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.
92 =======================
93 GENERATING SIGNING KEYS
94 =======================
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
103 Under normal conditions, the kernel build will automatically generate a new
104 keypair using openssl if one does not exist in the files:
109 during the building of vmlinux (the public part of the key needs to be built
110 into vmlinux) using parameters in the:
114 file (which is also generated if it does not already exist).
116 It is strongly recommended that you provide your own x509.genkey file.
118 Most notably, in the x509.genkey file, the req_distinguished_name section
119 should be altered from the default:
121 [ req_distinguished_name ]
122 #O = Unspecified company
123 CN = Build time autogenerated kernel key
124 #emailAddress = unspecified.user@unspecified.company
126 The generated RSA key size can also be set with:
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:
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
142 =========================
143 PUBLIC KEYS IN THE KERNEL
144 =========================
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:
149 [root@deneb ~]# cat /proc/keys
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 []
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.
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).
163 Finally, it is possible to add additional public keys by doing:
165 keyctl padd asymmetric "" [.system_keyring-ID] <[key-file]
169 keyctl padd asymmetric "" 0x223c7853 <my_public_key.x509
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.
176 =========================
177 MANUALLY SIGNING MODULES
178 =========================
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:
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
188 The following is an example to sign a kernel module:
190 scripts/sign-file sha512 kernel-signkey.priv \
191 kernel-signkey.x509 module.ko
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.
198 ============================
199 SIGNED MODULES AND STRIPPING
200 ============================
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!
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.
212 ======================
213 LOADING SIGNED MODULES
214 ======================
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.
221 =========================================
222 NON-VALID SIGNATURES AND UNSIGNED MODULES
223 =========================================
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.
231 Any module that has an unparseable signature will be rejected.
234 =========================================
235 ADMINISTERING/PROTECTING THE PRIVATE KEY
236 =========================================
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.