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 built-in facility currently only supports the RSA & NIST P-384 ECDSA
32 public key signing standard (though it is plug 32 public key signing standard (though it is pluggable and permits others to be
33 used). The possible hash algorithms that can 33 used). The possible hash algorithms that can be used are SHA-2 and SHA-3 of
34 sizes 256, 384, and 512 (the algorithm is sele 34 sizes 256, 384, and 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_SHA256`` :menus 84 ``CONFIG_MODULE_SIG_SHA256`` :menuselection:`Sign modules with SHA-256`
85 ``CONFIG_MODULE_SIG_SHA384`` :menus 85 ``CONFIG_MODULE_SIG_SHA384`` :menuselection:`Sign modules with SHA-384`
86 ``CONFIG_MODULE_SIG_SHA512`` :menus 86 ``CONFIG_MODULE_SIG_SHA512`` :menuselection:`Sign modules with SHA-512`
87 ``CONFIG_MODULE_SIG_SHA3_256`` :menus 87 ``CONFIG_MODULE_SIG_SHA3_256`` :menuselection:`Sign modules with SHA3-256`
88 ``CONFIG_MODULE_SIG_SHA3_384`` :menus 88 ``CONFIG_MODULE_SIG_SHA3_384`` :menuselection:`Sign modules with SHA3-384`
89 ``CONFIG_MODULE_SIG_SHA3_512`` :menus 89 ``CONFIG_MODULE_SIG_SHA3_512`` :menuselection:`Sign modules with SHA3-512`
90 =============================== ====== 90 =============================== ==========================================
91 91
92 The algorithm selected here will also be 92 The algorithm selected here will also be built into the kernel (rather
93 than being a module) so that modules sign 93 than being a module) so that modules signed with that algorithm can have
94 their signatures checked without causing 94 their signatures checked without causing a dependency loop.
95 95
96 96
97 (4) :menuselection:`File name or PKCS#11 URI 97 (4) :menuselection:`File name or PKCS#11 URI of module signing key`
98 (``CONFIG_MODULE_SIG_KEY``) 98 (``CONFIG_MODULE_SIG_KEY``)
99 99
100 Setting this option to something other th 100 Setting this option to something other than its default of
101 ``certs/signing_key.pem`` will disable th 101 ``certs/signing_key.pem`` will disable the autogeneration of signing keys
102 and allow the kernel modules to be signed 102 and allow the kernel modules to be signed with a key of your choosing.
103 The string provided should identify a fil 103 The string provided should identify a file containing both a private key
104 and its corresponding X.509 certificate i 104 and its corresponding X.509 certificate in PEM form, or — on systems where
105 the OpenSSL ENGINE_pkcs11 is functional â 105 the OpenSSL ENGINE_pkcs11 is functional — a PKCS#11 URI as defined by
106 RFC7512. In the latter case, the PKCS#11 106 RFC7512. In the latter case, the PKCS#11 URI should reference both a
107 certificate and a private key. 107 certificate and a private key.
108 108
109 If the PEM file containing the private ke 109 If the PEM file containing the private key is encrypted, or if the
110 PKCS#11 token requires a PIN, this can be 110 PKCS#11 token requires a PIN, this can be provided at build time by
111 means of the ``KBUILD_SIGN_PIN`` variable 111 means of the ``KBUILD_SIGN_PIN`` variable.
112 112
113 113
114 (5) :menuselection:`Additional X.509 keys for 114 (5) :menuselection:`Additional X.509 keys for default system keyring`
115 (``CONFIG_SYSTEM_TRUSTED_KEYS``) 115 (``CONFIG_SYSTEM_TRUSTED_KEYS``)
116 116
117 This option can be set to the filename of 117 This option can be set to the filename of a PEM-encoded file containing
118 additional certificates which will be inc 118 additional certificates which will be included in the system keyring by
119 default. 119 default.
120 120
121 Note that enabling module signing adds a depen 121 Note that enabling module signing adds a dependency on the OpenSSL devel
122 packages to the kernel build processes for the 122 packages to the kernel build processes for the tool that does the signing.
123 123
124 124
125 ======================= 125 =======================
126 Generating signing keys 126 Generating signing keys
127 ======================= 127 =======================
128 128
129 Cryptographic keypairs are required to generat 129 Cryptographic keypairs are required to generate and check signatures. A
130 private key is used to generate a signature an 130 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 131 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 132 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 133 kernel so that it can be used to check the signatures as the modules are
134 loaded. 134 loaded.
135 135
136 Under normal conditions, when ``CONFIG_MODULE_ 136 Under normal conditions, when ``CONFIG_MODULE_SIG_KEY`` is unchanged from its
137 default, the kernel build will automatically g 137 default, the kernel build will automatically generate a new keypair using
138 openssl if one does not exist in the file:: 138 openssl if one does not exist in the file::
139 139
140 certs/signing_key.pem 140 certs/signing_key.pem
141 141
142 during the building of vmlinux (the public par 142 during the building of vmlinux (the public part of the key needs to be built
143 into vmlinux) using parameters in the:: 143 into vmlinux) using parameters in the::
144 144
145 certs/x509.genkey 145 certs/x509.genkey
146 146
147 file (which is also generated if it does not a 147 file (which is also generated if it does not already exist).
148 148
149 One can select between RSA (``MODULE_SIG_KEY_T 149 One can select between RSA (``MODULE_SIG_KEY_TYPE_RSA``) and ECDSA
150 (``MODULE_SIG_KEY_TYPE_ECDSA``) to generate ei 150 (``MODULE_SIG_KEY_TYPE_ECDSA``) to generate either RSA 4k or NIST
151 P-384 keypair. 151 P-384 keypair.
152 152
153 It is strongly recommended that you provide yo 153 It is strongly recommended that you provide your own x509.genkey file.
154 154
155 Most notably, in the x509.genkey file, the req 155 Most notably, in the x509.genkey file, the req_distinguished_name section
156 should be altered from the default:: 156 should be altered from the default::
157 157
158 [ req_distinguished_name ] 158 [ req_distinguished_name ]
159 #O = Unspecified company 159 #O = Unspecified company
160 CN = Build time autogenerated kernel k 160 CN = Build time autogenerated kernel key
161 #emailAddress = unspecified.user@unspe 161 #emailAddress = unspecified.user@unspecified.company
162 162
163 The generated RSA key size can also be set wit 163 The generated RSA key size can also be set with::
164 164
165 [ req ] 165 [ req ]
166 default_bits = 4096 166 default_bits = 4096
167 167
168 168
169 It is also possible to manually generate the k 169 It is also possible to manually generate the key private/public files using the
170 x509.genkey key generation configuration file 170 x509.genkey key generation configuration file in the root node of the Linux
171 kernel sources tree and the openssl command. 171 kernel sources tree and the openssl command. The following is an example to
172 generate the public/private key files:: 172 generate the public/private key files::
173 173
174 openssl req -new -nodes -utf8 -sha256 174 openssl req -new -nodes -utf8 -sha256 -days 36500 -batch -x509 \
175 -config x509.genkey -outform PEM -o 175 -config x509.genkey -outform PEM -out kernel_key.pem \
176 -keyout kernel_key.pem 176 -keyout kernel_key.pem
177 177
178 The full pathname for the resulting kernel_key 178 The full pathname for the resulting kernel_key.pem file can then be specified
179 in the ``CONFIG_MODULE_SIG_KEY`` option, and t 179 in the ``CONFIG_MODULE_SIG_KEY`` option, and the certificate and key therein will
180 be used instead of an autogenerated keypair. 180 be used instead of an autogenerated keypair.
181 181
182 182
183 ========================= 183 =========================
184 Public keys in the kernel 184 Public keys in the kernel
185 ========================= 185 =========================
186 186
187 The kernel contains a ring of public keys that 187 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 188 in a keyring called ".builtin_trusted_keys" that can be seen by::
189 189
190 [root@deneb ~]# cat /proc/keys 190 [root@deneb ~]# cat /proc/keys
191 ... 191 ...
192 223c7853 I------ 1 perm 1f030000 192 223c7853 I------ 1 perm 1f030000 0 0 keyring .builtin_trusted_keys: 1
193 302d2d52 I------ 1 perm 1f010000 193 302d2d52 I------ 1 perm 1f010000 0 0 asymmetri Fedora kernel signing key: d69a84e6bce3d216b979e9505b3e3ef9a7118079: X509.RSA a7118079 []
194 ... 194 ...
195 195
196 Beyond the public key generated specifically f 196 Beyond the public key generated specifically for module signing, additional
197 trusted certificates can be provided in a PEM- 197 trusted certificates can be provided in a PEM-encoded file referenced by the
198 ``CONFIG_SYSTEM_TRUSTED_KEYS`` configuration o 198 ``CONFIG_SYSTEM_TRUSTED_KEYS`` configuration option.
199 199
200 Further, the architecture code may take public 200 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 201 add those in also (e.g. from the UEFI key database).
202 202
203 Finally, it is possible to add additional publ 203 Finally, it is possible to add additional public keys by doing::
204 204
205 keyctl padd asymmetric "" [.builtin_tr 205 keyctl padd asymmetric "" [.builtin_trusted_keys-ID] <[key-file]
206 206
207 e.g.:: 207 e.g.::
208 208
209 keyctl padd asymmetric "" 0x223c7853 < 209 keyctl padd asymmetric "" 0x223c7853 <my_public_key.x509
210 210
211 Note, however, that the kernel will only permi 211 Note, however, that the kernel will only permit keys to be added to
212 ``.builtin_trusted_keys`` **if** the new key's 212 ``.builtin_trusted_keys`` **if** the new key's X.509 wrapper is validly signed by a key
213 that is already resident in the ``.builtin_tru 213 that is already resident in the ``.builtin_trusted_keys`` at the time the key was added.
214 214
215 215
216 ======================== 216 ========================
217 Manually signing modules 217 Manually signing modules
218 ======================== 218 ========================
219 219
220 To manually sign a module, use the scripts/sig 220 To manually sign a module, use the scripts/sign-file tool available in
221 the Linux kernel source tree. The script requ 221 the Linux kernel source tree. The script requires 4 arguments:
222 222
223 1. The hash algorithm (e.g., sha256) 223 1. The hash algorithm (e.g., sha256)
224 2. The private key filename or PKCS#1 224 2. The private key filename or PKCS#11 URI
225 3. The public key filename 225 3. The public key filename
226 4. The kernel module to be signed 226 4. The kernel module to be signed
227 227
228 The following is an example to sign a kernel m 228 The following is an example to sign a kernel module::
229 229
230 scripts/sign-file sha512 kernel-signke 230 scripts/sign-file sha512 kernel-signkey.priv \
231 kernel-signkey.x509 module.ko 231 kernel-signkey.x509 module.ko
232 232
233 The hash algorithm used does not have to match 233 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 234 doesn't, you should make sure that hash algorithm is either built into the
235 kernel or can be loaded without requiring itse 235 kernel or can be loaded without requiring itself.
236 236
237 If the private key requires a passphrase or PI 237 If the private key requires a passphrase or PIN, it can be provided in the
238 $KBUILD_SIGN_PIN environment variable. 238 $KBUILD_SIGN_PIN environment variable.
239 239
240 240
241 ============================ 241 ============================
242 Signed modules and stripping 242 Signed modules and stripping
243 ============================ 243 ============================
244 244
245 A signed module has a digital signature simply 245 A signed module has a digital signature simply appended at the end. The string
246 ``~Module signature appended~.`` at the end of 246 ``~Module signature appended~.`` at the end of the module's file confirms that a
247 signature is present but it does not confirm t 247 signature is present but it does not confirm that the signature is valid!
248 248
249 Signed modules are BRITTLE as the signature is 249 Signed modules are BRITTLE as the signature is outside of the defined ELF
250 container. Thus they MAY NOT be stripped once 250 container. Thus they MAY NOT be stripped once the signature is computed and
251 attached. Note the entire module is the signe 251 attached. Note the entire module is the signed payload, including any and all
252 debug information present at the time of signi 252 debug information present at the time of signing.
253 253
254 254
255 ====================== 255 ======================
256 Loading signed modules 256 Loading signed modules
257 ====================== 257 ======================
258 258
259 Modules are loaded with insmod, modprobe, ``in 259 Modules are loaded with insmod, modprobe, ``init_module()`` or
260 ``finit_module()``, exactly as for unsigned mo 260 ``finit_module()``, exactly as for unsigned modules as no processing is
261 done in userspace. The signature checking is 261 done in userspace. The signature checking is all done within the kernel.
262 262
263 263
264 ========================================= 264 =========================================
265 Non-valid signatures and unsigned modules 265 Non-valid signatures and unsigned modules
266 ========================================= 266 =========================================
267 267
268 If ``CONFIG_MODULE_SIG_FORCE`` is enabled or m 268 If ``CONFIG_MODULE_SIG_FORCE`` is enabled or module.sig_enforce=1 is supplied on
269 the kernel command line, the kernel will only 269 the kernel command line, the kernel will only load validly signed modules
270 for which it has a public key. Otherwise, it 270 for which it has a public key. Otherwise, it will also load modules that are
271 unsigned. Any module for which the kernel ha 271 unsigned. Any module for which the kernel has a key, but which proves to have
272 a signature mismatch will not be permitted to 272 a signature mismatch will not be permitted to load.
273 273
274 Any module that has an unparsable signature wi 274 Any module that has an unparsable signature will be rejected.
275 275
276 276
277 ========================================= 277 =========================================
278 Administering/protecting the private key 278 Administering/protecting the private key
279 ========================================= 279 =========================================
280 280
281 Since the private key is used to sign modules, 281 Since the private key is used to sign modules, viruses and malware could use
282 the private key to sign modules and compromise 282 the private key to sign modules and compromise the operating system. The
283 private key must be either destroyed or moved 283 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. 284 in the root node of the kernel source tree.
285 285
286 If you use the same private key to sign module 286 If you use the same private key to sign modules for multiple kernel
287 configurations, you must ensure that the modul 287 configurations, you must ensure that the module version information is
288 sufficient to prevent loading a module into a 288 sufficient to prevent loading a module into a different kernel. Either
289 set ``CONFIG_MODVERSIONS=y`` or ensure that ea 289 set ``CONFIG_MODVERSIONS=y`` or ensure that each configuration has a different
290 kernel release string by changing ``EXTRAVERSI 290 kernel release string by changing ``EXTRAVERSION`` or ``CONFIG_LOCALVERSION``.
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