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 <[email protected]>
Signed-off-by: Sudhakar Kuppusamy <[email protected]>
Reviewed-by: Avnish Chouhan <[email protected]>
Reviewed-by: Daniel Kiper <[email protected]>
---
docs/grub.texi | 325 +++++++++++++++++++++++++++++++++++++++++++++++++
1 file changed, 325 insertions(+)
diff --git a/docs/grub.texi b/docs/grub.texi
index c462c68e7..af06bf237 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, post 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 shown using @command{append_list_db}
(@pxref{append_list_db}
+(@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
