SHA512`` :menus 88 ``CONFIG_MODULE_SIG_SHA512`` :menuselection:`Sign modules with SHA-512`
87 ``CONFIG_MODULE_SIG_SHA3_256`` :menus <<
88 ``CONFIG_MODULE_SIG_SHA3_384`` :menus <<
89 ``CONFIG_MODULE_SIG_SHA3_512`` :menus <<
90 =============================== ====== 89 =============================== ==========================================
91 90
92 The algorithm selected here will also be 91 The algorithm selected here will also be built into the kernel (rather
93 than being a module) so that modules sign 92 than being a module) so that modules signed with that algorithm can have
94 their signatures checked without causing 93 their signatures checked without causing a dependency loop.
95 94
96 95
97 (4) :menuselection:`File name or PKCS#11 URI 96 (4) :menuselection:`File name or PKCS#11 URI of module signing key`
98 (``CONFIG_MODULE_SIG_KEY``) 97 (``CONFIG_MODULE_SIG_KEY``)
99 98
100 Setting this option to something other th 99 Setting this option to something other than its default of
101 ``certs/signing_key.pem`` will disable th 100 ``certs/signing_key.pem`` will disable the autogeneration of signing keys
102 and allow the kernel modules to be signed 101 and allow the kernel modules to be signed with a key of your choosing.
103 The string provided should identify a fil 102 The string provided should identify a file containing both a private key
104 and its corresponding X.509 certificate i 103 and its corresponding X.509 certificate in PEM form, or — on systems where
105 the OpenSSL ENGINE_pkcs11 is functional â 104 the OpenSSL ENGINE_pkcs11 is functional — a PKCS#11 URI as defined by
106 RFC7512. In the latter case, the PKCS#11 105 RFC7512. In the latter case, the PKCS#11 URI should reference both a
107 certificate and a private key. 106 certificate and a private key.
108 107
109 If the PEM file containing the private ke 108 If the PEM file containing the private key is encrypted, or if the
110 PKCS#11 token requires a PIN, this can be !! 109 PKCS#11 token requries a PIN, this can be provided at build time by
111 means of the ``KBUILD_SIGN_PIN`` variable 110 means of the ``KBUILD_SIGN_PIN`` variable.
112 111
113 112
114 (5) :menuselection:`Additional X.509 keys for 113 (5) :menuselection:`Additional X.509 keys for default system keyring`
115 (``CONFIG_SYSTEM_TRUSTED_KEYS``) 114 (``CONFIG_SYSTEM_TRUSTED_KEYS``)
116 115
117 This option can be set to the filename of 116 This option can be set to the filename of a PEM-encoded file containing
118 additional certificates which will be inc 117 additional certificates which will be included in the system keyring by
119 default. 118 default.
120 119
121 Note that enabling module signing adds a depen 120 Note that enabling module signing adds a dependency on the OpenSSL devel
122 packages to the kernel build processes for the 121 packages to the kernel build processes for the tool that does the signing.
123 122
124 123
125 ======================= 124 =======================
126 Generating signing keys 125 Generating signing keys
127 ======================= 126 =======================
128 127
129 Cryptographic keypairs are required to generat 128 Cryptographic keypairs are required to generate and check signatures. A
130 private key is used to generate a signature an 129 private key is used to generate a signature and the corresponding public key is
131 used to check it. The private key is only nee 130 used to check it. The private key is only needed during the build, after which
132 it can be deleted or stored securely. The pub 131 it can be deleted or stored securely. The public key gets built into the
133 kernel so that it can be used to check the sig 132 kernel so that it can be used to check the signatures as the modules are
134 loaded. 133 loaded.
135 134
136 Under normal conditions, when ``CONFIG_MODULE_ 135 Under normal conditions, when ``CONFIG_MODULE_SIG_KEY`` is unchanged from its
137 default, the kernel build will automatically g 136 default, the kernel build will automatically generate a new keypair using
138 openssl if one does not exist in the file:: 137 openssl if one does not exist in the file::
139 138
140 certs/signing_key.pem 139 certs/signing_key.pem
141 140
142 during the building of vmlinux (the public par 141 during the building of vmlinux (the public part of the key needs to be built
143 into vmlinux) using parameters in the:: 142 into vmlinux) using parameters in the::
144 143
145 certs/x509.genkey 144 certs/x509.genkey
146 145
147 file (which is also generated if it does not a 146 file (which is also generated if it does not already exist).
148 147
149 One can select between RSA (``MODULE_SIG_KEY_T <<
150 (``MODULE_SIG_KEY_TYPE_ECDSA``) to generate ei <<
151 P-384 keypair. <<
152 <<
153 It is strongly recommended that you provide yo 148 It is strongly recommended that you provide your own x509.genkey file.
154 149
155 Most notably, in the x509.genkey file, the req 150 Most notably, in the x509.genkey file, the req_distinguished_name section
156 should be altered from the default:: 151 should be altered from the default::
157 152
158 [ req_distinguished_name ] 153 [ req_distinguished_name ]
159 #O = Unspecified company 154 #O = Unspecified company
160 CN = Build time autogenerated kernel k 155 CN = Build time autogenerated kernel key
161 #emailAddress = unspecified.user@unspe 156 #emailAddress = unspecified.user@unspecified.company
162 157
163 The generated RSA key size can also be set wit 158 The generated RSA key size can also be set with::
164 159
165 [ req ] 160 [ req ]
166 default_bits = 4096 161 default_bits = 4096
167 162
168 163
169 It is also possible to manually generate the k 164 It is also possible to manually generate the key private/public files using the
170 x509.genkey key generation configuration file 165 x509.genkey key generation configuration file in the root node of the Linux
171 kernel sources tree and the openssl command. 166 kernel sources tree and the openssl command. The following is an example to
172 generate the public/private key files:: 167 generate the public/private key files::
173 168
174 openssl req -new -nodes -utf8 -sha256 169 openssl req -new -nodes -utf8 -sha256 -days 36500 -batch -x509 \
175 -config x509.genkey -outform PEM -o 170 -config x509.genkey -outform PEM -out kernel_key.pem \
176 -keyout kernel_key.pem 171 -keyout kernel_key.pem
177 172
178 The full pathname for the resulting kernel_key 173 The full pathname for the resulting kernel_key.pem file can then be specified
179 in the ``CONFIG_MODULE_SIG_KEY`` option, and t 174 in the ``CONFIG_MODULE_SIG_KEY`` option, and the certificate and key therein will
180 be used instead of an autogenerated keypair. 175 be used instead of an autogenerated keypair.
181 176
182 177
183 ========================= 178 =========================
184 Public keys in the kernel 179 Public keys in the kernel
185 ========================= 180 =========================
186 181
187 The kernel contains a ring of public keys that 182 The kernel contains a ring of public keys that can be viewed by root. They're
188 in a keyring called ".builtin_trusted_keys" th !! 183 in a keyring called ".system_keyring" that can be seen by::
189 184
190 [root@deneb ~]# cat /proc/keys 185 [root@deneb ~]# cat /proc/keys
191 ... 186 ...
192 223c7853 I------ 1 perm 1f030000 !! 187 223c7853 I------ 1 perm 1f030000 0 0 keyring .system_keyring: 1
193 302d2d52 I------ 1 perm 1f010000 188 302d2d52 I------ 1 perm 1f010000 0 0 asymmetri Fedora kernel signing key: d69a84e6bce3d216b979e9505b3e3ef9a7118079: X509.RSA a7118079 []
194 ... 189 ...
195 190
196 Beyond the public key generated specifically f 191 Beyond the public key generated specifically for module signing, additional
197 trusted certificates can be provided in a PEM- 192 trusted certificates can be provided in a PEM-encoded file referenced by the
198 ``CONFIG_SYSTEM_TRUSTED_KEYS`` configuration o 193 ``CONFIG_SYSTEM_TRUSTED_KEYS`` configuration option.
199 194
200 Further, the architecture code may take public 195 Further, the architecture code may take public keys from a hardware store and
201 add those in also (e.g. from the UEFI key data 196 add those in also (e.g. from the UEFI key database).
202 197
203 Finally, it is possible to add additional publ 198 Finally, it is possible to add additional public keys by doing::
204 199
205 keyctl padd asymmetric "" [.builtin_tr !! 200 keyctl padd asymmetric "" [.system_keyring-ID] <[key-file]
206 201
207 e.g.:: 202 e.g.::
208 203
209 keyctl padd asymmetric "" 0x223c7853 < 204 keyctl padd asymmetric "" 0x223c7853 <my_public_key.x509
210 205
211 Note, however, that the kernel will only permi 206 Note, however, that the kernel will only permit keys to be added to
212 ``.builtin_trusted_keys`` **if** the new key's !! 207 ``.system_keyring _if_`` the new key's X.509 wrapper is validly signed by a key
213 that is already resident in the ``.builtin_tru !! 208 that is already resident in the .system_keyring at the time the key was added.
214 209
215 210
216 ======================== 211 ========================
217 Manually signing modules 212 Manually signing modules
218 ======================== 213 ========================
219 214
220 To manually sign a module, use the scripts/sig 215 To manually sign a module, use the scripts/sign-file tool available in
221 the Linux kernel source tree. The script requ 216 the Linux kernel source tree. The script requires 4 arguments:
222 217
223 1. The hash algorithm (e.g., sha256) 218 1. The hash algorithm (e.g., sha256)
224 2. The private key filename or PKCS#1 219 2. The private key filename or PKCS#11 URI
225 3. The public key filename 220 3. The public key filename
226 4. The kernel module to be signed 221 4. The kernel module to be signed
227 222
228 The following is an example to sign a kernel m 223 The following is an example to sign a kernel module::
229 224
230 scripts/sign-file sha512 kernel-signke 225 scripts/sign-file sha512 kernel-signkey.priv \
231 kernel-signkey.x509 module.ko 226 kernel-signkey.x509 module.ko
232 227
233 The hash algorithm used does not have to match 228 The hash algorithm used does not have to match the one configured, but if it
234 doesn't, you should make sure that hash algori 229 doesn't, you should make sure that hash algorithm is either built into the
235 kernel or can be loaded without requiring itse 230 kernel or can be loaded without requiring itself.
236 231
237 If the private key requires a passphrase or PI 232 If the private key requires a passphrase or PIN, it can be provided in the
238 $KBUILD_SIGN_PIN environment variable. 233 $KBUILD_SIGN_PIN environment variable.
239 234
240 235
241 ============================ 236 ============================
242 Signed modules and stripping 237 Signed modules and stripping
243 ============================ 238 ============================
244 239
245 A signed module has a digital signature simply 240 A signed module has a digital signature simply appended at the end. The string
246 ``~Module signature appended~.`` at the end of 241 ``~Module signature appended~.`` at the end of the module's file confirms that a
247 signature is present but it does not confirm t 242 signature is present but it does not confirm that the signature is valid!
248 243
249 Signed modules are BRITTLE as the signature is 244 Signed modules are BRITTLE as the signature is outside of the defined ELF
250 container. Thus they MAY NOT be stripped once 245 container. Thus they MAY NOT be stripped once the signature is computed and
251 attached. Note the entire module is the signe 246 attached. Note the entire module is the signed payload, including any and all
252 debug information present at the time of signi 247 debug information present at the time of signing.
253 248
254 249
255 ====================== 250 ======================
256 Loading signed modules 251 Loading signed modules
257 ====================== 252 ======================
258 253
259 Modules are loaded with insmod, modprobe, ``in 254 Modules are loaded with insmod, modprobe, ``init_module()`` or
260 ``finit_module()``, exactly as for unsigned mo 255 ``finit_module()``, exactly as for unsigned modules as no processing is
261 done in userspace. The signature checking is 256 done in userspace. The signature checking is all done within the kernel.
262 257
263 258
264 ========================================= 259 =========================================
265 Non-valid signatures and unsigned modules 260 Non-valid signatures and unsigned modules
266 ========================================= 261 =========================================
267 262
268 If ``CONFIG_MODULE_SIG_FORCE`` is enabled or m 263 If ``CONFIG_MODULE_SIG_FORCE`` is enabled or module.sig_enforce=1 is supplied on
269 the kernel command line, the kernel will only 264 the kernel command line, the kernel will only load validly signed modules
270 for which it has a public key. Otherwise, it 265 for which it has a public key. Otherwise, it will also load modules that are
271 unsigned. Any module for which the kernel ha 266 unsigned. Any module for which the kernel has a key, but which proves to have
272 a signature mismatch will not be permitted to 267 a signature mismatch will not be permitted to load.
273 268
274 Any module that has an unparsable signature wi !! 269 Any module that has an unparseable signature will be rejected.
275 270
276 271
277 ========================================= 272 =========================================
278 Administering/protecting the private key 273 Administering/protecting the private key
279 ========================================= 274 =========================================
280 275
281 Since the private key is used to sign modules, 276 Since the private key is used to sign modules, viruses and malware could use
282 the private key to sign modules and compromise 277 the private key to sign modules and compromise the operating system. The
283 private key must be either destroyed or moved 278 private key must be either destroyed or moved to a secure location and not kept
284 in the root node of the kernel source tree. 279 in the root node of the kernel source tree.
285 280
286 If you use the same private key to sign module 281 If you use the same private key to sign modules for multiple kernel
287 configurations, you must ensure that the modul 282 configurations, you must ensure that the module version information is
288 sufficient to prevent loading a module into a 283 sufficient to prevent loading a module into a different kernel. Either
289 set ``CONFIG_MODVERSIONS=y`` or ensure that ea 284 set ``CONFIG_MODVERSIONS=y`` or ensure that each configuration has a different
290 kernel release string by changing ``EXTRAVERSI 285 kernel release string by changing ``EXTRAVERSION`` or ``CONFIG_LOCALVERSION``.
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