[PATCH v3] Add Documentation/module-signing.txt file

[James Solner]

This patch adds the Documentation/module-signing.txt file that is
currently missing from the Documentation directory. The init/Kconfig
file references the Documentation/module-signing.txt file to explain
how kernel module signing works. This patch supplies this documentation. 

The initial version of this patch provided old documentation
that was a mixture of the old RHEL style GPG signing. 
Version 1 updated the documentation to described the current
implementation using x509 certificate signing. 
Version 2, fixes grammar/spelling mistakes and removes
trailing whitespaces. Version 3, fixes grammar/spelling mistakes. 

Signed-off-by: James Solner <solner@xxxxxxxxxxxxxxxxxx>

---
 Documentation/module-signing.txt | 115 +++++++++++++++++++++++++++++++++++++++
 1 file changed, 115 insertions(+)
 create mode 100644 Documentation/module-signing.txt

diff --git a/Documentation/module-signing.txt b/Documentation/module-signing.txt
new file mode 100644
index 0000000..42aa52e
--- /dev/null
+++ b/Documentation/module-signing.txt
@@ -0,0 +1,115 @@
+		==============================
+		KERNEL MODULE SIGNING FACILITY
+		==============================
+
+The module signing facility applies cryptographic signature checking
+when loading modules by checking its signature against a public key.
+This allows increased kernel security by disallowing loading unsigned
+modules or modules signed with an invalid key. Module signing increases
+the kernel security and reduces the odds of malicious modules being
+loading into Linux operating system.
+
+This facility uses X.509 ITU-T standard to perform the cryptographic
+work and to determine the format of the signatures and key data. The
+key type used is RSA and the possible hash algorithms that can be
+used are SHA-1, SHA-224, SHA-256, SHA-384, and SHA-512. These hash
+algorithms can be selected during the kernel configuration build:
+
+	CONFIG_SIG_SHA1
+	CONFIG_SIG_SHA224
+	CONFIG_SIG_SHA256
+	CONFIG_SIG_SHA384
+	CONFIG_SIG_SHA512
+
+The module signing facility is a kernel feature and is enabled through the
+Linux kernel configuration builder. In the "Enable Loadable Module Support"
+section of the kernel configuration, the CONFIG_MODULE_SIG symbol is enabled
+to activate this feature. This feature supports two options for signed
+module support: "permissive" and "restrictive".  The default is the
+"permissive" option and allows a module with a valid signature to be loaded.
+If the signature is invalid, the module is still loaded, but the kernel is
+marked as "tainted". The "restrictive" option (CONFIG_MODULE_SIG_FORCE)
+requires a valid signature before the module can be loaded.
+
+Modules can be signed using two methods: "automatically" or "manually".
+The CONFIG_MODULE_SIG_ALL symbol will automatically sign the modules
+during the "modules_install" part of the kernel build. A module can also
+be signed manually using the scripts/sign-file tool.
+
+================================================
+AUTOMATICALLY GENERATING PUBLIC AND PRIVATE KEYS
+================================================
+As part of "modules_install" kernel build, the Linux kernel build
+infrastructure will automatically create two files in the root node
+of the Linux kernel source tree. These files contain the public/private
+keys and are called "signing_key.x509" and "signing_key.priv".
+The public key is built into the kernel and used to verify modules'
+signatures when the modules are loaded.
+
+=================================================
+MANUALLY GENERATING PUBLIC AND PRIVATE KEYS
+=================================================
+To manually generate the key private/public files, use 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 DER -out signing_key.x509 \
+	   -keyout signing_key.priv
+
+=========================
+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
+	3. The public key
+	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
+
+============================
+SIGNED MODULES AND STRIPPING
+============================
+
+A signed module has a digital signature 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
+all the debug information present at the time of signing.
+
+======================
+LOADING SIGNED MODULES
+======================
+
+Modules are loaded with insmod, exactly as for unsigned modules.
+The signature checker checks at the end of the file for the signature
+marker and applies signature checking.
+
+=========================================
+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.
+
+=========================================
+ADMINISTERING/PROTECTING THE PRIVATE KEY
+=========================================
+Since the private key is used to sign modules, malware can use
+the private key to sign modules and compromise the operating system.
+The private key must be moved to a secure location and not kept in
+the root node of the kernel source tree.
-- 
1.7.12.4
---


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