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- ==============================
- 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
- "certs/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:
- certs/signing_key.pem
- during the building of vmlinux (the public part of the key needs to be built
- into 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 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 <my_public_key.x509
- Note, however, that the kernel will only permit keys to be added to
- .system_keyring _if_ the new key's X.509 wrapper is validly signed by a key
- that is already resident in the .system_keyring at the time the key was added.
- =========================
- MANUALLY SIGNING MODULES
- =========================
- To manually sign a module, use the scripts/sign-file tool available in
- the 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 it
- doesn't, you should make sure that hash algorithm is either built into the
- kernel 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
- "~Module signature appended~." at the end of the module's file confirms that a
- signature is present but it does not confirm that the signature is valid!
- Signed modules are BRITTLE as the signature is outside of the defined ELF
- container. Thus they MAY NOT be stripped once the signature is computed and
- attached. Note the entire module is the signed payload, including any and all
- debug information present at the time of signing.
- ======================
- LOADING SIGNED MODULES
- ======================
- Modules are loaded with insmod, modprobe, init_module() or finit_module(),
- exactly as for unsigned modules as no processing is done in userspace. The
- signature checking is all done within the kernel.
- =========================================
- NON-VALID SIGNATURES AND UNSIGNED MODULES
- =========================================
- If CONFIG_MODULE_SIG_FORCE is enabled or enforcemodulesig=1 is supplied on
- the kernel command line, the kernel will only load validly signed modules
- for which it has a public key. Otherwise, it will also load modules that are
- unsigned. Any module for which the kernel has a key, but which proves to have
- a 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 use
- the private key to sign modules and compromise the operating system. The
- private key must be either destroyed or moved to a secure location and not kept
- in the root node of the kernel source tree.
- If you use the same private key to sign modules for multiple kernel
- configurations, you must ensure that the module version information is
- sufficient to prevent loading a module into a different kernel. Either
- set CONFIG_MODVERSIONS=y or ensure that each configuration has a different
- kernel release string by changing EXTRAVERSION or CONFIG_LOCALVERSION.
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