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Documentation / module-signing.txt




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Based on kernel version 3.16. Page generated on 2014-08-06 21:40 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	=======================
93	GENERATING SIGNING KEYS
94	=======================
95	
96	Cryptographic keypairs are required to generate and check signatures.  A
97	private key is used to generate a signature and the corresponding public key is
98	used to check it.  The private key is only needed during the build, after which
99	it can be deleted or stored securely.  The public key gets built into the
100	kernel so that it can be used to check the signatures as the modules are
101	loaded.
102	
103	Under normal conditions, the kernel build will automatically generate a new
104	keypair using openssl if one does not exist in the files:
105	
106		signing_key.priv
107		signing_key.x509
108	
109	during the building of vmlinux (the public part of the key needs to be built
110	into vmlinux) using parameters in the:
111	
112		x509.genkey
113	
114	file (which is also generated if it does not already exist).
115	
116	It is strongly recommended that you provide your own x509.genkey file.
117	
118	Most notably, in the x509.genkey file, the req_distinguished_name section
119	should be altered from the default:
120	
121		[ req_distinguished_name ]
122		O = Magrathea
123		CN = Glacier signing key
124		emailAddress = slartibartfast@magrathea.h2g2
125	
126	The generated RSA key size can also be set with:
127	
128		[ req ]
129		default_bits = 4096
130	
131	
132	It is also possible to manually generate the key private/public files using the
133	x509.genkey key generation configuration file in the root node of the Linux
134	kernel sources tree and the openssl command.  The following is an example to
135	generate the public/private key files:
136	
137		openssl req -new -nodes -utf8 -sha256 -days 36500 -batch -x509 \
138		   -config x509.genkey -outform DER -out signing_key.x509 \
139		   -keyout signing_key.priv
140	
141	
142	=========================
143	PUBLIC KEYS IN THE KERNEL
144	=========================
145	
146	The kernel contains a ring of public keys that can be viewed by root.  They're
147	in a keyring called ".system_keyring" that can be seen by:
148	
149		[root@deneb ~]# cat /proc/keys
150		...
151		223c7853 I------     1 perm 1f030000     0     0 keyring   .system_keyring: 1
152		302d2d52 I------     1 perm 1f010000     0     0 asymmetri Fedora kernel signing key: d69a84e6bce3d216b979e9505b3e3ef9a7118079: X509.RSA a7118079 []
153		...
154	
155	Beyond the public key generated specifically for module signing, any file
156	placed in the kernel source root directory or the kernel build root directory
157	whose name is suffixed with ".x509" will be assumed to be an X.509 public key
158	and will be added to the keyring.
159	
160	Further, the architecture code may take public keys from a hardware store and
161	add those in also (e.g. from the UEFI key database).
162	
163	Finally, it is possible to add additional public keys by doing:
164	
165		keyctl padd asymmetric "" [.system_keyring-ID] <[key-file]
166	
167	e.g.:
168	
169		keyctl padd asymmetric "" 0x223c7853 <my_public_key.x509
170	
171	Note, however, that the kernel will only permit keys to be added to
172	.system_keyring _if_ the new key's X.509 wrapper is validly signed by a key
173	that is already resident in the .system_keyring at the time the key was added.
174	
175	
176	=========================
177	MANUALLY SIGNING MODULES
178	=========================
179	
180	To manually sign a module, use the scripts/sign-file tool available in
181	the Linux kernel source tree.  The script requires 4 arguments:
182	
183		1.  The hash algorithm (e.g., sha256)
184		2.  The private key filename
185		3.  The public key filename
186		4.  The kernel module to be signed
187	
188	The following is an example to sign a kernel module:
189	
190		scripts/sign-file sha512 kernel-signkey.priv \
191			kernel-signkey.x509 module.ko
192	
193	The hash algorithm used does not have to match the one configured, but if it
194	doesn't, you should make sure that hash algorithm is either built into the
195	kernel or can be loaded without requiring itself.
196	
197	
198	============================
199	SIGNED MODULES AND STRIPPING
200	============================
201	
202	A signed module has a digital signature simply appended at the end.  The string
203	"~Module signature appended~." at the end of the module's file confirms that a
204	signature is present but it does not confirm that the signature is valid!
205	
206	Signed modules are BRITTLE as the signature is outside of the defined ELF
207	container.  Thus they MAY NOT be stripped once the signature is computed and
208	attached.  Note the entire module is the signed payload, including any and all
209	debug information present at the time of signing.
210	
211	
212	======================
213	LOADING SIGNED MODULES
214	======================
215	
216	Modules are loaded with insmod, modprobe, init_module() or finit_module(),
217	exactly as for unsigned modules as no processing is done in userspace.  The
218	signature checking is all done within the kernel.
219	
220	
221	=========================================
222	NON-VALID SIGNATURES AND UNSIGNED MODULES
223	=========================================
224	
225	If CONFIG_MODULE_SIG_FORCE is enabled or enforcemodulesig=1 is supplied on
226	the kernel command line, the kernel will only load validly signed modules
227	for which it has a public key.   Otherwise, it will also load modules that are
228	unsigned.   Any module for which the kernel has a key, but which proves to have
229	a signature mismatch will not be permitted to load.
230	
231	Any module that has an unparseable signature will be rejected.
232	
233	
234	=========================================
235	ADMINISTERING/PROTECTING THE PRIVATE KEY
236	=========================================
237	
238	Since the private key is used to sign modules, viruses and malware could use
239	the private key to sign modules and compromise the operating system.  The
240	private key must be either destroyed or moved to a secure location and not kept
241	in the root node of the kernel source tree.
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