1 ============================== 2 KERNEL MODULE SIGNING FACILITY 3 ============================== 4 5CONTENTS 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======== 19OVERVIEW 20======== 21 22The kernel module signing facility cryptographically signs modules during 23installation and then checks the signature upon loading the module. This 24allows increased kernel security by disallowing the loading of unsigned modules 25or modules signed with an invalid key. Module signing increases security by 26making it harder to load a malicious module into the kernel. The module 27signature checking is done by the kernel so that it is not necessary to have 28trusted userspace bits. 29 30This facility uses X.509 ITU-T standard certificates to encode the public keys 31involved. The signatures are not themselves encoded in any industrial standard 32type. The facility currently only supports the RSA public key encryption 33standard (though it is pluggable and permits others to be used). The possible 34hash algorithms that can be used are SHA-1, SHA-224, SHA-256, SHA-384, and 35SHA-512 (the algorithm is selected by data in the signature). 36 37 38========================== 39CONFIGURING MODULE SIGNING 40========================== 41 42The module signing facility is enabled by going to the "Enable Loadable Module 43Support" section of the kernel configuration and turning on 44 45 CONFIG_MODULE_SIG "Module signature verification" 46 47This 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======================= 93GENERATING SIGNING KEYS 94======================= 95 96Cryptographic keypairs are required to generate and check signatures. A 97private key is used to generate a signature and the corresponding public key is 98used to check it. The private key is only needed during the build, after which 99it can be deleted or stored securely. The public key gets built into the 100kernel so that it can be used to check the signatures as the modules are 101loaded. 102 103Under normal conditions, the kernel build will automatically generate a new 104keypair using openssl if one does not exist in the files: 105 106 signing_key.priv 107 signing_key.x509 108 109during the building of vmlinux (the public part of the key needs to be built 110into vmlinux) using parameters in the: 111 112 x509.genkey 113 114file (which is also generated if it does not already exist). 115 116It is strongly recommended that you provide your own x509.genkey file. 117 118Most notably, in the x509.genkey file, the req_distinguished_name section 119should be altered from the default: 120 121 [ req_distinguished_name ] 122 #O = Unspecified company 123 CN = Build time autogenerated kernel key 124 #emailAddress = unspecified.user@unspecified.company 125 126The generated RSA key size can also be set with: 127 128 [ req ] 129 default_bits = 4096 130 131 132It is also possible to manually generate the key private/public files using the 133x509.genkey key generation configuration file in the root node of the Linux 134kernel sources tree and the openssl command. The following is an example to 135generate 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========================= 143PUBLIC KEYS IN THE KERNEL 144========================= 145 146The kernel contains a ring of public keys that can be viewed by root. They're 147in 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 155Beyond the public key generated specifically for module signing, any file 156placed in the kernel source root directory or the kernel build root directory 157whose name is suffixed with ".x509" will be assumed to be an X.509 public key 158and will be added to the keyring. 159 160Further, the architecture code may take public keys from a hardware store and 161add those in also (e.g. from the UEFI key database). 162 163Finally, it is possible to add additional public keys by doing: 164 165 keyctl padd asymmetric "" [.system_keyring-ID] <[key-file] 166 167e.g.: 168 169 keyctl padd asymmetric "" 0x223c7853 <my_public_key.x509 170 171Note, 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 173that is already resident in the .system_keyring at the time the key was added. 174 175 176========================= 177MANUALLY SIGNING MODULES 178========================= 179 180To manually sign a module, use the scripts/sign-file tool available in 181the 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 188The 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 193The hash algorithm used does not have to match the one configured, but if it 194doesn't, you should make sure that hash algorithm is either built into the 195kernel or can be loaded without requiring itself. 196 197 198============================ 199SIGNED MODULES AND STRIPPING 200============================ 201 202A 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 204signature is present but it does not confirm that the signature is valid! 205 206Signed modules are BRITTLE as the signature is outside of the defined ELF 207container. Thus they MAY NOT be stripped once the signature is computed and 208attached. Note the entire module is the signed payload, including any and all 209debug information present at the time of signing. 210 211 212====================== 213LOADING SIGNED MODULES 214====================== 215 216Modules are loaded with insmod, modprobe, init_module() or finit_module(), 217exactly as for unsigned modules as no processing is done in userspace. The 218signature checking is all done within the kernel. 219 220 221========================================= 222NON-VALID SIGNATURES AND UNSIGNED MODULES 223========================================= 224 225If CONFIG_MODULE_SIG_FORCE is enabled or enforcemodulesig=1 is supplied on 226the kernel command line, the kernel will only load validly signed modules 227for which it has a public key. Otherwise, it will also load modules that are 228unsigned. Any module for which the kernel has a key, but which proves to have 229a signature mismatch will not be permitted to load. 230 231Any module that has an unparseable signature will be rejected. 232 233 234========================================= 235ADMINISTERING/PROTECTING THE PRIVATE KEY 236========================================= 237 238Since the private key is used to sign modules, viruses and malware could use 239the private key to sign modules and compromise the operating system. The 240private key must be either destroyed or moved to a secure location and not kept 241in the root node of the kernel source tree. 242