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.
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.
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.
66 (2) "Automatically sign all modules" (CONFIG_MODULE_SIG_ALL)
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:
75 (3) "Which hash algorithm should modules be signed with?"
77 This presents a choice of which hash algorithm the installation phase will
78 sign the modules with:
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"
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.
91 =======================
92 GENERATING SIGNING KEYS
93 =======================
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
102 Under normal conditions, the kernel build will automatically generate a new
103 keypair using openssl if one does not exist in the files:
108 during the building of vmlinux (the public part of the key needs to be built
109 into vmlinux) using parameters in the:
113 file (which is also generated if it does not already exist).
115 It is strongly recommended that you provide your own x509.genkey file.
117 Most notably, in the x509.genkey file, the req_distinguished_name section
118 should be altered from the default:
120 [ req_distinguished_name ]
122 CN = Glacier signing key
123 emailAddress = slartibartfast@magrathea.h2g2
125 The generated RSA key size can also be set with:
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:
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
141 =========================
142 PUBLIC KEYS IN THE KERNEL
143 =========================
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:
148 [root@deneb ~]# cat /proc/keys
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 []
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.
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).
162 Finally, it is possible to add additional public keys by doing:
164 keyctl padd asymmetric "" [.system_keyring-ID] <[key-file]
168 keyctl padd asymmetric "" 0x223c7853 <my_public_key.x509
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.
175 =========================
176 MANUALLY SIGNING MODULES
177 =========================
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:
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
187 The following is an example to sign a kernel module:
189 scripts/sign-file sha512 kernel-signkey.priv \
190 kernel-signkey.x509 module.ko
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.
197 ============================
198 SIGNED MODULES AND STRIPPING
199 ============================
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!
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.
211 ======================
212 LOADING SIGNED MODULES
213 ======================
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.
220 =========================================
221 NON-VALID SIGNATURES AND UNSIGNED MODULES
222 =========================================
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.
230 Any module that has an unparseable signature will be rejected.
233 =========================================
234 ADMINISTERING/PROTECTING THE PRIVATE KEY
235 =========================================
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.