[PATCH v18 20/20] docs/grub: Document appended signature

[Sudhakar Kuppusamy <sudhakar@linux.ibm.com>]

This explains how appended signatures can be used to form part of
a secure boot chain, and documents the commands and variables
introduced.

Signed-off-by: Daniel Axtens <dja@axtens.net>
Signed-off-by: Sudhakar Kuppusamy <sudhakar@linux.ibm.com>
Reviewed-by: Stefan Berger <stefanb@linux.ibm.com>
Reviewed-by: Avnish Chouhan <avnish@linux.ibm.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>
---
 docs/grub.texi | 325 +++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 325 insertions(+)

diff --git a/docs/grub.texi b/docs/grub.texi
index c462c68e7..7cd5dee06 100644
--- a/docs/grub.texi
+++ b/docs/grub.texi
@@ -3281,8 +3281,10 @@ GRUB.  Others may be used freely in GRUB configuration files.
 These variables have special meaning to GRUB.
 
 @menu
+* appendedsig_key_mgmt::
 * biosnum::
 * blsuki_save_default::
+* check_appended_signatures::
 * check_signatures::
 * chosen::
 * cmdpath::
@@ -3333,6 +3335,19 @@ These variables have special meaning to GRUB.
 @end menu
 
 
+@node appendedsig_key_mgmt
+@subsection appendedsig_key_mgmt
+
+This variable controls whether GRUB enforces appended signature validation
+using either @code{static} or @code{dynamic} key management. It is automatically
+set by GRUB to either @code{static} or @code{dynamic} based on the
+@strong{'ibm,secure-boot'} device tree property and Platform KeyStore (PKS).
+Also, it can be explicitly set to either @code{static} or @code{dynamic} by
+setting the @code{appendedsig_key_mgmt} variable from the GRUB console
+when the GRUB is not locked down.
+
+@xref{Using appended signatures} for more information.
+
 @node biosnum
 @subsection biosnum
 
@@ -3353,6 +3368,17 @@ If this variable is set, menu entries generated from BLS config files
 (@pxref{blscfg}) or UKI files (@pxref{uki}) will be set as the default boot
 entry when selected.
 
+@node check_appended_signatures
+@subsection check_appended_signatures
+
+This variable controls whether GRUB enforces appended signature validation on
+loaded kernel and GRUB module files. It is automatically set by GRUB
+to either @code{no} or @code{yes} based on the @strong{'ibm,secure-boot'} device
+tree property. Also, it can be explicitly set to either @code{no} or @code{yes} by
+setting the @code{check_appended_signatures} variable from the GRUB console
+when the GRUB is not locked down.
+
+@xref{Using appended signatures} for more information.
 
 @node check_signatures
 @subsection check_signatures
@@ -6546,6 +6572,13 @@ you forget a command, you can run the command @command{help}
 @menu
 * [::                           Check file types and compare values
 * acpi::                        Load ACPI tables
+* append_add_db_cert::          Add trusted certificate to the db list
+* append_add_db_hash::          Add trusted certificate/binary hash to the db list
+* append_add_dbx_cert::         Add distrusted certificate to the dbx list
+* append_add_dbx_hash::         Add distrusted certificate/binary hash to the dbx list
+* append_list_db::              List all trusted certificates from the db list
+* append_list_dbx::             List all distrusted certificates and binary/certificate hashes from the dbx list
+* append_verify::               Verify appended digital signature using db and dbx lists
 * authenticate::                Check whether user is in user list
 * background_color::            Set background color for active terminal
 * background_image::            Load background image for active terminal
@@ -6669,6 +6702,140 @@ Note: The command is not allowed when lockdown is enforced (@pxref{Lockdown}).
       unsigned code.
 @end deffn
 
+@node append_add_db_cert
+@subsection append_add_db_cert
+
+@deffn Command append_add_db_cert <X509_certificate>
+Read an X.509 certificate from the file @var{X509_certificate}
+and add it to GRUB's internal db list of trusted certificates.
+These certificates are used to validate appended signatures when the
+environment variable @code{check_appended_signatures} (@pxref{check_appended_signatures})
+is set to @code{yes} or the @command{append_verify} (@pxref{append_verify})
+command is executed from the GRUB console.
+
+@xref{Using appended signatures} for more information.
+@end deffn
+
+@node append_add_db_hash
+@subsection append_add_db_hash
+
+@deffn Command append_add_db_hash <hash_file>
+Read a binary hash from the file @var{hash_file}
+and add it to GRUB's internal db list of trusted binary hashes. These
+hashes are used to validate the Linux kernel/GRUB module binary hashes when the
+environment variable @code{check_appended_signatures}
+(@pxref{check_appended_signatures}) is set to @code{yes} or the
+@command{append_verify} (@pxref{append_verify}) command is executed
+from the GRUB console.
+
+Here is an example for how to generate a SHA-256 hash for a file. The hash
+will be in binary format:
+
+@example
+
+# The vmlinux (kernel image) file is your binary file, and
+# it should be unsigned.
+#
+# Generate the binary_hash.bin file from the vmlinux file
+# using OpenSSL command
+
+openssl dgst -binary -sha256 -out binary_hash.bin vmlinux
+
+@end example
+
+@xref{Using appended signatures} for more information.
+@end deffn
+
+@node append_add_dbx_cert
+@subsection append_add_dbx_cert
+
+@deffn Command append_add_dbx_cert <X509_certificate>
+Read an X.509 certificate from the file @var{X509_certificate}
+and add it to GRUB's internal dbx list of distrusted certificates.
+These certificates are used to ensure that the distrusted certificates
+are rejected during appended signatures validation when the environment
+variable @code{check_appended_signatures} is set to @code{yes}
+(@pxref{check_appended_signatures}) or the @command{append_verify}
+(@pxref{append_verify}) command is executed from the GRUB console.
+Also, these certificates are used to prevent distrusted certificates from
+being added to the db list later on.
+
+@xref{Using appended signatures} for more information.
+@end deffn
+
+@node append_add_dbx_hash
+@subsection append_add_dbx_hash
+
+@deffn Command append_add_dbx_hash [@option{-b}|@option{-c}] <hash_file>
+Read a binary/certificate hash from the file @var{hash_file}
+and add it to GRUB's internal dbx list of distrusted binary/certificate hashes.
+When the environment variable @code{check_appended_signatures} (@pxref{check_appended_signatures})
+is set to @code{yes} or the @command{append_verify} (@pxref{append_verify}) command
+is executed from the GRUB console, then matching distrusted binary hashes or the signature
+validation with distrusted certificates may lead to the rejection of the Linux kernel or GRUB modules.
+Also, these hashes are used to prevent distrusted certificates and binary hashes from being
+added to the db list later on.
+
+The @option{-b} (@option{--binary-hash}) can be used to specify a binary hash file and
+@option{-c} (@option{--cert-hash}) can be used to specify a certificate hash file.
+
+Here is an example for how to generate a SHA-256 hash for a binary and a
+certificate file. The hash will be in binary format:
+
+@example
+
+# The vmlinux (kernel image) file is your binary file, and
+# it should be unsigned. The kernel.der is your certificate file.
+#
+# Generate the cert_hash.bin file from the kernel.der file
+
+openssl dgst -binary -sha256 -out cert_hash.bin kernel.der
+
+# Generate the binary_hash.bin file from the vmlinux file
+
+openssl dgst -binary -sha256 -out binary_hash.bin vmlinux
+
+@end example
+
+@xref{Using appended signatures} for more information.
+@end deffn
+
+@node append_list_db
+@subsection append_list_db
+
+@deffn Command append_list_db
+List all X.509 certificates and binary hashes trusted by GRUB for validating
+appended signatures. The output is a numbered list of certificates and binary hashes,
+showing the certificate's version, serial number, issuer, subject,
+public key algorithm, RSA public key size, and certificate fingerprint.
+
+@xref{Using appended signatures} for more information.
+@end deffn
+
+@node append_list_dbx
+@subsection append_list_dbx
+
+@deffn Command append_list_dbx
+List all the distrusted X.509 certificates and binary/certificate hashes.
+The output is a numbered list of certificates and binary/certificate hashes,
+showing the certificate's version, serial number, issuer, subject,
+public key algorithm, RSA public key size, and certificate fingerprint.
+
+@xref{Using appended signatures} for more information.
+@end deffn
+
+@node append_verify
+@subsection append_verify
+
+@deffn Command append_verify <signed_file>
+Verifies an appended signature on @var{signed_file} against the trusted X.509 certificates
+and hashes known to GRUB (@pxref{append_list_db},@pxref{append_list_dbx}, @pxref{append_add_db_cert},
+@pxref{append_add_db_hash}, @pxref{append_add_dbx_hash} and @pxref{append_add_dbx_cert}).
+Exit code @code{$?} is set to 0 if the signature validates successfully.
+If validation fails, it is set to a non-zero value.
+
+@xref{Using appended signatures} for more information.
+@end deffn
 
 @node authenticate
 @subsection authenticate
@@ -7507,6 +7674,13 @@ configurations, but to allow the user to select from among multiple
 configurations, and to enable ``one-shot'' boot attempts and
 ``savedefault'' behavior.  @xref{Using GPG-style digital signatures}, for more
 information.
+
+If the environment variable @code{check_appended_signatures} value is set to
+@code{yes} and GRUB is in lockeddown mode, the user is not allowed to set
+@code{check_appended_signatures} to @code{no} and @code{appendedsig_key_mgmt}
+to @code{static} or @code{dynamic} either directly using @command{load_env}
+command or via environment block file. @xref{Using appended signatures}, for
+more information.
 @end deffn
 
 
@@ -8902,11 +9076,13 @@ environment variables and commands are listed in the same order.
 @menu
 * Authentication and authorisation::   Users and access control
 * Using GPG-style digital signatures:: Booting digitally signed code
+* Using appended signatures::          An alternative approach to booting digitally signed code
 * UEFI secure boot and shim::          Booting digitally signed PE files
 * Secure Boot Advanced Targeting::     Embedded information for generation number based revocation
 * Measured Boot::                      Measuring boot components
 * Lockdown::                           Lockdown when booting on a secure setup
 * TPM2 key protector::                 Managing disk key with TPM2 key protector
+* Signing certificate and hash files:: Certificate and hash file signing
 * Signing GRUB itself::                Ensuring the integrity of the GRUB core image
 @end menu
 
@@ -9067,6 +9243,127 @@ or BIOS) configuration to cause the machine to boot from a different
 (attacker-controlled) device.  GRUB is at best only one link in a
 secure boot chain.
 
+@node Using appended signatures
+@section Using appended signatures in GRUB
+
+GRUB supports verifying Linux-style 'appended signatures' for Linux on Power LPAR
+secure boot. Appended signatures are PKCS#7 messages containing a signature over the
+contents of a file, plus some metadata, appended to the end of a file. A file
+with an appended signature ends with the magic string:
+
+@example
+~Module signature appended~\n
+@end example
+
+where @code{\n} represents the line feed character, @code{0x0a}.
+
+Linux on Power LPAR secure boot is controlled by @strong{'ibm,secure-boot'}
+device tree property and if this property is set to @code{2} (@samp{enforce}),
+GRUB enters lockdown mode. There are three secure boot modes. They are
+
+@itemize
+@item @samp{0 - disabled}: Secure boot is disabled. This is the default.
+@item @samp{1 - audit}: Enforce signature verification by setting
+      @code{check_appended_signatures} (@pxref{check_appended_signatures}) to
+      @code{yes} and do not enter lockdown mode. Signature verification
+      is performed and if signature verification fails, display the errors and
+      allow the boot to continue.
+@item @samp{2 - enforce}: Enter lockdown mode and enforce signature verification by setting
+      @code{check_appended_signatures} (@pxref{check_appended_signatures}) to @code{yes}.
+@end itemize
+
+Note that Linux on Power LPAR only supports @samp{0 - disabled} and @samp{2 - enforce},
+and @samp{1 - audit} is considered as secure boot being disabled.
+
+Enforcement of signature verification is controlled by the environment variable
+@code{check_appended_signatures} (@pxref{check_appended_signatures}).
+
+@itemize
+@item @samp{no}: No verification is performed. This is the default.
+@item @samp{yes}: Signature verification is performed and if signature verification fails,
+      display the errors and stop the boot. Signature verification cannot be disabled by setting
+      the @code{check_appended_signatures} variable back to @samp{no}.
+@end itemize
+
+To enable appended signature verification, load the appendedsig module and an
+X.509 certificate for verification. It is recommended to build the appendedsig module
+into the core GRUB image.
+
+Key management is controlled by the environment variable @code{appendedsig_key_mgmt}
+(@pxref{appendedsig_key_mgmt}).
+
+@itemize
+@item @samp{static}: Enforce static key management signature verification. This is the default.
+      When GRUB is in lockdown mode, then the user cannot change the value of the
+      @code{appendedsig_key_mgmt}.
+@item @samp{dynamic}: Enforce dynamic key management signature verification. When GRUB is in
+      lockdown mode, then the user cannot change the value of the @code{appendedsig_key_mgmt}.
+@end itemize
+
+In static key management mode, certificates will be built into the core image using
+the @code{--x509} parameter to @command{grub-mkimage}. The list of trusted certificates
+available at boot time can be shown using @command{append_list_db} (@pxref{append_list_db}).
+Distrusted certificates can be explicitly removed from the db using @command{append_add_dbx_cert}
+(@pxref{append_add_dbx_cert}). Also, trusted certificates can be explicitly added to the db using
+@command{append_add_db_cert} (@pxref{append_add_db_cert}).
+
+In dynamic key management mode, db and dbx are read from the Platform KeyStore (PKS). If
+db does not exist in PKS, static keys (built-in keys) are used as the default keys.
+The list of trusted certificates and binary hashes available at boot time can be shown using
+@command{append_list_db} (@pxref{append_list_db}) and the list of distrusted certificates and
+binary/certificate hashes available at boot time can be shown using @command{append_list_dbx}
+(@pxref{append_list_dbx}). The trusted certificates and binary hashes can be explicitly added
+to the db using @command{append_add_db_cert} (@pxref{append_add_db_cert}) and
+@command{append_add_db_hash} (@pxref{append_add_db_hash}). Distrusted certificates can be explicitly
+added to the dbx using @command{append_add_dbx_cert} (@pxref{append_add_dbx_cert}) and distrusted
+certificate/binary hashes can be explicitly added to the dbx using @command{append_add_dbx_hash}
+(@pxref{append_add_dbx_hash}).
+
+A file can be explicitly verified using @command{append_verify} (@pxref{append_verify}).
+
+Note that when the environment variable @code{check_appended_signatures} is set to @code{yes},
+the @command{append_add_db_cert} and @command{append_add_dbx_cert} commands only accept
+the file @samp{@var{X509_certificate}} that is signed with an appended signature
+(@pxref{Signing certificate and hash files}), and the @command{append_add_db_hash} and
+@command{append_add_dbx_hash} commands only accept the file @samp{@var{hash_file}} that is
+signed with an appended signature (@pxref{Signing certificate and hash files}).
+The signature is verified by the appendedsig module.
+When the environment variable @code{check_appended_signatures} is set to @code{no},
+these commands accept files without an appended signature.
+
+Also, note that @samp{@var{X509_certificate}} should be in DER-format and @samp{@var{hash_file}}
+should be in binary format. Only SHA-256, SHA-384, or SHA-512 hashes of binary/certificate are allowed.
+Certificates/hashes of certificates/binaries added through @command{append_add_db_cert},
+@command{append_add_dbx_cert}, @command{append_add_db_hash}, and @command{append_add_dbx_hash}
+will not be persisted across boots.
+
+Only signatures created using SHA-256 or SHA-512 hash algorithm along with RSA keys of size 2048,
+3072, or 4096 bits are supported.
+
+A file can be signed with the @command{sign-file} utility supplied with the
+Linux kernel source. For example, if you have @code{signing.key} as the private
+key and @code{certificate.der} as the X.509 certificate containing the public key:
+
+@example
+sign-file SHA256 signing.key certificate.der vmlinux vmlinux.signed
+@end example
+
+Once signature verification is turned on, the following file types must carry
+appended signatures:
+
+@enumerate
+@item Linux kernels
+@item GRUB modules, except those built in to the core image
+@item Any new certificate or binary hash files to be trusted
+@item Any new certificate/binary hash files to be distrusted
+@end enumerate
+
+When GRUB is in lockdown mode (when secure boot mode is set to @code{enforce}),
+signature verification cannot be @strong{disabled} by setting the
+@code{check_appended_signatures} (@pxref{check_appended_signatures}) variable
+to @code{no} or using the @command{load_env} (@pxref{load_env}) command from
+the GRUB console.
+
 @node UEFI secure boot and shim
 @section UEFI secure boot and shim support
 
@@ -9596,6 +9893,34 @@ which increases the risk of password leakage during the process. Moreover, the
 superuser list must be well maintained, and the password used cannot be
 synchronized with LUKS key rotation.
 
+@node Signing certificate and hash files
+@section Signing certificate and hash files
+X.509 certificate (public key) files and hash files (binary/certificate hash files)
+can be signed with a Linux kernel module-style appended signature.
+
+The signer.key is a private key used for signing and signer.der is the corresponding
+public key (certificate) used for appended signature verification. Note that the
+signer.der (certificate) should exist in the db (@pxref{Using appended signatures}).
+
+@itemize
+@item Signing the X.509 certificate file using @file{sign-file}.
+The kernel.der is an X.509 certificate file.
+@example
+
+sign-file SHA256 signer.key signer.der kernel.der \
+  kernel.der.signed
+
+@end example
+@item Signing the hash file using @file{sign-file}.
+The binary_hash.bin is a binary hash file.
+@example
+
+sign-file SHA256 signer.key signer.der binary_hash.bin \
+  binary_hash.signed
+
+@end example
+@end itemize
+
 @node Signing GRUB itself
 @section Signing GRUB itself
 To ensure a complete secure-boot chain, there must be a way for the code that
-- 
2.50.1 (Apple Git-155)


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