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James Solner3cafea32013-11-06 12:53:36 -06001 ==============================
2 KERNEL MODULE SIGNING FACILITY
3 ==============================
4
5CONTENTS
6
7 - Overview.
8 - Configuring module signing.
9 - Generating signing keys.
10 - Public keys in the kernel.
11 - Manually signing modules.
12 - Signed modules and stripping.
13 - Loading signed modules.
14 - Non-valid signatures and unsigned modules.
15 - Administering/protecting the private key.
16
17
18========
19OVERVIEW
20========
21
22The kernel module signing facility cryptographically signs modules during
23installation and then checks the signature upon loading the module. This
24allows increased kernel security by disallowing the loading of unsigned modules
25or modules signed with an invalid key. Module signing increases security by
26making it harder to load a malicious module into the kernel. The module
27signature checking is done by the kernel so that it is not necessary to have
28trusted userspace bits.
29
30This facility uses X.509 ITU-T standard certificates to encode the public keys
31involved. The signatures are not themselves encoded in any industrial standard
32type. The facility currently only supports the RSA public key encryption
33standard (though it is pluggable and permits others to be used). The possible
34hash algorithms that can be used are SHA-1, SHA-224, SHA-256, SHA-384, and
35SHA-512 (the algorithm is selected by data in the signature).
36
37
38==========================
39CONFIGURING MODULE SIGNING
40==========================
41
42The module signing facility is enabled by going to the "Enable Loadable Module
43Support" section of the kernel configuration and turning on
44
45 CONFIG_MODULE_SIG "Module signature verification"
46
47This has a number of options available:
48
49 (1) "Require modules to be validly signed" (CONFIG_MODULE_SIG_FORCE)
50
51 This specifies how the kernel should deal with a module that has a
52 signature for which the key is not known or a module that is unsigned.
53
54 If this is off (ie. "permissive"), then modules for which the key is not
55 available and modules that are unsigned are permitted, but the kernel will
Mathieu Desnoyers66cc69e2014-03-13 12:11:30 +103056 be marked as being tainted, and the concerned modules will be marked as
Rusty Russell57673c22014-03-31 14:39:57 +103057 tainted, shown with the character 'E'.
James Solner3cafea32013-11-06 12:53:36 -060058
59 If this is on (ie. "restrictive"), only modules that have a valid
60 signature that can be verified by a public key in the kernel's possession
61 will be loaded. All other modules will generate an error.
62
63 Irrespective of the setting here, if the module has a signature block that
64 cannot be parsed, it will be rejected out of hand.
65
66
67 (2) "Automatically sign all modules" (CONFIG_MODULE_SIG_ALL)
68
69 If this is on then modules will be automatically signed during the
70 modules_install phase of a build. If this is off, then the modules must
71 be signed manually using:
72
73 scripts/sign-file
74
75
76 (3) "Which hash algorithm should modules be signed with?"
77
78 This presents a choice of which hash algorithm the installation phase will
79 sign the modules with:
80
Paul Bolle7df24822014-02-12 10:28:05 +010081 CONFIG_MODULE_SIG_SHA1 "Sign modules with SHA-1"
82 CONFIG_MODULE_SIG_SHA224 "Sign modules with SHA-224"
83 CONFIG_MODULE_SIG_SHA256 "Sign modules with SHA-256"
84 CONFIG_MODULE_SIG_SHA384 "Sign modules with SHA-384"
85 CONFIG_MODULE_SIG_SHA512 "Sign modules with SHA-512"
James Solner3cafea32013-11-06 12:53:36 -060086
87 The algorithm selected here will also be built into the kernel (rather
88 than being a module) so that modules signed with that algorithm can have
89 their signatures checked without causing a dependency loop.
90
David Woodhouse99d27b12015-07-20 21:16:31 +010091
David Woodhouse19e91b62015-07-20 21:16:29 +010092 (4) "File name or PKCS#11 URI of module signing key" (CONFIG_MODULE_SIG_KEY)
93
94 Setting this option to something other than its default of
David Howellscfc411e2015-08-14 15:20:41 +010095 "certs/signing_key.pem" will disable the autogeneration of signing keys
96 and allow the kernel modules to be signed with a key of your choosing.
97 The string provided should identify a file containing both a private key
98 and its corresponding X.509 certificate in PEM form, or — on systems where
99 the OpenSSL ENGINE_pkcs11 is functional — a PKCS#11 URI as defined by
100 RFC7512. In the latter case, the PKCS#11 URI should reference both a
101 certificate and a private key.
David Woodhouse19e91b62015-07-20 21:16:29 +0100102
103 If the PEM file containing the private key is encrypted, or if the
104 PKCS#11 token requries a PIN, this can be provided at build time by
105 means of the KBUILD_SIGN_PIN variable.
106
James Solner3cafea32013-11-06 12:53:36 -0600107
David Woodhouse99d27b12015-07-20 21:16:31 +0100108 (5) "Additional X.509 keys for default system keyring" (CONFIG_SYSTEM_TRUSTED_KEYS)
109
110 This option can be set to the filename of a PEM-encoded file containing
111 additional certificates which will be included in the system keyring by
112 default.
113
David Howells228c37f2015-08-11 12:38:54 +0100114Note that enabling module signing adds a dependency on the OpenSSL devel
115packages to the kernel build processes for the tool that does the signing.
116
David Woodhouse99d27b12015-07-20 21:16:31 +0100117
James Solner3cafea32013-11-06 12:53:36 -0600118=======================
119GENERATING SIGNING KEYS
120=======================
121
122Cryptographic keypairs are required to generate and check signatures. A
123private key is used to generate a signature and the corresponding public key is
124used to check it. The private key is only needed during the build, after which
125it can be deleted or stored securely. The public key gets built into the
126kernel so that it can be used to check the signatures as the modules are
127loaded.
128
David Woodhouse19e91b62015-07-20 21:16:29 +0100129Under normal conditions, when CONFIG_MODULE_SIG_KEY is unchanged from its
David Woodhousefb117942015-07-20 21:16:30 +0100130default, the kernel build will automatically generate a new keypair using
131openssl if one does not exist in the file:
James Solner3cafea32013-11-06 12:53:36 -0600132
David Howellscfc411e2015-08-14 15:20:41 +0100133 certs/signing_key.pem
James Solner3cafea32013-11-06 12:53:36 -0600134
135during the building of vmlinux (the public part of the key needs to be built
136into vmlinux) using parameters in the:
137
David Howellscfc411e2015-08-14 15:20:41 +0100138 certs/x509.genkey
James Solner3cafea32013-11-06 12:53:36 -0600139
140file (which is also generated if it does not already exist).
141
142It is strongly recommended that you provide your own x509.genkey file.
143
144Most notably, in the x509.genkey file, the req_distinguished_name section
145should be altered from the default:
146
147 [ req_distinguished_name ]
David Howells9c4249c2015-04-30 14:58:43 +0100148 #O = Unspecified company
149 CN = Build time autogenerated kernel key
150 #emailAddress = unspecified.user@unspecified.company
James Solner3cafea32013-11-06 12:53:36 -0600151
152The generated RSA key size can also be set with:
153
154 [ req ]
155 default_bits = 4096
156
157
158It is also possible to manually generate the key private/public files using the
159x509.genkey key generation configuration file in the root node of the Linux
160kernel sources tree and the openssl command. The following is an example to
161generate the public/private key files:
162
163 openssl req -new -nodes -utf8 -sha256 -days 36500 -batch -x509 \
David Woodhouse19e91b62015-07-20 21:16:29 +0100164 -config x509.genkey -outform PEM -out kernel_key.pem \
165 -keyout kernel_key.pem
166
167The full pathname for the resulting kernel_key.pem file can then be specified
168in the CONFIG_MODULE_SIG_KEY option, and the certificate and key therein will
169be used instead of an autogenerated keypair.
James Solner3cafea32013-11-06 12:53:36 -0600170
171
172=========================
173PUBLIC KEYS IN THE KERNEL
174=========================
175
176The kernel contains a ring of public keys that can be viewed by root. They're
177in a keyring called ".system_keyring" that can be seen by:
178
179 [root@deneb ~]# cat /proc/keys
180 ...
181 223c7853 I------ 1 perm 1f030000 0 0 keyring .system_keyring: 1
182 302d2d52 I------ 1 perm 1f010000 0 0 asymmetri Fedora kernel signing key: d69a84e6bce3d216b979e9505b3e3ef9a7118079: X509.RSA a7118079 []
183 ...
184
David Woodhouse99d27b12015-07-20 21:16:31 +0100185Beyond the public key generated specifically for module signing, additional
186trusted certificates can be provided in a PEM-encoded file referenced by the
187CONFIG_SYSTEM_TRUSTED_KEYS configuration option.
James Solner3cafea32013-11-06 12:53:36 -0600188
189Further, the architecture code may take public keys from a hardware store and
190add those in also (e.g. from the UEFI key database).
191
192Finally, it is possible to add additional public keys by doing:
193
194 keyctl padd asymmetric "" [.system_keyring-ID] <[key-file]
195
196e.g.:
197
198 keyctl padd asymmetric "" 0x223c7853 <my_public_key.x509
199
200Note, however, that the kernel will only permit keys to be added to
201.system_keyring _if_ the new key's X.509 wrapper is validly signed by a key
202that is already resident in the .system_keyring at the time the key was added.
203
204
205=========================
206MANUALLY SIGNING MODULES
207=========================
208
209To manually sign a module, use the scripts/sign-file tool available in
210the Linux kernel source tree. The script requires 4 arguments:
211
212 1. The hash algorithm (e.g., sha256)
David Woodhouse19e91b62015-07-20 21:16:29 +0100213 2. The private key filename or PKCS#11 URI
James Solner3cafea32013-11-06 12:53:36 -0600214 3. The public key filename
215 4. The kernel module to be signed
216
217The following is an example to sign a kernel module:
218
219 scripts/sign-file sha512 kernel-signkey.priv \
220 kernel-signkey.x509 module.ko
221
222The hash algorithm used does not have to match the one configured, but if it
223doesn't, you should make sure that hash algorithm is either built into the
224kernel or can be loaded without requiring itself.
225
David Woodhouseaf1eb292015-07-20 21:16:28 +0100226If the private key requires a passphrase or PIN, it can be provided in the
227$KBUILD_SIGN_PIN environment variable.
228
James Solner3cafea32013-11-06 12:53:36 -0600229
230============================
231SIGNED MODULES AND STRIPPING
232============================
233
234A signed module has a digital signature simply appended at the end. The string
235"~Module signature appended~." at the end of the module's file confirms that a
236signature is present but it does not confirm that the signature is valid!
237
238Signed modules are BRITTLE as the signature is outside of the defined ELF
239container. Thus they MAY NOT be stripped once the signature is computed and
240attached. Note the entire module is the signed payload, including any and all
241debug information present at the time of signing.
242
243
244======================
245LOADING SIGNED MODULES
246======================
247
248Modules are loaded with insmod, modprobe, init_module() or finit_module(),
249exactly as for unsigned modules as no processing is done in userspace. The
250signature checking is all done within the kernel.
251
252
253=========================================
254NON-VALID SIGNATURES AND UNSIGNED MODULES
255=========================================
256
257If CONFIG_MODULE_SIG_FORCE is enabled or enforcemodulesig=1 is supplied on
258the kernel command line, the kernel will only load validly signed modules
259for which it has a public key. Otherwise, it will also load modules that are
260unsigned. Any module for which the kernel has a key, but which proves to have
261a signature mismatch will not be permitted to load.
262
263Any module that has an unparseable signature will be rejected.
264
265
266=========================================
267ADMINISTERING/PROTECTING THE PRIVATE KEY
268=========================================
269
270Since the private key is used to sign modules, viruses and malware could use
271the private key to sign modules and compromise the operating system. The
272private key must be either destroyed or moved to a secure location and not kept
273in the root node of the kernel source tree.