Kernel module signing facility

Overview

The kernel module signing facility cryptographically signs modules duringinstallation and then checks the signature upon loading the module. Thisallows increased kernel security by disallowing the loading of unsigned modulesor modules signed with an invalid key. Module signing increases security bymaking it harder to load a malicious module into the kernel. The modulesignature checking is done by the kernel so that it is not necessary to havetrusted userspace bits.

This facility uses X.509 ITU-T standard certificates to encode the public keysinvolved. The signatures are not themselves encoded in any industrial standardtype. The facility currently only supports the RSA public key encryptionstandard (though it is pluggable and permits others to be used). The possiblehash algorithms that can be used are SHA-1, SHA-224, SHA-256, SHA-384, andSHA-512 (the algorithm is selected by data in the signature).

Configuring module signing

The module signing facility is enabled by going to theEnable Loadable Module Support section ofthe 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 asignature 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 notavailable and modules that are unsigned are permitted, but the kernel willbe marked as being tainted, and the concerned modules will be marked astainted, shown with the character ‘E’.

    If this is on (ie. “restrictive”), only modules that have a validsignature that can be verified by a public key in the kernel’s possessionwill be loaded. All other modules will generate an error.

    Irrespective of the setting here, if the module has a signature block thatcannot 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 themodules_install phase of a build. If this is off, then the modules mustbe 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 willsign the modules with:

    CONFIG_MODULE_SIG_SHA1Sign modules with SHA-1
    CONFIG_MODULE_SIG_SHA224Sign modules with SHA-224
    CONFIG_MODULE_SIG_SHA256Sign modules with SHA-256
    CONFIG_MODULE_SIG_SHA384Sign modules with SHA-384
    CONFIG_MODULE_SIG_SHA512Sign modules with SHA-512

    The algorithm selected here will also be built into the kernel (ratherthan being a module) so that modules signed with that algorithm can havetheir 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 ofcerts/signing_key.pem will disable the autogeneration of signing keysand allow the kernel modules to be signed with a key of your choosing.The string provided should identify a file containing both a private keyand its corresponding X.509 certificate in PEM form, or — on systems wherethe OpenSSL ENGINE_pkcs11 is functional — a PKCS#11 URI as defined byRFC7512. In the latter case, the PKCS#11 URI should reference both acertificate and a private key.

    If the PEM file containing the private key is encrypted, or if thePKCS#11 token requries a PIN, this can be provided at build time bymeans of theKBUILD_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 containingadditional certificates which will be included in the system keyring bydefault.

Note that enabling module signing adds a dependency on the OpenSSL develpackages to the kernel build processes for the tool that does the signing.

Generating signing keys

Cryptographic keypairs are required to generate and check signatures. Aprivate key is used to generate a signature and the corresponding public key isused to check it. The private key is only needed during the build, after whichit can be deleted or stored securely. The public key gets built into thekernel so that it can be used to check the signatures as the modules areloaded.

Under normal conditions, whenCONFIG_MODULE_SIG_KEY is unchanged from itsdefault, the kernel build will automatically generate a new keypair usingopenssl if one does not exist in the file:

certs/signing_key.pem

during the building of vmlinux (the public part of the key needs to be builtinto vmlinux) using parameters in the:

certs/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 sectionshould be altered from the default:

[ req_distinguished_name ]#O = Unspecified companyCN = 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 thex509.genkey key generation configuration file in the root node of the Linuxkernel sources tree and the openssl command. The following is an example togenerate 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 specifiedin theCONFIG_MODULE_SIG_KEY option, and the certificate and key therein willbe 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’rein a keyring called “.builtin_trusted_keys” that can be seen by:

[root@deneb ~]# cat /proc/keys...223c7853 I------     1 perm 1f030000     0     0 keyring   .builtin_trusted_keys: 1302d2d52 I------     1 perm 1f010000     0     0 asymmetri Fedora kernel signing key: d69a84e6bce3d216b979e9505b3e3ef9a7118079: X509.RSA a7118079 []...

Beyond the public key generated specifically for module signing, additionaltrusted certificates can be provided in a PEM-encoded file referenced by theCONFIG_SYSTEM_TRUSTED_KEYS configuration option.

Further, the architecture code may take public keys from a hardware store andadd those in also (e.g. from the UEFI key database).

Finally, it is possible to add additional public keys by doing:

keyctl padd asymmetric "" [.builtin_trusted_keys-ID] <[key-file]

e.g.:

keyctl padd asymmetric "" 0x223c7853 <my_public_key.x509

Note, however, that the kernel will only permit keys to be added to.builtin_trusted_keysif the new key’s X.509 wrapper is validly signed by a keythat is already resident in the.builtin_trusted_keys at the time the key was added.

Manually signing modules

To manually sign a module, use the scripts/sign-file tool available inthe Linux kernel source tree. The script requires 4 arguments:

  1. The hash algorithm (e.g., sha256)
  2. The private key filename or PKCS#11 URI
  3. The public key filename
  4. The kernel module to be signed

The following is an example to sign a kernel module:

scripts/sign-file sha512 kernel-signkey.priv \        kernel-signkey.x509 module.ko

The hash algorithm used does not have to match the one configured, but if itdoesn’t, you should make sure that hash algorithm is either built into thekernel or can be loaded without requiring itself.

If the private key requires a passphrase or PIN, it can be provided in the$KBUILD_SIGN_PIN environment variable.

Signed modules and stripping

A signed module has a digital signature simply appended at the end. The string~Modulesignatureappended~. at the end of the module’s file confirms that asignature is present but it does not confirm that the signature is valid!

Signed modules are BRITTLE as the signature is outside of the defined ELFcontainer. Thus they MAY NOT be stripped once the signature is computed andattached. Note the entire module is the signed payload, including any and alldebug information present at the time of signing.

Loading signed modules

Modules are loaded with insmod, modprobe,init_module() orfinit_module(), exactly as for unsigned modules as no processing isdone in userspace. The signature checking is all done within the kernel.

Non-valid signatures and unsigned modules

IfCONFIG_MODULE_SIG_FORCE is enabled or module.sig_enforce=1 is supplied onthe kernel command line, the kernel will only load validly signed modulesfor which it has a public key. Otherwise, it will also load modules that areunsigned. Any module for which the kernel has a key, but which proves to havea signature mismatch will not be permitted to load.

Any module that has an unparseable signature will be rejected.

Administering/protecting the private key

Since the private key is used to sign modules, viruses and malware could usethe private key to sign modules and compromise the operating system. Theprivate key must be either destroyed or moved to a secure location and not keptin the root node of the kernel source tree.

If you use the same private key to sign modules for multiple kernelconfigurations, you must ensure that the module version information issufficient to prevent loading a module into a different kernel. EithersetCONFIG_MODVERSIONS=y or ensure that each configuration has a differentkernel release string by changingEXTRAVERSION orCONFIG_LOCALVERSION.