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Documentation / module-signing.txt




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Based on kernel version 3.13. Page generated on 2014-01-20 22:03 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.
57	
58	     If this is on (ie. "restrictive"), only modules that have a valid
59	     signature that can be verified by a public key in the kernel's possession
60	     will be loaded.  All other modules will generate an error.
61	
62	     Irrespective of the setting here, if the module has a signature block that
63	     cannot be parsed, it will be rejected out of hand.
64	
65	
66	 (2) "Automatically sign all modules" (CONFIG_MODULE_SIG_ALL)
67	
68	     If this is on then modules will be automatically signed during the
69	     modules_install phase of a build.  If this is off, then the modules must
70	     be signed manually using:
71	
72		scripts/sign-file
73	
74	
75	 (3) "Which hash algorithm should modules be signed with?"
76	
77	     This presents a choice of which hash algorithm the installation phase will
78	     sign the modules with:
79	
80		CONFIG_SIG_SHA1		"Sign modules with SHA-1"
81		CONFIG_SIG_SHA224	"Sign modules with SHA-224"
82		CONFIG_SIG_SHA256	"Sign modules with SHA-256"
83		CONFIG_SIG_SHA384	"Sign modules with SHA-384"
84		CONFIG_SIG_SHA512	"Sign modules with SHA-512"
85	
86	     The algorithm selected here will also be built into the kernel (rather
87	     than being a module) so that modules signed with that algorithm can have
88	     their signatures checked without causing a dependency loop.
89	
90	
91	=======================
92	GENERATING SIGNING KEYS
93	=======================
94	
95	Cryptographic keypairs are required to generate and check signatures.  A
96	private key is used to generate a signature and the corresponding public key is
97	used to check it.  The private key is only needed during the build, after which
98	it can be deleted or stored securely.  The public key gets built into the
99	kernel so that it can be used to check the signatures as the modules are
100	loaded.
101	
102	Under normal conditions, the kernel build will automatically generate a new
103	keypair using openssl if one does not exist in the files:
104	
105		signing_key.priv
106		signing_key.x509
107	
108	during the building of vmlinux (the public part of the key needs to be built
109	into vmlinux) using parameters in the:
110	
111		x509.genkey
112	
113	file (which is also generated if it does not already exist).
114	
115	It is strongly recommended that you provide your own x509.genkey file.
116	
117	Most notably, in the x509.genkey file, the req_distinguished_name section
118	should be altered from the default:
119	
120		[ req_distinguished_name ]
121		O = Magrathea
122		CN = Glacier signing key
123		emailAddress = slartibartfast@magrathea.h2g2
124	
125	The generated RSA key size can also be set with:
126	
127		[ req ]
128		default_bits = 4096
129	
130	
131	It is also possible to manually generate the key private/public files using the
132	x509.genkey key generation configuration file in the root node of the Linux
133	kernel sources tree and the openssl command.  The following is an example to
134	generate the public/private key files:
135	
136		openssl req -new -nodes -utf8 -sha256 -days 36500 -batch -x509 \
137		   -config x509.genkey -outform DER -out signing_key.x509 \
138		   -keyout signing_key.priv
139	
140	
141	=========================
142	PUBLIC KEYS IN THE KERNEL
143	=========================
144	
145	The kernel contains a ring of public keys that can be viewed by root.  They're
146	in a keyring called ".system_keyring" that can be seen by:
147	
148		[root@deneb ~]# cat /proc/keys
149		...
150		223c7853 I------     1 perm 1f030000     0     0 keyring   .system_keyring: 1
151		302d2d52 I------     1 perm 1f010000     0     0 asymmetri Fedora kernel signing key: d69a84e6bce3d216b979e9505b3e3ef9a7118079: X509.RSA a7118079 []
152		...
153	
154	Beyond the public key generated specifically for module signing, any file
155	placed in the kernel source root directory or the kernel build root directory
156	whose name is suffixed with ".x509" will be assumed to be an X.509 public key
157	and will be added to the keyring.
158	
159	Further, the architecture code may take public keys from a hardware store and
160	add those in also (e.g. from the UEFI key database).
161	
162	Finally, it is possible to add additional public keys by doing:
163	
164		keyctl padd asymmetric "" [.system_keyring-ID] <[key-file]
165	
166	e.g.:
167	
168		keyctl padd asymmetric "" 0x223c7853 <my_public_key.x509
169	
170	Note, however, that the kernel will only permit keys to be added to
171	.system_keyring _if_ the new key's X.509 wrapper is validly signed by a key
172	that is already resident in the .system_keyring at the time the key was added.
173	
174	
175	=========================
176	MANUALLY SIGNING MODULES
177	=========================
178	
179	To manually sign a module, use the scripts/sign-file tool available in
180	the Linux kernel source tree.  The script requires 4 arguments:
181	
182		1.  The hash algorithm (e.g., sha256)
183		2.  The private key filename
184		3.  The public key filename
185		4.  The kernel module to be signed
186	
187	The following is an example to sign a kernel module:
188	
189		scripts/sign-file sha512 kernel-signkey.priv \
190			kernel-signkey.x509 module.ko
191	
192	The hash algorithm used does not have to match the one configured, but if it
193	doesn't, you should make sure that hash algorithm is either built into the
194	kernel or can be loaded without requiring itself.
195	
196	
197	============================
198	SIGNED MODULES AND STRIPPING
199	============================
200	
201	A signed module has a digital signature simply appended at the end.  The string
202	"~Module signature appended~." at the end of the module's file confirms that a
203	signature is present but it does not confirm that the signature is valid!
204	
205	Signed modules are BRITTLE as the signature is outside of the defined ELF
206	container.  Thus they MAY NOT be stripped once the signature is computed and
207	attached.  Note the entire module is the signed payload, including any and all
208	debug information present at the time of signing.
209	
210	
211	======================
212	LOADING SIGNED MODULES
213	======================
214	
215	Modules are loaded with insmod, modprobe, init_module() or finit_module(),
216	exactly as for unsigned modules as no processing is done in userspace.  The
217	signature checking is all done within the kernel.
218	
219	
220	=========================================
221	NON-VALID SIGNATURES AND UNSIGNED MODULES
222	=========================================
223	
224	If CONFIG_MODULE_SIG_FORCE is enabled or enforcemodulesig=1 is supplied on
225	the kernel command line, the kernel will only load validly signed modules
226	for which it has a public key.   Otherwise, it will also load modules that are
227	unsigned.   Any module for which the kernel has a key, but which proves to have
228	a signature mismatch will not be permitted to load.
229	
230	Any module that has an unparseable signature will be rejected.
231	
232	
233	=========================================
234	ADMINISTERING/PROTECTING THE PRIVATE KEY
235	=========================================
236	
237	Since the private key is used to sign modules, viruses and malware could use
238	the private key to sign modules and compromise the operating system.  The
239	private key must be either destroyed or moved to a secure location and not kept
240	in the root node of the kernel source tree.
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