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