============================== KERNEL MODULE SIGNING FACILITY ============================== CONTENTS - Overview. - Configuring module signing. - Generating signing keys. - Public keys in the kernel. - Manually signing modules. - Signed modules and stripping. - Loading signed modules. - Non-valid signatures and unsigned modules. - Administering/protecting the private key. ======== OVERVIEW ======== The kernel module signing facility cryptographically signs modules during installation and then checks the signature upon loading the module. This allows increased kernel security by disallowing the loading of unsigned modules or modules signed with an invalid key. Module signing increases security by making it harder to load a malicious module into the kernel. The module signature checking is done by the kernel so that it is not necessary to have trusted userspace bits. This facility uses X.509 ITU-T standard certificates to encode the public keys involved. The signatures are not themselves encoded in any industrial standard type. The facility currently only supports the RSA public key encryption standard (though it is pluggable and permits others to be used). The possible hash algorithms that can be used are SHA-1, SHA-224, SHA-256, SHA-384, and SHA-512 (the algorithm is selected by data in the signature). ========================== CONFIGURING MODULE SIGNING ========================== The module signing facility is enabled by going to the "Enable Loadable Module Support" section of the kernel configuration and turning on CONFIG_MODULE_SIG "Module signature verification" This has a number of options available: (1) "Require modules to be validly signed" (CONFIG_MODULE_SIG_FORCE) This specifies how the kernel should deal with a module that has a signature for which the key is not known or a module that is unsigned. If this is off (ie. "permissive"), then modules for which the key is not available and modules that are unsigned are permitted, but the kernel will be marked as being tainted, and the concerned modules will be marked as tainted, shown with the character 'E'. If this is on (ie. "restrictive"), only modules that have a valid signature that can be verified by a public key in the kernel's possession will be loaded. All other modules will generate an error. Irrespective of the setting here, if the module has a signature block that cannot be parsed, it will be rejected out of hand. (2) "Automatically sign all modules" (CONFIG_MODULE_SIG_ALL) If this is on then modules will be automatically signed during the modules_install phase of a build. If this is off, then the modules must be signed manually using: scripts/sign-file (3) "Which hash algorithm should modules be signed with?" This presents a choice of which hash algorithm the installation phase will sign the modules with: CONFIG_MODULE_SIG_SHA1 "Sign modules with SHA-1" CONFIG_MODULE_SIG_SHA224 "Sign modules with SHA-224" CONFIG_MODULE_SIG_SHA256 "Sign modules with SHA-256" CONFIG_MODULE_SIG_SHA384 "Sign modules with SHA-384" CONFIG_MODULE_SIG_SHA512 "Sign modules with SHA-512" The algorithm selected here will also be built into the kernel (rather than being a module) so that modules signed with that algorithm can have their signatures checked without causing a dependency loop. (4) "File name or PKCS#11 URI of module signing key" (CONFIG_MODULE_SIG_KEY) Setting this option to something other than its default of "signing_key.pem" will disable the autogeneration of signing keys and allow the kernel modules to be signed with a key of your choosing. The string provided should identify a file containing both a private key and its corresponding X.509 certificate in PEM form, or — on systems where the OpenSSL ENGINE_pkcs11 is functional — a PKCS#11 URI as defined by RFC7512. In the latter case, the PKCS#11 URI should reference both a certificate and a private key. If the PEM file containing the private key is encrypted, or if the PKCS#11 token requries a PIN, this can be provided at build time by means of the KBUILD_SIGN_PIN variable. (5) "Additional X.509 keys for default system keyring" (CONFIG_SYSTEM_TRUSTED_KEYS) This option can be set to the filename of a PEM-encoded file containing additional certificates which will be included in the system keyring by default. Note that enabling module signing adds a dependency on the OpenSSL devel packages to the kernel build processes for the tool that does the signing. ======================= GENERATING SIGNING KEYS ======================= Cryptographic keypairs are required to generate and check signatures. A private key is used to generate a signature and the corresponding public key is used to check it. The private key is only needed during the build, after which it can be deleted or stored securely. The public key gets built into the kernel so that it can be used to check the signatures as the modules are loaded. Under normal conditions, when CONFIG_MODULE_SIG_KEY is unchanged from its default, the kernel build will automatically generate a new keypair using openssl if one does not exist in the file: signing_key.pem during the building of vmlinux (the public part of the key needs to be built into vmlinux) using parameters in the: x509.genkey file (which is also generated if it does not already exist). It is strongly recommended that you provide your own x509.genkey file. Most notably, in the x509.genkey file, the req_distinguished_name section should be altered from the default: [ req_distinguished_name ] #O = Unspecified company CN = Build time autogenerated kernel key #emailAddress = unspecified.user@unspecified.company The generated RSA key size can also be set with: [ req ] default_bits = 4096 It is also possible to manually generate the key private/public files using the x509.genkey key generation configuration file in the root node of the Linux kernel sources tree and the openssl command. The following is an example to generate the public/private key files: openssl req -new -nodes -utf8 -sha256 -days 36500 -batch -x509 \ -config x509.genkey -outform PEM -out kernel_key.pem \ -keyout kernel_key.pem The full pathname for the resulting kernel_key.pem file can then be specified in the CONFIG_MODULE_SIG_KEY option, and the certificate and key therein will be used instead of an autogenerated keypair. ========================= PUBLIC KEYS IN THE KERNEL ========================= The kernel contains a ring of public keys that can be viewed by root. They're in a keyring called ".system_keyring" that can be seen by: [root@deneb ~]# cat /proc/keys ... 223c7853 I------ 1 perm 1f030000 0 0 keyring .system_keyring: 1 302d2d52 I------ 1 perm 1f010000 0 0 asymmetri Fedora kernel signing key: d69a84e6bce3d216b979e9505b3e3ef9a7118079: X509.RSA a7118079 [] ... Beyond the public key generated specifically for module signing, additional trusted certificates can be provided in a PEM-encoded file referenced by the CONFIG_SYSTEM_TRUSTED_KEYS configuration option. Further, the architecture code may take public keys from a hardware store and add those in also (e.g. from the UEFI key database). Finally, it is possible to add additional public keys by doing: keyctl padd asymmetric "" [.system_keyring-ID] <[key-file] e.g.: keyctl padd asymmetric "" 0x223c7853