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Based on kernel version 4.3. Page generated on 2015-11-02 12:50 EST.

1				==============================
2				KERNEL MODULE SIGNING FACILITY
3				==============================
4	
5	CONTENTS
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	========
19	OVERVIEW
20	========
21	
22	The kernel module signing facility cryptographically signs modules during
23	installation and then checks the signature upon loading the module.  This
24	allows increased kernel security by disallowing the loading of unsigned modules
25	or modules signed with an invalid key.  Module signing increases security by
26	making it harder to load a malicious module into the kernel.  The module
27	signature checking is done by the kernel so that it is not necessary to have
28	trusted userspace bits.
29	
30	This facility uses X.509 ITU-T standard certificates to encode the public keys
31	involved.  The signatures are not themselves encoded in any industrial standard
32	type.  The facility currently only supports the RSA public key encryption
33	standard (though it is pluggable and permits others to be used).  The possible
34	hash algorithms that can be used are SHA-1, SHA-224, SHA-256, SHA-384, and
35	SHA-512 (the algorithm is selected by data in the signature).
36	
37	
38	==========================
39	CONFIGURING MODULE SIGNING
40	==========================
41	
42	The module signing facility is enabled by going to the "Enable Loadable Module
43	Support" section of the kernel configuration and turning on
44	
45		CONFIG_MODULE_SIG	"Module signature verification"
46	
47	This 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
56	     be marked as being tainted, and the concerned modules will be marked as
57	     tainted, shown with the character 'E'.
58	
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	
81		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"
86	
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	
91	
92	 (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
95	     "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.
102	
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	
107	
108	 (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	
114	Note that enabling module signing adds a dependency on the OpenSSL devel
115	packages to the kernel build processes for the tool that does the signing.
116	
117	
118	=======================
119	GENERATING SIGNING KEYS
120	=======================
121	
122	Cryptographic keypairs are required to generate and check signatures.  A
123	private key is used to generate a signature and the corresponding public key is
124	used to check it.  The private key is only needed during the build, after which
125	it can be deleted or stored securely.  The public key gets built into the
126	kernel so that it can be used to check the signatures as the modules are
127	loaded.
128	
129	Under normal conditions, when CONFIG_MODULE_SIG_KEY is unchanged from its
130	default, the kernel build will automatically generate a new keypair using
131	openssl if one does not exist in the file:
132	
133		certs/signing_key.pem
134	
135	during the building of vmlinux (the public part of the key needs to be built
136	into vmlinux) using parameters in the:
137	
138		certs/x509.genkey
139	
140	file (which is also generated if it does not already exist).
141	
142	It is strongly recommended that you provide your own x509.genkey file.
143	
144	Most notably, in the x509.genkey file, the req_distinguished_name section
145	should be altered from the default:
146	
147		[ req_distinguished_name ]
148		#O = Unspecified company
149		CN = Build time autogenerated kernel key
150		#emailAddress = unspecified.user@unspecified.company
151	
152	The generated RSA key size can also be set with:
153	
154		[ req ]
155		default_bits = 4096
156	
157	
158	It is also possible to manually generate the key private/public files using the
159	x509.genkey key generation configuration file in the root node of the Linux
160	kernel sources tree and the openssl command.  The following is an example to
161	generate the public/private key files:
162	
163		openssl req -new -nodes -utf8 -sha256 -days 36500 -batch -x509 \
164		   -config x509.genkey -outform PEM -out kernel_key.pem \
165		   -keyout kernel_key.pem
166	
167	The full pathname for the resulting kernel_key.pem file can then be specified
168	in the CONFIG_MODULE_SIG_KEY option, and the certificate and key therein will
169	be used instead of an autogenerated keypair.
170	
171	
172	=========================
173	PUBLIC KEYS IN THE KERNEL
174	=========================
175	
176	The kernel contains a ring of public keys that can be viewed by root.  They're
177	in 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	
185	Beyond the public key generated specifically for module signing, additional
186	trusted certificates can be provided in a PEM-encoded file referenced by the
187	CONFIG_SYSTEM_TRUSTED_KEYS configuration option.
188	
189	Further, the architecture code may take public keys from a hardware store and
190	add those in also (e.g. from the UEFI key database).
191	
192	Finally, it is possible to add additional public keys by doing:
193	
194		keyctl padd asymmetric "" [.system_keyring-ID] <[key-file]
195	
196	e.g.:
197	
198		keyctl padd asymmetric "" 0x223c7853 <my_public_key.x509
199	
200	Note, 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
202	that is already resident in the .system_keyring at the time the key was added.
203	
204	
205	=========================
206	MANUALLY SIGNING MODULES
207	=========================
208	
209	To manually sign a module, use the scripts/sign-file tool available in
210	the Linux kernel source tree.  The script requires 4 arguments:
211	
212		1.  The hash algorithm (e.g., sha256)
213		2.  The private key filename or PKCS#11 URI
214		3.  The public key filename
215		4.  The kernel module to be signed
216	
217	The 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	
222	The hash algorithm used does not have to match the one configured, but if it
223	doesn't, you should make sure that hash algorithm is either built into the
224	kernel or can be loaded without requiring itself.
225	
226	If the private key requires a passphrase or PIN, it can be provided in the
227	$KBUILD_SIGN_PIN environment variable.
228	
229	
230	============================
231	SIGNED MODULES AND STRIPPING
232	============================
233	
234	A 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
236	signature is present but it does not confirm that the signature is valid!
237	
238	Signed modules are BRITTLE as the signature is outside of the defined ELF
239	container.  Thus they MAY NOT be stripped once the signature is computed and
240	attached.  Note the entire module is the signed payload, including any and all
241	debug information present at the time of signing.
242	
243	
244	======================
245	LOADING SIGNED MODULES
246	======================
247	
248	Modules are loaded with insmod, modprobe, init_module() or finit_module(),
249	exactly as for unsigned modules as no processing is done in userspace.  The
250	signature checking is all done within the kernel.
251	
252	
253	=========================================
254	NON-VALID SIGNATURES AND UNSIGNED MODULES
255	=========================================
256	
257	If CONFIG_MODULE_SIG_FORCE is enabled or enforcemodulesig=1 is supplied on
258	the kernel command line, the kernel will only load validly signed modules
259	for which it has a public key.   Otherwise, it will also load modules that are
260	unsigned.   Any module for which the kernel has a key, but which proves to have
261	a signature mismatch will not be permitted to load.
262	
263	Any module that has an unparseable signature will be rejected.
264	
265	
266	=========================================
267	ADMINISTERING/PROTECTING THE PRIVATE KEY
268	=========================================
269	
270	Since the private key is used to sign modules, viruses and malware could use
271	the private key to sign modules and compromise the operating system.  The
272	private key must be either destroyed or moved to a secure location and not kept
273	in the root node of the kernel source tree.
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