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 facility currently only supports the RSA public key encryption
32standard (though it is pluggable and permits others to be used).  The possible
33hash algorithms that can be used are SHA-1, SHA-224, SHA-256, SHA-384, and
34SHA-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_SHA1``	:menuselection:`Sign modules with SHA-1`
85	``CONFIG_MODULE_SIG_SHA224``	:menuselection:`Sign modules with SHA-224`
86	``CONFIG_MODULE_SIG_SHA256``	:menuselection:`Sign modules with SHA-256`
87	``CONFIG_MODULE_SIG_SHA384``	:menuselection:`Sign modules with SHA-384`
88	``CONFIG_MODULE_SIG_SHA512``	:menuselection:`Sign modules with SHA-512`
89        =============================== ==========================================
90
91     The algorithm selected here will also be built into the kernel (rather
92     than being a module) so that modules signed with that algorithm can have
93     their signatures checked without causing a dependency loop.
94
95
96 (4) :menuselection:`File name or PKCS#11 URI of module signing key`
97     (``CONFIG_MODULE_SIG_KEY``)
98
99     Setting this option to something other than its default of
100     ``certs/signing_key.pem`` will disable the autogeneration of signing keys
101     and allow the kernel modules to be signed with a key of your choosing.
102     The string provided should identify a file containing both a private key
103     and its corresponding X.509 certificate in PEM form, or — on systems where
104     the OpenSSL ENGINE_pkcs11 is functional — a PKCS#11 URI as defined by
105     RFC7512. In the latter case, the PKCS#11 URI should reference both a
106     certificate and a private key.
107
108     If the PEM file containing the private key is encrypted, or if the
109     PKCS#11 token requries a PIN, this can be provided at build time by
110     means of the ``KBUILD_SIGN_PIN`` variable.
111
112
113 (5) :menuselection:`Additional X.509 keys for default system keyring`
114     (``CONFIG_SYSTEM_TRUSTED_KEYS``)
115
116     This option can be set to the filename of a PEM-encoded file containing
117     additional certificates which will be included in the system keyring by
118     default.
119
120Note that enabling module signing adds a dependency on the OpenSSL devel
121packages to the kernel build processes for the tool that does the signing.
122
123
124=======================
125Generating signing keys
126=======================
127
128Cryptographic keypairs are required to generate and check signatures.  A
129private key is used to generate a signature and the corresponding public key is
130used to check it.  The private key is only needed during the build, after which
131it can be deleted or stored securely.  The public key gets built into the
132kernel so that it can be used to check the signatures as the modules are
133loaded.
134
135Under normal conditions, when ``CONFIG_MODULE_SIG_KEY`` is unchanged from its
136default, the kernel build will automatically generate a new keypair using
137openssl if one does not exist in the file::
138
139	certs/signing_key.pem
140
141during the building of vmlinux (the public part of the key needs to be built
142into vmlinux) using parameters in the::
143
144	certs/x509.genkey
145
146file (which is also generated if it does not already exist).
147
148It is strongly recommended that you provide your own x509.genkey file.
149
150Most notably, in the x509.genkey file, the req_distinguished_name section
151should be altered from the default::
152
153	[ req_distinguished_name ]
154	#O = Unspecified company
155	CN = Build time autogenerated kernel key
156	#emailAddress = unspecified.user@unspecified.company
157
158The generated RSA key size can also be set with::
159
160	[ req ]
161	default_bits = 4096
162
163
164It is also possible to manually generate the key private/public files using the
165x509.genkey key generation configuration file in the root node of the Linux
166kernel sources tree and the openssl command.  The following is an example to
167generate the public/private key files::
168
169	openssl req -new -nodes -utf8 -sha256 -days 36500 -batch -x509 \
170	   -config x509.genkey -outform PEM -out kernel_key.pem \
171	   -keyout kernel_key.pem
172
173The full pathname for the resulting kernel_key.pem file can then be specified
174in the ``CONFIG_MODULE_SIG_KEY`` option, and the certificate and key therein will
175be used instead of an autogenerated keypair.
176
177
178=========================
179Public keys in the kernel
180=========================
181
182The kernel contains a ring of public keys that can be viewed by root.  They're
183in a keyring called ".builtin_trusted_keys" that can be seen by::
184
185	[root@deneb ~]# cat /proc/keys
186	...
187	223c7853 I------     1 perm 1f030000     0     0 keyring   .builtin_trusted_keys: 1
188	302d2d52 I------     1 perm 1f010000     0     0 asymmetri Fedora kernel signing key: d69a84e6bce3d216b979e9505b3e3ef9a7118079: X509.RSA a7118079 []
189	...
190
191Beyond the public key generated specifically for module signing, additional
192trusted certificates can be provided in a PEM-encoded file referenced by the
193``CONFIG_SYSTEM_TRUSTED_KEYS`` configuration option.
194
195Further, the architecture code may take public keys from a hardware store and
196add those in also (e.g. from the UEFI key database).
197
198Finally, it is possible to add additional public keys by doing::
199
200	keyctl padd asymmetric "" [.builtin_trusted_keys-ID] <[key-file]
201
202e.g.::
203
204	keyctl padd asymmetric "" 0x223c7853 <my_public_key.x509
205
206Note, however, that the kernel will only permit keys to be added to
207``.builtin_trusted_keys`` **if** the new key's X.509 wrapper is validly signed by a key
208that is already resident in the ``.builtin_trusted_keys`` at the time the key was added.
209
210
211========================
212Manually signing modules
213========================
214
215To manually sign a module, use the scripts/sign-file tool available in
216the Linux kernel source tree.  The script requires 4 arguments:
217
218	1.  The hash algorithm (e.g., sha256)
219	2.  The private key filename or PKCS#11 URI
220	3.  The public key filename
221	4.  The kernel module to be signed
222
223The following is an example to sign a kernel module::
224
225	scripts/sign-file sha512 kernel-signkey.priv \
226		kernel-signkey.x509 module.ko
227
228The hash algorithm used does not have to match the one configured, but if it
229doesn't, you should make sure that hash algorithm is either built into the
230kernel or can be loaded without requiring itself.
231
232If the private key requires a passphrase or PIN, it can be provided in the
233$KBUILD_SIGN_PIN environment variable.
234
235
236============================
237Signed modules and stripping
238============================
239
240A signed module has a digital signature simply appended at the end.  The string
241``~Module signature appended~.`` at the end of the module's file confirms that a
242signature is present but it does not confirm that the signature is valid!
243
244Signed modules are BRITTLE as the signature is outside of the defined ELF
245container.  Thus they MAY NOT be stripped once the signature is computed and
246attached.  Note the entire module is the signed payload, including any and all
247debug information present at the time of signing.
248
249
250======================
251Loading signed modules
252======================
253
254Modules are loaded with insmod, modprobe, ``init_module()`` or
255``finit_module()``, exactly as for unsigned modules as no processing is
256done in userspace.  The signature checking is all done within the kernel.
257
258
259=========================================
260Non-valid signatures and unsigned modules
261=========================================
262
263If ``CONFIG_MODULE_SIG_FORCE`` is enabled or module.sig_enforce=1 is supplied on
264the kernel command line, the kernel will only load validly signed modules
265for which it has a public key.   Otherwise, it will also load modules that are
266unsigned.   Any module for which the kernel has a key, but which proves to have
267a signature mismatch will not be permitted to load.
268
269Any module that has an unparseable signature will be rejected.
270
271
272=========================================
273Administering/protecting the private key
274=========================================
275
276Since the private key is used to sign modules, viruses and malware could use
277the private key to sign modules and compromise the operating system.  The
278private key must be either destroyed or moved to a secure location and not kept
279in the root node of the kernel source tree.
280
281If you use the same private key to sign modules for multiple kernel
282configurations, you must ensure that the module version information is
283sufficient to prevent loading a module into a different kernel.  Either
284set ``CONFIG_MODVERSIONS=y`` or ensure that each configuration has a different
285kernel release string by changing ``EXTRAVERSION`` or ``CONFIG_LOCALVERSION``.
286