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Documentation/module-signing.txt: convert to ReST markup
- Fix identatio for the document title; - remove its index; - create a table for hash algorithm to be used; - use quote blocks where needed; - use monotonic fonts for parameters; - adjust whitespaces and blank lines; - Fix case on section titles; - add it to the user's book. Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>
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Mauro Carvalho Chehab
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@ -1,22 +1,21 @@
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==============================
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KERNEL MODULE SIGNING FACILITY
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==============================
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Kernel module signing facility
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------------------------------
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CONTENTS
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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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.. 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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OVERVIEW
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Overview
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========
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The kernel module signing facility cryptographically signs modules during
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@ -36,17 +35,19 @@ SHA-512 (the algorithm is selected by data in the signature).
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==========================
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CONFIGURING MODULE SIGNING
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Configuring module signing
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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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The module signing facility is enabled by going to the
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:menuselection:`Enable Loadable Module Support` section of
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the kernel configuration and turning on::
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CONFIG_MODULE_SIG "Module signature verification"
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This has a number of options available:
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(1) "Require modules to be validly signed" (CONFIG_MODULE_SIG_FORCE)
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(1) :menuselection:`Require modules to be validly signed`
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(``CONFIG_MODULE_SIG_FORCE``)
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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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@ -64,35 +65,39 @@ This has a number of options available:
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cannot be parsed, it will be rejected out of hand.
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(2) "Automatically sign all modules" (CONFIG_MODULE_SIG_ALL)
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(2) :menuselection:`Automatically sign all modules`
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(``CONFIG_MODULE_SIG_ALL``)
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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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be signed manually using::
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scripts/sign-file
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(3) "Which hash algorithm should modules be signed with?"
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(3) :menuselection:`Which hash algorithm should modules be signed with?`
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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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CONFIG_MODULE_SIG_SHA1 "Sign modules with SHA-1"
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CONFIG_MODULE_SIG_SHA224 "Sign modules with SHA-224"
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CONFIG_MODULE_SIG_SHA256 "Sign modules with SHA-256"
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CONFIG_MODULE_SIG_SHA384 "Sign modules with SHA-384"
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CONFIG_MODULE_SIG_SHA512 "Sign modules with SHA-512"
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=============================== ==========================================
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``CONFIG_MODULE_SIG_SHA1`` :menuselection:`Sign modules with SHA-1`
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``CONFIG_MODULE_SIG_SHA224`` :menuselection:`Sign modules with SHA-224`
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``CONFIG_MODULE_SIG_SHA256`` :menuselection:`Sign modules with SHA-256`
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``CONFIG_MODULE_SIG_SHA384`` :menuselection:`Sign modules with SHA-384`
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``CONFIG_MODULE_SIG_SHA512`` :menuselection:`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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(4) "File name or PKCS#11 URI of module signing key" (CONFIG_MODULE_SIG_KEY)
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(4) :menuselection:`File name or PKCS#11 URI of module signing key`
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(``CONFIG_MODULE_SIG_KEY``)
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Setting this option to something other than its default of
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"certs/signing_key.pem" will disable the autogeneration of signing keys
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``certs/signing_key.pem`` will disable the autogeneration of signing keys
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and allow the kernel modules to be signed with a key of your choosing.
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The string provided should identify a file containing both a private key
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and its corresponding X.509 certificate in PEM form, or — on systems where
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@ -102,10 +107,11 @@ This has a number of options available:
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If the PEM file containing the private key is encrypted, or if the
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PKCS#11 token requries a PIN, this can be provided at build time by
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means of the KBUILD_SIGN_PIN variable.
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means of the ``KBUILD_SIGN_PIN`` variable.
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(5) "Additional X.509 keys for default system keyring" (CONFIG_SYSTEM_TRUSTED_KEYS)
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(5) :menuselection:`Additional X.509 keys for default system keyring`
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(``CONFIG_SYSTEM_TRUSTED_KEYS``)
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This option can be set to the filename of a PEM-encoded file containing
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additional certificates which will be included in the system keyring by
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@ -116,7 +122,7 @@ packages to the kernel build processes for the tool that does the signing.
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=======================
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GENERATING SIGNING KEYS
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Generating signing keys
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=======================
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Cryptographic keypairs are required to generate and check signatures. A
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@ -126,14 +132,14 @@ 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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Under normal conditions, when CONFIG_MODULE_SIG_KEY is unchanged from its
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Under normal conditions, when ``CONFIG_MODULE_SIG_KEY`` is unchanged from its
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default, the kernel build will automatically generate a new keypair using
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openssl if one does not exist in the file:
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openssl if one does not exist in the file::
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certs/signing_key.pem
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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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into vmlinux) using parameters in the::
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certs/x509.genkey
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@ -142,14 +148,14 @@ file (which is also generated if it does not already exist).
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It is strongly recommended that you provide your own x509.genkey file.
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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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should be altered from the default::
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[ req_distinguished_name ]
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#O = Unspecified company
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CN = Build time autogenerated kernel key
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#emailAddress = unspecified.user@unspecified.company
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The generated RSA key size can also be set with:
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The generated RSA key size can also be set with::
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[ req ]
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default_bits = 4096
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@ -158,23 +164,23 @@ The generated RSA key size can also be set with:
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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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generate the public/private key files::
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openssl req -new -nodes -utf8 -sha256 -days 36500 -batch -x509 \
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-config x509.genkey -outform PEM -out kernel_key.pem \
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-keyout kernel_key.pem
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The full pathname for the resulting kernel_key.pem file can then be specified
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in the CONFIG_MODULE_SIG_KEY option, and the certificate and key therein will
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in the ``CONFIG_MODULE_SIG_KEY`` option, and the certificate and key therein will
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be used instead of an autogenerated keypair.
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=========================
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PUBLIC KEYS IN THE KERNEL
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Public keys in the kernel
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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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in a keyring called ".system_keyring" that can be seen by::
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[root@deneb ~]# cat /proc/keys
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...
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@ -184,27 +190,27 @@ in a keyring called ".system_keyring" that can be seen by:
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Beyond the public key generated specifically for module signing, additional
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trusted certificates can be provided in a PEM-encoded file referenced by the
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CONFIG_SYSTEM_TRUSTED_KEYS configuration option.
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``CONFIG_SYSTEM_TRUSTED_KEYS`` configuration option.
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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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Finally, it is possible to add additional public keys by doing:
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Finally, it is possible to add additional public keys by doing::
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keyctl padd asymmetric "" [.system_keyring-ID] <[key-file]
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e.g.:
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e.g.::
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keyctl padd asymmetric "" 0x223c7853 <my_public_key.x509
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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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``.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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MANUALLY SIGNING MODULES
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=========================
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========================
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Manually signing modules
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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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@ -214,7 +220,7 @@ the Linux kernel source tree. The script requires 4 arguments:
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3. The public key filename
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4. The kernel module to be signed
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The following is an example to sign a kernel module:
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The following is an example to sign a kernel module::
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scripts/sign-file sha512 kernel-signkey.priv \
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kernel-signkey.x509 module.ko
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@ -228,11 +234,11 @@ $KBUILD_SIGN_PIN environment variable.
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============================
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SIGNED MODULES AND STRIPPING
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Signed modules and stripping
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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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``~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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Signed modules are BRITTLE as the signature is outside of the defined ELF
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@ -242,19 +248,19 @@ debug information present at the time of signing.
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======================
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LOADING SIGNED MODULES
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Loading signed modules
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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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Modules are loaded with insmod, modprobe, ``init_module()`` or
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``finit_module()``, exactly as for unsigned modules as no processing is
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done in userspace. The signature checking is all done within the kernel.
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=========================================
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NON-VALID SIGNATURES AND UNSIGNED MODULES
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Non-valid signatures and unsigned modules
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=========================================
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If CONFIG_MODULE_SIG_FORCE is enabled or module.sig_enforce=1 is supplied on
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If ``CONFIG_MODULE_SIG_FORCE`` is enabled or module.sig_enforce=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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@ -264,7 +270,7 @@ Any module that has an unparseable signature will be rejected.
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=========================================
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ADMINISTERING/PROTECTING THE PRIVATE KEY
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Administering/protecting the private key
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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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@ -275,5 +281,5 @@ in the root node of the kernel source tree.
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If you use the same private key to sign modules for multiple kernel
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configurations, you must ensure that the module version information is
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sufficient to prevent loading a module into a different kernel. Either
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set CONFIG_MODVERSIONS=y or ensure that each configuration has a different
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kernel release string by changing EXTRAVERSION or CONFIG_LOCALVERSION.
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set ``CONFIG_MODVERSIONS=y`` or ensure that each configuration has a different
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kernel release string by changing ``EXTRAVERSION`` or ``CONFIG_LOCALVERSION``.
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