module-signing.rst 11 KB

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  1. Kernel module signing facility
  2. ------------------------------
  3. .. CONTENTS
  4. ..
  5. .. - Overview.
  6. .. - Configuring module signing.
  7. .. - Generating signing keys.
  8. .. - Public keys in the kernel.
  9. .. - Manually signing modules.
  10. .. - Signed modules and stripping.
  11. .. - Loading signed modules.
  12. .. - Non-valid signatures and unsigned modules.
  13. .. - Administering/protecting the private key.
  14. ========
  15. Overview
  16. ========
  17. The kernel module signing facility cryptographically signs modules during
  18. installation and then checks the signature upon loading the module. This
  19. allows increased kernel security by disallowing the loading of unsigned modules
  20. or modules signed with an invalid key. Module signing increases security by
  21. making it harder to load a malicious module into the kernel. The module
  22. signature checking is done by the kernel so that it is not necessary to have
  23. trusted userspace bits.
  24. This facility uses X.509 ITU-T standard certificates to encode the public keys
  25. involved. The signatures are not themselves encoded in any industrial standard
  26. type. The facility currently only supports the RSA public key encryption
  27. standard (though it is pluggable and permits others to be used). The possible
  28. hash algorithms that can be used are SHA-1, SHA-224, SHA-256, SHA-384, and
  29. SHA-512 (the algorithm is selected by data in the signature).
  30. ==========================
  31. Configuring module signing
  32. ==========================
  33. The module signing facility is enabled by going to the
  34. :menuselection:`Enable Loadable Module Support` section of
  35. the kernel configuration and turning on::
  36. CONFIG_MODULE_SIG "Module signature verification"
  37. This has a number of options available:
  38. (1) :menuselection:`Require modules to be validly signed`
  39. (``CONFIG_MODULE_SIG_FORCE``)
  40. This specifies how the kernel should deal with a module that has a
  41. signature for which the key is not known or a module that is unsigned.
  42. If this is off (ie. "permissive"), then modules for which the key is not
  43. available and modules that are unsigned are permitted, but the kernel will
  44. be marked as being tainted, and the concerned modules will be marked as
  45. tainted, shown with the character 'E'.
  46. If this is on (ie. "restrictive"), only modules that have a valid
  47. signature that can be verified by a public key in the kernel's possession
  48. will be loaded. All other modules will generate an error.
  49. Irrespective of the setting here, if the module has a signature block that
  50. cannot be parsed, it will be rejected out of hand.
  51. (2) :menuselection:`Automatically sign all modules`
  52. (``CONFIG_MODULE_SIG_ALL``)
  53. If this is on then modules will be automatically signed during the
  54. modules_install phase of a build. If this is off, then the modules must
  55. be signed manually using::
  56. scripts/sign-file
  57. (3) :menuselection:`Which hash algorithm should modules be signed with?`
  58. This presents a choice of which hash algorithm the installation phase will
  59. sign the modules with:
  60. =============================== ==========================================
  61. ``CONFIG_MODULE_SIG_SHA1`` :menuselection:`Sign modules with SHA-1`
  62. ``CONFIG_MODULE_SIG_SHA224`` :menuselection:`Sign modules with SHA-224`
  63. ``CONFIG_MODULE_SIG_SHA256`` :menuselection:`Sign modules with SHA-256`
  64. ``CONFIG_MODULE_SIG_SHA384`` :menuselection:`Sign modules with SHA-384`
  65. ``CONFIG_MODULE_SIG_SHA512`` :menuselection:`Sign modules with SHA-512`
  66. =============================== ==========================================
  67. The algorithm selected here will also be built into the kernel (rather
  68. than being a module) so that modules signed with that algorithm can have
  69. their signatures checked without causing a dependency loop.
  70. (4) :menuselection:`File name or PKCS#11 URI of module signing key`
  71. (``CONFIG_MODULE_SIG_KEY``)
  72. Setting this option to something other than its default of
  73. ``certs/signing_key.pem`` will disable the autogeneration of signing keys
  74. and allow the kernel modules to be signed with a key of your choosing.
  75. The string provided should identify a file containing both a private key
  76. and its corresponding X.509 certificate in PEM form, or — on systems where
  77. the OpenSSL ENGINE_pkcs11 is functional — a PKCS#11 URI as defined by
  78. RFC7512. In the latter case, the PKCS#11 URI should reference both a
  79. certificate and a private key.
  80. If the PEM file containing the private key is encrypted, or if the
  81. PKCS#11 token requries a PIN, this can be provided at build time by
  82. means of the ``KBUILD_SIGN_PIN`` variable.
  83. (5) :menuselection:`Additional X.509 keys for default system keyring`
  84. (``CONFIG_SYSTEM_TRUSTED_KEYS``)
  85. This option can be set to the filename of a PEM-encoded file containing
  86. additional certificates which will be included in the system keyring by
  87. default.
  88. Note that enabling module signing adds a dependency on the OpenSSL devel
  89. packages to the kernel build processes for the tool that does the signing.
  90. =======================
  91. Generating signing keys
  92. =======================
  93. Cryptographic keypairs are required to generate and check signatures. A
  94. private key is used to generate a signature and the corresponding public key is
  95. used to check it. The private key is only needed during the build, after which
  96. it can be deleted or stored securely. The public key gets built into the
  97. kernel so that it can be used to check the signatures as the modules are
  98. loaded.
  99. Under normal conditions, when ``CONFIG_MODULE_SIG_KEY`` is unchanged from its
  100. default, the kernel build will automatically generate a new keypair using
  101. openssl if one does not exist in the file::
  102. certs/signing_key.pem
  103. during the building of vmlinux (the public part of the key needs to be built
  104. into vmlinux) using parameters in the::
  105. certs/x509.genkey
  106. file (which is also generated if it does not already exist).
  107. It is strongly recommended that you provide your own x509.genkey file.
  108. Most notably, in the x509.genkey file, the req_distinguished_name section
  109. should be altered from the default::
  110. [ req_distinguished_name ]
  111. #O = Unspecified company
  112. CN = Build time autogenerated kernel key
  113. #emailAddress = unspecified.user@unspecified.company
  114. The generated RSA key size can also be set with::
  115. [ req ]
  116. default_bits = 4096
  117. It is also possible to manually generate the key private/public files using the
  118. x509.genkey key generation configuration file in the root node of the Linux
  119. kernel sources tree and the openssl command. The following is an example to
  120. generate the public/private key files::
  121. openssl req -new -nodes -utf8 -sha256 -days 36500 -batch -x509 \
  122. -config x509.genkey -outform PEM -out kernel_key.pem \
  123. -keyout kernel_key.pem
  124. The full pathname for the resulting kernel_key.pem file can then be specified
  125. in the ``CONFIG_MODULE_SIG_KEY`` option, and the certificate and key therein will
  126. be used instead of an autogenerated keypair.
  127. =========================
  128. Public keys in the kernel
  129. =========================
  130. The kernel contains a ring of public keys that can be viewed by root. They're
  131. in a keyring called ".builtin_trusted_keys" that can be seen by::
  132. [root@deneb ~]# cat /proc/keys
  133. ...
  134. 223c7853 I------ 1 perm 1f030000 0 0 keyring .builtin_trusted_keys: 1
  135. 302d2d52 I------ 1 perm 1f010000 0 0 asymmetri Fedora kernel signing key: d69a84e6bce3d216b979e9505b3e3ef9a7118079: X509.RSA a7118079 []
  136. ...
  137. Beyond the public key generated specifically for module signing, additional
  138. trusted certificates can be provided in a PEM-encoded file referenced by the
  139. ``CONFIG_SYSTEM_TRUSTED_KEYS`` configuration option.
  140. Further, the architecture code may take public keys from a hardware store and
  141. add those in also (e.g. from the UEFI key database).
  142. Finally, it is possible to add additional public keys by doing::
  143. keyctl padd asymmetric "" [.builtin_trusted_keys-ID] <[key-file]
  144. e.g.::
  145. keyctl padd asymmetric "" 0x223c7853 <my_public_key.x509
  146. Note, however, that the kernel will only permit keys to be added to
  147. ``.builtin_trusted_keys`` **if** the new key's X.509 wrapper is validly signed by a key
  148. that is already resident in the ``.builtin_trusted_keys`` at the time the key was added.
  149. ========================
  150. Manually signing modules
  151. ========================
  152. To manually sign a module, use the scripts/sign-file tool available in
  153. the Linux kernel source tree. The script requires 4 arguments:
  154. 1. The hash algorithm (e.g., sha256)
  155. 2. The private key filename or PKCS#11 URI
  156. 3. The public key filename
  157. 4. The kernel module to be signed
  158. The following is an example to sign a kernel module::
  159. scripts/sign-file sha512 kernel-signkey.priv \
  160. kernel-signkey.x509 module.ko
  161. The hash algorithm used does not have to match the one configured, but if it
  162. doesn't, you should make sure that hash algorithm is either built into the
  163. kernel or can be loaded without requiring itself.
  164. If the private key requires a passphrase or PIN, it can be provided in the
  165. $KBUILD_SIGN_PIN environment variable.
  166. ============================
  167. Signed modules and stripping
  168. ============================
  169. A signed module has a digital signature simply appended at the end. The string
  170. ``~Module signature appended~.`` at the end of the module's file confirms that a
  171. signature is present but it does not confirm that the signature is valid!
  172. Signed modules are BRITTLE as the signature is outside of the defined ELF
  173. container. Thus they MAY NOT be stripped once the signature is computed and
  174. attached. Note the entire module is the signed payload, including any and all
  175. debug information present at the time of signing.
  176. ======================
  177. Loading signed modules
  178. ======================
  179. Modules are loaded with insmod, modprobe, ``init_module()`` or
  180. ``finit_module()``, exactly as for unsigned modules as no processing is
  181. done in userspace. The signature checking is all done within the kernel.
  182. =========================================
  183. Non-valid signatures and unsigned modules
  184. =========================================
  185. If ``CONFIG_MODULE_SIG_FORCE`` is enabled or module.sig_enforce=1 is supplied on
  186. the kernel command line, the kernel will only load validly signed modules
  187. for which it has a public key. Otherwise, it will also load modules that are
  188. unsigned. Any module for which the kernel has a key, but which proves to have
  189. a signature mismatch will not be permitted to load.
  190. Any module that has an unparseable signature will be rejected.
  191. =========================================
  192. Administering/protecting the private key
  193. =========================================
  194. Since the private key is used to sign modules, viruses and malware could use
  195. the private key to sign modules and compromise the operating system. The
  196. private key must be either destroyed or moved to a secure location and not kept
  197. in the root node of the kernel source tree.
  198. If you use the same private key to sign modules for multiple kernel
  199. configurations, you must ensure that the module version information is
  200. sufficient to prevent loading a module into a different kernel. Either
  201. set ``CONFIG_MODVERSIONS=y`` or ensure that each configuration has a different
  202. kernel release string by changing ``EXTRAVERSION`` or ``CONFIG_LOCALVERSION``.