1 ==============================
2 KERNEL MODULE SIGNING FACILITY
3 ==============================
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.
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.
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).
38 ==========================
39 CONFIGURING MODULE SIGNING
40 ==========================
42 The module signing facility is enabled by going to the "Enable Loadable Module
43 Support" section of the kernel configuration and turning on
45 CONFIG_MODULE_SIG "Module signature verification"
47 This has a number of options available:
49 (1) "Require modules to be validly signed" (CONFIG_MODULE_SIG_FORCE)
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.
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'.
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.
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.
67 (2) "Automatically sign all modules" (CONFIG_MODULE_SIG_ALL)
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:
76 (3) "Which hash algorithm should modules be signed with?"
78 This presents a choice of which hash algorithm the installation phase will
79 sign the modules with:
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"
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.
91 (4) "File name or PKCS#11 URI of module signing key" (CONFIG_MODULE_SIG_KEY)
93 Setting this option to something other than its default of
94 "signing_key.pem" will disable the autogeneration of signing keys and
95 allow the kernel modules to be signed with a key of your choosing.
96 The string provided should identify a file containing both a private
97 key and its corresponding X.509 certificate in PEM form, or — on
98 systems where the OpenSSL ENGINE_pkcs11 is functional — a PKCS#11 URI
99 as defined by RFC7512. In the latter case, the PKCS#11 URI should
100 reference both a certificate and a private key.
102 If the PEM file containing the private key is encrypted, or if the
103 PKCS#11 token requries a PIN, this can be provided at build time by
104 means of the KBUILD_SIGN_PIN variable.
107 =======================
108 GENERATING SIGNING KEYS
109 =======================
111 Cryptographic keypairs are required to generate and check signatures. A
112 private key is used to generate a signature and the corresponding public key is
113 used to check it. The private key is only needed during the build, after which
114 it can be deleted or stored securely. The public key gets built into the
115 kernel so that it can be used to check the signatures as the modules are
118 Under normal conditions, when CONFIG_MODULE_SIG_KEY is unchanged from its
119 default, the kernel build will automatically generate a new keypair using
120 openssl if one does not exist in the file:
124 during the building of vmlinux (the public part of the key needs to be built
125 into vmlinux) using parameters in the:
129 file (which is also generated if it does not already exist).
131 It is strongly recommended that you provide your own x509.genkey file.
133 Most notably, in the x509.genkey file, the req_distinguished_name section
134 should be altered from the default:
136 [ req_distinguished_name ]
137 #O = Unspecified company
138 CN = Build time autogenerated kernel key
139 #emailAddress = unspecified.user@unspecified.company
141 The generated RSA key size can also be set with:
147 It is also possible to manually generate the key private/public files using the
148 x509.genkey key generation configuration file in the root node of the Linux
149 kernel sources tree and the openssl command. The following is an example to
150 generate the public/private key files:
152 openssl req -new -nodes -utf8 -sha256 -days 36500 -batch -x509 \
153 -config x509.genkey -outform PEM -out kernel_key.pem \
154 -keyout kernel_key.pem
156 The full pathname for the resulting kernel_key.pem file can then be specified
157 in the CONFIG_MODULE_SIG_KEY option, and the certificate and key therein will
158 be used instead of an autogenerated keypair.
161 =========================
162 PUBLIC KEYS IN THE KERNEL
163 =========================
165 The kernel contains a ring of public keys that can be viewed by root. They're
166 in a keyring called ".system_keyring" that can be seen by:
168 [root@deneb ~]# cat /proc/keys
170 223c7853 I------ 1 perm 1f030000 0 0 keyring .system_keyring: 1
171 302d2d52 I------ 1 perm 1f010000 0 0 asymmetri Fedora kernel signing key: d69a84e6bce3d216b979e9505b3e3ef9a7118079: X509.RSA a7118079 []
174 Beyond the public key generated specifically for module signing, any file
175 placed in the kernel source root directory or the kernel build root directory
176 whose name is suffixed with ".x509" will be assumed to be an X.509 public key
177 and will be added to the keyring.
179 Further, the architecture code may take public keys from a hardware store and
180 add those in also (e.g. from the UEFI key database).
182 Finally, it is possible to add additional public keys by doing:
184 keyctl padd asymmetric "" [.system_keyring-ID] <[key-file]
188 keyctl padd asymmetric "" 0x223c7853 <my_public_key.x509
190 Note, however, that the kernel will only permit keys to be added to
191 .system_keyring _if_ the new key's X.509 wrapper is validly signed by a key
192 that is already resident in the .system_keyring at the time the key was added.
195 =========================
196 MANUALLY SIGNING MODULES
197 =========================
199 To manually sign a module, use the scripts/sign-file tool available in
200 the Linux kernel source tree. The script requires 4 arguments:
202 1. The hash algorithm (e.g., sha256)
203 2. The private key filename or PKCS#11 URI
204 3. The public key filename
205 4. The kernel module to be signed
207 The following is an example to sign a kernel module:
209 scripts/sign-file sha512 kernel-signkey.priv \
210 kernel-signkey.x509 module.ko
212 The hash algorithm used does not have to match the one configured, but if it
213 doesn't, you should make sure that hash algorithm is either built into the
214 kernel or can be loaded without requiring itself.
216 If the private key requires a passphrase or PIN, it can be provided in the
217 $KBUILD_SIGN_PIN environment variable.
220 ============================
221 SIGNED MODULES AND STRIPPING
222 ============================
224 A signed module has a digital signature simply appended at the end. The string
225 "~Module signature appended~." at the end of the module's file confirms that a
226 signature is present but it does not confirm that the signature is valid!
228 Signed modules are BRITTLE as the signature is outside of the defined ELF
229 container. Thus they MAY NOT be stripped once the signature is computed and
230 attached. Note the entire module is the signed payload, including any and all
231 debug information present at the time of signing.
234 ======================
235 LOADING SIGNED MODULES
236 ======================
238 Modules are loaded with insmod, modprobe, init_module() or finit_module(),
239 exactly as for unsigned modules as no processing is done in userspace. The
240 signature checking is all done within the kernel.
243 =========================================
244 NON-VALID SIGNATURES AND UNSIGNED MODULES
245 =========================================
247 If CONFIG_MODULE_SIG_FORCE is enabled or enforcemodulesig=1 is supplied on
248 the kernel command line, the kernel will only load validly signed modules
249 for which it has a public key. Otherwise, it will also load modules that are
250 unsigned. Any module for which the kernel has a key, but which proves to have
251 a signature mismatch will not be permitted to load.
253 Any module that has an unparseable signature will be rejected.
256 =========================================
257 ADMINISTERING/PROTECTING THE PRIVATE KEY
258 =========================================
260 Since the private key is used to sign modules, viruses and malware could use
261 the private key to sign modules and compromise the operating system. The
262 private key must be either destroyed or moved to a secure location and not kept
263 in the root node of the kernel source tree.