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1Kernel module signing facility
2------------------------------
3
4.. CONTENTS
5..
6.. - Overview.
7.. - Configuring module signing.
8.. - Generating signing keys.
9.. - Public keys in the kernel.
10.. - Manually signing modules.
11.. - Signed modules and stripping.
12.. - Loading signed modules.
13.. - Non-valid signatures and unsigned modules.
14.. - Administering/protecting the private key.
15
16
17========
18Overview
19========
20
21The kernel module signing facility cryptographically signs modules during
22installation and then checks the signature upon loading the module. This
23allows increased kernel security by disallowing the loading of unsigned modules
24or modules signed with an invalid key. Module signing increases security by
25making it harder to load a malicious module into the kernel. The module
26signature checking is done by the kernel so that it is not necessary to have
27trusted userspace bits.
28
29This facility uses X.509 ITU-T standard certificates to encode the public keys
30involved. The signatures are not themselves encoded in any industrial standard
31type. The built-in facility currently only supports the RSA & NIST P-384 ECDSA
32public key signing standard (though it is pluggable and permits others to be
33used). The possible hash algorithms that can be used are SHA-2 and SHA-3 of
34sizes 256, 384, and 512 (the algorithm is selected by data in the signature).
35
36
37==========================
38Configuring module signing
39==========================
40
41The module signing facility is enabled by going to the
42:menuselection:`Enable Loadable Module Support` section of
43the kernel configuration and turning on::
44
45 CONFIG_MODULE_SIG "Module signature verification"
46
47This has a number of options available:
48
49 (1) :menuselection:`Require modules to be validly signed`
50 (``CONFIG_MODULE_SIG_FORCE``)
51
52 This specifies how the kernel should deal with a module that has a
53 signature for which the key is not known or a module that is unsigned.
54
55 If this is off (ie. "permissive"), then modules for which the key is not
56 available and modules that are unsigned are permitted, but the kernel will
57 be marked as being tainted, and the concerned modules will be marked as
58 tainted, shown with the character 'E'.
59
60 If this is on (ie. "restrictive"), only modules that have a valid
61 signature that can be verified by a public key in the kernel's possession
62 will be loaded. All other modules will generate an error.
63
64 Irrespective of the setting here, if the module has a signature block that
65 cannot be parsed, it will be rejected out of hand.
66
67
68 (2) :menuselection:`Automatically sign all modules`
69 (``CONFIG_MODULE_SIG_ALL``)
70
71 If this is on then modules will be automatically signed during the
72 modules_install phase of a build. If this is off, then the modules must
73 be signed manually using::
74
75 scripts/sign-file
76
77
78 (3) :menuselection:`Which hash algorithm should modules be signed with?`
79
80 This presents a choice of which hash algorithm the installation phase will
81 sign the modules with:
82
83 =============================== ==========================================
84 ``CONFIG_MODULE_SIG_SHA256`` :menuselection:`Sign modules with SHA-256`
85 ``CONFIG_MODULE_SIG_SHA384`` :menuselection:`Sign modules with SHA-384`
86 ``CONFIG_MODULE_SIG_SHA512`` :menuselection:`Sign modules with SHA-512`
87 ``CONFIG_MODULE_SIG_SHA3_256`` :menuselection:`Sign modules with SHA3-256`
88 ``CONFIG_MODULE_SIG_SHA3_384`` :menuselection:`Sign modules with SHA3-384`
89 ``CONFIG_MODULE_SIG_SHA3_512`` :menuselection:`Sign modules with SHA3-512`
90 =============================== ==========================================
91
92 The algorithm selected here will also be built into the kernel (rather
93 than being a module) so that modules signed with that algorithm can have
94 their signatures checked without causing a dependency loop.
95
96
97 (4) :menuselection:`File name or PKCS#11 URI of module signing key`
98 (``CONFIG_MODULE_SIG_KEY``)
99
100 Setting this option to something other than its default of
101 ``certs/signing_key.pem`` will disable the autogeneration of signing keys
102 and allow the kernel modules to be signed with a key of your choosing.
103 The string provided should identify a file containing both a private key
104 and its corresponding X.509 certificate in PEM form, or — on systems where
105 the OpenSSL ENGINE_pkcs11 is functional — a PKCS#11 URI as defined by
106 RFC7512. In the latter case, the PKCS#11 URI should reference both a
107 certificate and a private key.
108
109 If the PEM file containing the private key is encrypted, or if the
110 PKCS#11 token requires a PIN, this can be provided at build time by
111 means of the ``KBUILD_SIGN_PIN`` variable.
112
113
114 (5) :menuselection:`Additional X.509 keys for default system keyring`
115 (``CONFIG_SYSTEM_TRUSTED_KEYS``)
116
117 This option can be set to the filename of a PEM-encoded file containing
118 additional certificates which will be included in the system keyring by
119 default.
120
121Note that enabling module signing adds a dependency on the OpenSSL devel
122packages to the kernel build processes for the tool that does the signing.
123
124
125=======================
126Generating signing keys
127=======================
128
129Cryptographic keypairs are required to generate and check signatures. A
130private key is used to generate a signature and the corresponding public key is
131used to check it. The private key is only needed during the build, after which
132it can be deleted or stored securely. The public key gets built into the
133kernel so that it can be used to check the signatures as the modules are
134loaded.
135
136Under normal conditions, when ``CONFIG_MODULE_SIG_KEY`` is unchanged from its
137default, the kernel build will automatically generate a new keypair using
138openssl if one does not exist in the file::
139
140 certs/signing_key.pem
141
142during the building of vmlinux (the public part of the key needs to be built
143into vmlinux) using parameters in the::
144
145 certs/x509.genkey
146
147file (which is also generated if it does not already exist).
148
149One can select between RSA (``MODULE_SIG_KEY_TYPE_RSA``) and ECDSA
150(``MODULE_SIG_KEY_TYPE_ECDSA``) to generate either RSA 4k or NIST
151P-384 keypair.
152
153It is strongly recommended that you provide your own x509.genkey file.
154
155Most notably, in the x509.genkey file, the req_distinguished_name section
156should be altered from the default::
157
158 [ req_distinguished_name ]
159 #O = Unspecified company
160 CN = Build time autogenerated kernel key
161 #emailAddress = unspecified.user@unspecified.company
162
163The generated RSA key size can also be set with::
164
165 [ req ]
166 default_bits = 4096
167
168
169It is also possible to manually generate the key private/public files using the
170x509.genkey key generation configuration file in the root node of the Linux
171kernel sources tree and the openssl command. The following is an example to
172generate the public/private key files::
173
174 openssl req -new -nodes -utf8 -sha256 -days 36500 -batch -x509 \
175 -config x509.genkey -outform PEM -out kernel_key.pem \
176 -keyout kernel_key.pem
177
178The full pathname for the resulting kernel_key.pem file can then be specified
179in the ``CONFIG_MODULE_SIG_KEY`` option, and the certificate and key therein will
180be used instead of an autogenerated keypair.
181
182
183=========================
184Public keys in the kernel
185=========================
186
187The kernel contains a ring of public keys that can be viewed by root. They're
188in a keyring called ".builtin_trusted_keys" that can be seen by::
189
190 [root@deneb ~]# cat /proc/keys
191 ...
192 223c7853 I------ 1 perm 1f030000 0 0 keyring .builtin_trusted_keys: 1
193 302d2d52 I------ 1 perm 1f010000 0 0 asymmetri Fedora kernel signing key: d69a84e6bce3d216b979e9505b3e3ef9a7118079: X509.RSA a7118079 []
194 ...
195
196Beyond the public key generated specifically for module signing, additional
197trusted certificates can be provided in a PEM-encoded file referenced by the
198``CONFIG_SYSTEM_TRUSTED_KEYS`` configuration option.
199
200Further, the architecture code may take public keys from a hardware store and
201add those in also (e.g. from the UEFI key database).
202
203Finally, it is possible to add additional public keys by doing::
204
205 keyctl padd asymmetric "" [.builtin_trusted_keys-ID] <[key-file]
206
207e.g.::
208
209 keyctl padd asymmetric "" 0x223c7853 <my_public_key.x509
210
211Note, however, that the kernel will only permit keys to be added to
212``.builtin_trusted_keys`` **if** the new key's X.509 wrapper is validly signed by a key
213that is already resident in the ``.builtin_trusted_keys`` at the time the key was added.
214
215
216========================
217Manually signing modules
218========================
219
220To manually sign a module, use the scripts/sign-file tool available in
221the Linux kernel source tree. The script requires 4 arguments:
222
223 1. The hash algorithm (e.g., sha256)
224 2. The private key filename or PKCS#11 URI
225 3. The public key filename
226 4. The kernel module to be signed
227
228The following is an example to sign a kernel module::
229
230 scripts/sign-file sha512 kernel-signkey.priv \
231 kernel-signkey.x509 module.ko
232
233The hash algorithm used does not have to match the one configured, but if it
234doesn't, you should make sure that hash algorithm is either built into the
235kernel or can be loaded without requiring itself.
236
237If the private key requires a passphrase or PIN, it can be provided in the
238$KBUILD_SIGN_PIN environment variable.
239
240
241============================
242Signed modules and stripping
243============================
244
245A signed module has a digital signature simply appended at the end. The string
246``~Module signature appended~.`` at the end of the module's file confirms that a
247signature is present but it does not confirm that the signature is valid!
248
249Signed modules are BRITTLE as the signature is outside of the defined ELF
250container. Thus they MAY NOT be stripped once the signature is computed and
251attached. Note the entire module is the signed payload, including any and all
252debug information present at the time of signing.
253
254
255======================
256Loading signed modules
257======================
258
259Modules are loaded with insmod, modprobe, ``init_module()`` or
260``finit_module()``, exactly as for unsigned modules as no processing is
261done in userspace. The signature checking is all done within the kernel.
262
263
264=========================================
265Non-valid signatures and unsigned modules
266=========================================
267
268If ``CONFIG_MODULE_SIG_FORCE`` is enabled or module.sig_enforce=1 is supplied on
269the kernel command line, the kernel will only load validly signed modules
270for which it has a public key. Otherwise, it will also load modules that are
271unsigned. Any module for which the kernel has a key, but which proves to have
272a signature mismatch will not be permitted to load.
273
274Any module that has an unparsable signature will be rejected.
275
276
277=========================================
278Administering/protecting the private key
279=========================================
280
281Since the private key is used to sign modules, viruses and malware could use
282the private key to sign modules and compromise the operating system. The
283private key must be either destroyed or moved to a secure location and not kept
284in the root node of the kernel source tree.
285
286If you use the same private key to sign modules for multiple kernel
287configurations, you must ensure that the module version information is
288sufficient to prevent loading a module into a different kernel. Either
289set ``CONFIG_MODVERSIONS=y`` or ensure that each configuration has a different
290kernel release string by changing ``EXTRAVERSION`` or ``CONFIG_LOCALVERSION``.