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+ ==============================
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+ KERNEL MODULE SIGNING FACILITY
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+ ==============================
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+
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+CONTENTS
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+
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+ - Overview.
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+ - Configuring module signing.
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+ - Generating signing keys.
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+ - Public keys in the kernel.
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+ - Manually signing modules.
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+ - Signed modules and stripping.
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+ - Loading signed modules.
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+ - Non-valid signatures and unsigned modules.
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+ - Administering/protecting the private key.
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+
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+
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+========
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+OVERVIEW
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+========
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+
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+The kernel module signing facility cryptographically signs modules during
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+installation and then checks the signature upon loading the module. This
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+allows increased kernel security by disallowing the loading of unsigned modules
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+or modules signed with an invalid key. Module signing increases security by
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+making it harder to load a malicious module into the kernel. The module
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+signature checking is done by the kernel so that it is not necessary to have
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+trusted userspace bits.
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+
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+This facility uses X.509 ITU-T standard certificates to encode the public keys
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+involved. The signatures are not themselves encoded in any industrial standard
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+type. The facility currently only supports the RSA public key encryption
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+standard (though it is pluggable and permits others to be used). The possible
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+hash algorithms that can be used are SHA-1, SHA-224, SHA-256, SHA-384, and
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+SHA-512 (the algorithm is selected by data in the signature).
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+
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+
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+==========================
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+CONFIGURING MODULE SIGNING
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+==========================
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+
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+The module signing facility is enabled by going to the "Enable Loadable Module
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+Support" section of the kernel configuration and turning on
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+
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+ CONFIG_MODULE_SIG "Module signature verification"
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+
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+This has a number of options available:
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+
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+ (1) "Require modules to be validly signed" (CONFIG_MODULE_SIG_FORCE)
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+
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+ This specifies how the kernel should deal with a module that has a
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+ signature for which the key is not known or a module that is unsigned.
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+
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+ If this is off (ie. "permissive"), then modules for which the key is not
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+ available and modules that are unsigned are permitted, but the kernel will
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+ be marked as being tainted.
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+
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+ If this is on (ie. "restrictive"), only modules that have a valid
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+ signature that can be verified by a public key in the kernel's possession
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+ will be loaded. All other modules will generate an error.
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+
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+ Irrespective of the setting here, if the module has a signature block that
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+ cannot be parsed, it will be rejected out of hand.
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+
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+
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+ (2) "Automatically sign all modules" (CONFIG_MODULE_SIG_ALL)
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+
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+ If this is on then modules will be automatically signed during the
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+ modules_install phase of a build. If this is off, then the modules must
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+ be signed manually using:
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+
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+ scripts/sign-file
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+
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+
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+ (3) "Which hash algorithm should modules be signed with?"
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+
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+ This presents a choice of which hash algorithm the installation phase will
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+ sign the modules with:
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+
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+ CONFIG_SIG_SHA1 "Sign modules with SHA-1"
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+ CONFIG_SIG_SHA224 "Sign modules with SHA-224"
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+ CONFIG_SIG_SHA256 "Sign modules with SHA-256"
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+ CONFIG_SIG_SHA384 "Sign modules with SHA-384"
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+ CONFIG_SIG_SHA512 "Sign modules with SHA-512"
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+
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+ The algorithm selected here will also be built into the kernel (rather
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+ than being a module) so that modules signed with that algorithm can have
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+ their signatures checked without causing a dependency loop.
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+
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+
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+=======================
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+GENERATING SIGNING KEYS
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+=======================
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+
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+Cryptographic keypairs are required to generate and check signatures. A
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+private key is used to generate a signature and the corresponding public key is
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+used to check it. The private key is only needed during the build, after which
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+it can be deleted or stored securely. The public key gets built into the
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+kernel so that it can be used to check the signatures as the modules are
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+loaded.
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+
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+Under normal conditions, the kernel build will automatically generate a new
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+keypair using openssl if one does not exist in the files:
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+
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+ signing_key.priv
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+ signing_key.x509
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+
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+during the building of vmlinux (the public part of the key needs to be built
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+into vmlinux) using parameters in the:
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+
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+ x509.genkey
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+
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+file (which is also generated if it does not already exist).
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+
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+It is strongly recommended that you provide your own x509.genkey file.
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+
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+Most notably, in the x509.genkey file, the req_distinguished_name section
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+should be altered from the default:
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+
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+ [ req_distinguished_name ]
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+ O = Magrathea
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+ CN = Glacier signing key
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+ emailAddress = slartibartfast@magrathea.h2g2
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+
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+The generated RSA key size can also be set with:
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+
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+ [ req ]
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+ default_bits = 4096
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+
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+
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+It is also possible to manually generate the key private/public files using the
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+x509.genkey key generation configuration file in the root node of the Linux
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+kernel sources tree and the openssl command. The following is an example to
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+generate the public/private key files:
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+
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+ openssl req -new -nodes -utf8 -sha256 -days 36500 -batch -x509 \
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+ -config x509.genkey -outform DER -out signing_key.x509 \
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+ -keyout signing_key.priv
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+
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+
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+=========================
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+PUBLIC KEYS IN THE KERNEL
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+=========================
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+
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+The kernel contains a ring of public keys that can be viewed by root. They're
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+in a keyring called ".system_keyring" that can be seen by:
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+
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+ [root@deneb ~]# cat /proc/keys
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+ ...
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+ 223c7853 I------ 1 perm 1f030000 0 0 keyring .system_keyring: 1
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+ 302d2d52 I------ 1 perm 1f010000 0 0 asymmetri Fedora kernel signing key: d69a84e6bce3d216b979e9505b3e3ef9a7118079: X509.RSA a7118079 []
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+ ...
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+
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+Beyond the public key generated specifically for module signing, any file
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+placed in the kernel source root directory or the kernel build root directory
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+whose name is suffixed with ".x509" will be assumed to be an X.509 public key
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+and will be added to the keyring.
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+
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+Further, the architecture code may take public keys from a hardware store and
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+add those in also (e.g. from the UEFI key database).
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+
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+Finally, it is possible to add additional public keys by doing:
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+
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+ keyctl padd asymmetric "" [.system_keyring-ID] <[key-file]
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+
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+e.g.:
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+
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+ keyctl padd asymmetric "" 0x223c7853 <my_public_key.x509
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+
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+Note, however, that the kernel will only permit keys to be added to
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+.system_keyring _if_ the new key's X.509 wrapper is validly signed by a key
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+that is already resident in the .system_keyring at the time the key was added.
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+
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+
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+=========================
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+MANUALLY SIGNING MODULES
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+=========================
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+
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+To manually sign a module, use the scripts/sign-file tool available in
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+the Linux kernel source tree. The script requires 4 arguments:
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+
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+ 1. The hash algorithm (e.g., sha256)
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+ 2. The private key filename
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+ 3. The public key filename
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+ 4. The kernel module to be signed
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+
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+The following is an example to sign a kernel module:
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+
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+ scripts/sign-file sha512 kernel-signkey.priv \
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+ kernel-signkey.x509 module.ko
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+
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+The hash algorithm used does not have to match the one configured, but if it
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+doesn't, you should make sure that hash algorithm is either built into the
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+kernel or can be loaded without requiring itself.
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+
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+
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+============================
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+SIGNED MODULES AND STRIPPING
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+============================
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+
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+A signed module has a digital signature simply appended at the end. The string
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+"~Module signature appended~." at the end of the module's file confirms that a
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+signature is present but it does not confirm that the signature is valid!
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+
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+Signed modules are BRITTLE as the signature is outside of the defined ELF
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+container. Thus they MAY NOT be stripped once the signature is computed and
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+attached. Note the entire module is the signed payload, including any and all
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+debug information present at the time of signing.
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+
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+
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+======================
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+LOADING SIGNED MODULES
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+======================
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+
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+Modules are loaded with insmod, modprobe, init_module() or finit_module(),
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+exactly as for unsigned modules as no processing is done in userspace. The
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+signature checking is all done within the kernel.
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+
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+
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+=========================================
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+NON-VALID SIGNATURES AND UNSIGNED MODULES
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+=========================================
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+
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+If CONFIG_MODULE_SIG_FORCE is enabled or enforcemodulesig=1 is supplied on
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+the kernel command line, the kernel will only load validly signed modules
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+for which it has a public key. Otherwise, it will also load modules that are
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+unsigned. Any module for which the kernel has a key, but which proves to have
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+a signature mismatch will not be permitted to load.
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+
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+Any module that has an unparseable signature will be rejected.
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+
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+
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+=========================================
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+ADMINISTERING/PROTECTING THE PRIVATE KEY
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+=========================================
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+
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+Since the private key is used to sign modules, viruses and malware could use
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+the private key to sign modules and compromise the operating system. The
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+private key must be either destroyed or moved to a secure location and not kept
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+in the root node of the kernel source tree.
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