docs/grub: Document appended signature
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>
This commit is contained in:
Sudhakar Kuppusamy
Daniel Kiper
parent
0f2dda8cf6
commit
3dff10a979
325
docs/grub.texi
325
docs/grub.texi
@ -3281,8 +3281,10 @@ GRUB. Others may be used freely in GRUB configuration files.
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These variables have special meaning to GRUB.
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@menu
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* appendedsig_key_mgmt::
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* biosnum::
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* blsuki_save_default::
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* check_appended_signatures::
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* check_signatures::
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* chosen::
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* cmdpath::
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@ -3333,6 +3335,19 @@ These variables have special meaning to GRUB.
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@end menu
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@node appendedsig_key_mgmt
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@subsection appendedsig_key_mgmt
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This variable controls whether GRUB enforces appended signature validation
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using either @code{static} or @code{dynamic} key management. It is automatically
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set by GRUB to either @code{static} or @code{dynamic} based on the
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@strong{'ibm,secure-boot'} device tree property and Platform KeyStore (PKS).
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Also, it can be explicitly set to either @code{static} or @code{dynamic} by
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setting the @code{appendedsig_key_mgmt} variable from the GRUB console
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when the GRUB is not locked down.
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@xref{Using appended signatures} for more information.
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@node biosnum
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@subsection biosnum
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@ -3353,6 +3368,17 @@ If this variable is set, menu entries generated from BLS config files
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(@pxref{blscfg}) or UKI files (@pxref{uki}) will be set as the default boot
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entry when selected.
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@node check_appended_signatures
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@subsection check_appended_signatures
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This variable controls whether GRUB enforces appended signature validation on
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loaded kernel and GRUB module files. It is automatically set by GRUB
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to either @code{no} or @code{yes} based on the @strong{'ibm,secure-boot'} device
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tree property. Also, it can be explicitly set to either @code{no} or @code{yes} by
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setting the @code{check_appended_signatures} variable from the GRUB console
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when the GRUB is not locked down.
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@xref{Using appended signatures} for more information.
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@node check_signatures
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@subsection check_signatures
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@ -6546,6 +6572,13 @@ you forget a command, you can run the command @command{help}
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@menu
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* [:: Check file types and compare values
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* acpi:: Load ACPI tables
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* append_add_db_cert:: Add trusted certificate to the db list
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* append_add_db_hash:: Add trusted certificate/binary hash to the db list
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* append_add_dbx_cert:: Add distrusted certificate to the dbx list
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* append_add_dbx_hash:: Add distrusted certificate/binary hash to the dbx list
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* append_list_db:: List all trusted certificates from the db list
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* append_list_dbx:: List all distrusted certificates and binary/certificate hashes from the dbx list
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* append_verify:: Verify appended digital signature using db and dbx lists
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* authenticate:: Check whether user is in user list
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* background_color:: Set background color for active terminal
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* background_image:: Load background image for active terminal
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@ -6669,6 +6702,140 @@ Note: The command is not allowed when lockdown is enforced (@pxref{Lockdown}).
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unsigned code.
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@end deffn
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@node append_add_db_cert
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@subsection append_add_db_cert
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@deffn Command append_add_db_cert <X509_certificate>
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Read an X.509 certificate from the file @var{X509_certificate}
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and add it to GRUB's internal db list of trusted certificates.
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These certificates are used to validate appended signatures when the
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environment variable @code{check_appended_signatures} (@pxref{check_appended_signatures})
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is set to @code{yes} or the @command{append_verify} (@pxref{append_verify})
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command is executed from the GRUB console.
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@xref{Using appended signatures} for more information.
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@end deffn
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@node append_add_db_hash
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@subsection append_add_db_hash
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@deffn Command append_add_db_hash <hash_file>
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Read a binary hash from the file @var{hash_file}
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and add it to GRUB's internal db list of trusted binary hashes. These
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hashes are used to validate the Linux kernel/GRUB module binary hashes when the
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environment variable @code{check_appended_signatures}
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(@pxref{check_appended_signatures}) is set to @code{yes} or the
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@command{append_verify} (@pxref{append_verify}) command is executed
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from the GRUB console.
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Here is an example for how to generate a SHA-256 hash for a file. The hash
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will be in binary format:
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@example
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# The vmlinux (kernel image) file is your binary file, and
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# it should be unsigned.
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#
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# Generate the binary_hash.bin file from the vmlinux file
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# using OpenSSL command
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openssl dgst -binary -sha256 -out binary_hash.bin vmlinux
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@end example
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@xref{Using appended signatures} for more information.
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@end deffn
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@node append_add_dbx_cert
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@subsection append_add_dbx_cert
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@deffn Command append_add_dbx_cert <X509_certificate>
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Read an X.509 certificate from the file @var{X509_certificate}
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and add it to GRUB's internal dbx list of distrusted certificates.
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These certificates are used to ensure that the distrusted certificates
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are rejected during appended signatures validation when the environment
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variable @code{check_appended_signatures} is set to @code{yes}
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(@pxref{check_appended_signatures}) or the @command{append_verify}
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(@pxref{append_verify}) command is executed from the GRUB console.
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Also, these certificates are used to prevent distrusted certificates from
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being added to the db list later on.
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@xref{Using appended signatures} for more information.
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@end deffn
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@node append_add_dbx_hash
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@subsection append_add_dbx_hash
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@deffn Command append_add_dbx_hash [@option{-b}|@option{-c}] <hash_file>
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Read a binary/certificate hash from the file @var{hash_file}
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and add it to GRUB's internal dbx list of distrusted binary/certificate hashes.
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When the environment variable @code{check_appended_signatures} (@pxref{check_appended_signatures})
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is set to @code{yes} or the @command{append_verify} (@pxref{append_verify}) command
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is executed from the GRUB console, then matching distrusted binary hashes or the signature
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validation with distrusted certificates may lead to the rejection of the Linux kernel or GRUB modules.
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Also, these hashes are used to prevent distrusted certificates and binary hashes from being
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added to the db list later on.
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The @option{-b} (@option{--binary-hash}) can be used to specify a binary hash file and
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@option{-c} (@option{--cert-hash}) can be used to specify a certificate hash file.
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Here is an example for how to generate a SHA-256 hash for a binary and a
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certificate file. The hash will be in binary format:
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@example
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# The vmlinux (kernel image) file is your binary file, and
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# it should be unsigned. The kernel.der is your certificate file.
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#
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# Generate the cert_hash.bin file from the kernel.der file
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openssl dgst -binary -sha256 -out cert_hash.bin kernel.der
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# Generate the binary_hash.bin file from the vmlinux file
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openssl dgst -binary -sha256 -out binary_hash.bin vmlinux
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@end example
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@xref{Using appended signatures} for more information.
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@end deffn
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@node append_list_db
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@subsection append_list_db
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@deffn Command append_list_db
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List all X.509 certificates and binary hashes trusted by GRUB for validating
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appended signatures. The output is a numbered list of certificates and binary hashes,
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showing the certificate's version, serial number, issuer, subject,
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public key algorithm, RSA public key size, and certificate fingerprint.
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@xref{Using appended signatures} for more information.
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@end deffn
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@node append_list_dbx
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@subsection append_list_dbx
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@deffn Command append_list_dbx
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List all the distrusted X.509 certificates and binary/certificate hashes.
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The output is a numbered list of certificates and binary/certificate hashes,
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showing the certificate's version, serial number, issuer, subject,
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public key algorithm, RSA public key size, and certificate fingerprint.
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@xref{Using appended signatures} for more information.
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@end deffn
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@node append_verify
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@subsection append_verify
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@deffn Command append_verify <signed_file>
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Verifies an appended signature on @var{signed_file} against the trusted X.509 certificates
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and hashes known to GRUB (@pxref{append_list_db},@pxref{append_list_dbx}, @pxref{append_add_db_cert},
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@pxref{append_add_db_hash}, @pxref{append_add_dbx_hash} and @pxref{append_add_dbx_cert}).
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Exit code @code{$?} is set to 0 if the signature validates successfully.
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If validation fails, it is set to a non-zero value.
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@xref{Using appended signatures} for more information.
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@end deffn
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@node authenticate
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@subsection authenticate
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@ -7507,6 +7674,13 @@ configurations, but to allow the user to select from among multiple
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configurations, and to enable ``one-shot'' boot attempts and
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``savedefault'' behavior. @xref{Using GPG-style digital signatures}, for more
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information.
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If the environment variable @code{check_appended_signatures} value is set to
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@code{yes} and GRUB is in lockeddown mode, the user is not allowed to set
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@code{check_appended_signatures} to @code{no} and @code{appendedsig_key_mgmt}
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to @code{static} or @code{dynamic} either directly using @command{load_env}
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command or via environment block file. @xref{Using appended signatures}, for
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more information.
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@end deffn
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@ -8902,11 +9076,13 @@ environment variables and commands are listed in the same order.
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@menu
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* Authentication and authorisation:: Users and access control
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* Using GPG-style digital signatures:: Booting digitally signed code
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* Using appended signatures:: An alternative approach to booting digitally signed code
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* UEFI secure boot and shim:: Booting digitally signed PE files
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* Secure Boot Advanced Targeting:: Embedded information for generation number based revocation
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* Measured Boot:: Measuring boot components
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* Lockdown:: Lockdown when booting on a secure setup
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* TPM2 key protector:: Managing disk key with TPM2 key protector
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* Signing certificate and hash files:: Certificate and hash file signing
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* Signing GRUB itself:: Ensuring the integrity of the GRUB core image
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@end menu
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@ -9067,6 +9243,127 @@ or BIOS) configuration to cause the machine to boot from a different
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(attacker-controlled) device. GRUB is at best only one link in a
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secure boot chain.
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@node Using appended signatures
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@section Using appended signatures in GRUB
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GRUB supports verifying Linux-style 'appended signatures' for Linux on Power LPAR
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secure boot. Appended signatures are PKCS#7 messages containing a signature over the
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contents of a file, plus some metadata, appended to the end of a file. A file
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with an appended signature ends with the magic string:
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@example
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~Module signature appended~\n
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@end example
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where @code{\n} represents the line feed character, @code{0x0a}.
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Linux on Power LPAR secure boot is controlled by @strong{'ibm,secure-boot'}
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device tree property and if this property is set to @code{2} (@samp{enforce}),
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GRUB enters lockdown mode. There are three secure boot modes. They are
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@itemize
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@item @samp{0 - disabled}: Secure boot is disabled. This is the default.
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@item @samp{1 - audit}: Enforce signature verification by setting
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@code{check_appended_signatures} (@pxref{check_appended_signatures}) to
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@code{yes} and do not enter lockdown mode. Signature verification
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is performed and if signature verification fails, display the errors and
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allow the boot to continue.
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@item @samp{2 - enforce}: Enter lockdown mode and enforce signature verification by setting
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@code{check_appended_signatures} (@pxref{check_appended_signatures}) to @code{yes}.
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@end itemize
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Note that Linux on Power LPAR only supports @samp{0 - disabled} and @samp{2 - enforce},
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and @samp{1 - audit} is considered as secure boot being disabled.
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Enforcement of signature verification is controlled by the environment variable
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@code{check_appended_signatures} (@pxref{check_appended_signatures}).
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@itemize
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@item @samp{no}: No verification is performed. This is the default.
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@item @samp{yes}: Signature verification is performed and if signature verification fails,
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display the errors and stop the boot. Signature verification cannot be disabled by setting
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the @code{check_appended_signatures} variable back to @samp{no}.
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@end itemize
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To enable appended signature verification, load the appendedsig module and an
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X.509 certificate for verification. It is recommended to build the appendedsig module
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into the core GRUB image.
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Key management is controlled by the environment variable @code{appendedsig_key_mgmt}
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(@pxref{appendedsig_key_mgmt}).
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@itemize
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@item @samp{static}: Enforce static key management signature verification. This is the default.
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When GRUB is in lockdown mode, then the user cannot change the value of the
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@code{appendedsig_key_mgmt}.
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@item @samp{dynamic}: Enforce dynamic key management signature verification. When GRUB is in
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lockdown mode, then the user cannot change the value of the @code{appendedsig_key_mgmt}.
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@end itemize
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In static key management mode, certificates will be built into the core image using
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the @code{--x509} parameter to @command{grub-mkimage}. The list of trusted certificates
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available at boot time can be shown using @command{append_list_db} (@pxref{append_list_db}).
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Distrusted certificates can be explicitly removed from the db using @command{append_add_dbx_cert}
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(@pxref{append_add_dbx_cert}). Also, trusted certificates can be explicitly added to the db using
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@command{append_add_db_cert} (@pxref{append_add_db_cert}).
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In dynamic key management mode, db and dbx are read from the Platform KeyStore (PKS). If
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db does not exist in PKS, static keys (built-in keys) are used as the default keys.
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The list of trusted certificates and binary hashes available at boot time can be shown using
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@command{append_list_db} (@pxref{append_list_db}) and the list of distrusted certificates and
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binary/certificate hashes available at boot time can be shown using @command{append_list_dbx}
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(@pxref{append_list_dbx}). The trusted certificates and binary hashes can be explicitly added
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to the db using @command{append_add_db_cert} (@pxref{append_add_db_cert}) and
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@command{append_add_db_hash} (@pxref{append_add_db_hash}). Distrusted certificates can be explicitly
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added to the dbx using @command{append_add_dbx_cert} (@pxref{append_add_dbx_cert}) and distrusted
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certificate/binary hashes can be explicitly added to the dbx using @command{append_add_dbx_hash}
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(@pxref{append_add_dbx_hash}).
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A file can be explicitly verified using @command{append_verify} (@pxref{append_verify}).
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Note that when the environment variable @code{check_appended_signatures} is set to @code{yes},
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the @command{append_add_db_cert} and @command{append_add_dbx_cert} commands only accept
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the file @samp{@var{X509_certificate}} that is signed with an appended signature
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(@pxref{Signing certificate and hash files}), and the @command{append_add_db_hash} and
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@command{append_add_dbx_hash} commands only accept the file @samp{@var{hash_file}} that is
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signed with an appended signature (@pxref{Signing certificate and hash files}).
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The signature is verified by the appendedsig module.
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When the environment variable @code{check_appended_signatures} is set to @code{no},
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these commands accept files without an appended signature.
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Also, note that @samp{@var{X509_certificate}} should be in DER-format and @samp{@var{hash_file}}
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should be in binary format. Only SHA-256, SHA-384, or SHA-512 hashes of binary/certificate are allowed.
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Certificates/hashes of certificates/binaries added through @command{append_add_db_cert},
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@command{append_add_dbx_cert}, @command{append_add_db_hash}, and @command{append_add_dbx_hash}
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will not be persisted across boots.
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Only signatures created using SHA-256 or SHA-512 hash algorithm along with RSA keys of size 2048,
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3072, or 4096 bits are supported.
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A file can be signed with the @command{sign-file} utility supplied with the
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Linux kernel source. For example, if you have @code{signing.key} as the private
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key and @code{certificate.der} as the X.509 certificate containing the public key:
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@example
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sign-file SHA256 signing.key certificate.der vmlinux vmlinux.signed
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@end example
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Once signature verification is turned on, the following file types must carry
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appended signatures:
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@enumerate
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@item Linux kernels
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@item GRUB modules, except those built in to the core image
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@item Any new certificate or binary hash files to be trusted
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@item Any new certificate/binary hash files to be distrusted
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@end enumerate
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When GRUB is in lockdown mode (when secure boot mode is set to @code{enforce}),
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signature verification cannot be @strong{disabled} by setting the
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@code{check_appended_signatures} (@pxref{check_appended_signatures}) variable
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to @code{no} or using the @command{load_env} (@pxref{load_env}) command from
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the GRUB console.
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@node UEFI secure boot and shim
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@section UEFI secure boot and shim support
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@ -9596,6 +9893,34 @@ which increases the risk of password leakage during the process. Moreover, the
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superuser list must be well maintained, and the password used cannot be
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synchronized with LUKS key rotation.
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@node Signing certificate and hash files
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@section Signing certificate and hash files
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X.509 certificate (public key) files and hash files (binary/certificate hash files)
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can be signed with a Linux kernel module-style appended signature.
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The signer.key is a private key used for signing and signer.der is the corresponding
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public key (certificate) used for appended signature verification. Note that the
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signer.der (certificate) should exist in the db (@pxref{Using appended signatures}).
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@itemize
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@item Signing the X.509 certificate file using @file{sign-file}.
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The kernel.der is an X.509 certificate file.
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@example
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sign-file SHA256 signer.key signer.der kernel.der \
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kernel.der.signed
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@end example
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@item Signing the hash file using @file{sign-file}.
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The binary_hash.bin is a binary hash file.
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@example
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sign-file SHA256 signer.key signer.der binary_hash.bin \
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binary_hash.signed
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@end example
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@end itemize
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@node Signing GRUB itself
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@section Signing GRUB itself
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To ensure a complete secure-boot chain, there must be a way for the code that
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