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342 lines
14 KiB
342 lines
14 KiB
From 0000000000000000000000000000000000000000 Mon Sep 17 00:00:00 2001
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From: Daniel Axtens <dja@axtens.net>
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Date: Thu, 1 Oct 2020 13:02:09 +1000
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Subject: [PATCH] appended signatures: documentation
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This explains how appended signatures can be used to form part of
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a secure boot chain, and documents the commands and variables
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introduced.
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Signed-off-by: Daniel Axtens <dja@axtens.net>
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---
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docs/grub.texi | 199 ++++++++++++++++++++++++++++++++++++++++++++++++++++-----
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1 file changed, 182 insertions(+), 17 deletions(-)
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diff --git a/docs/grub.texi b/docs/grub.texi
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index afbde7c1f7b..4816be85611 100644
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--- a/docs/grub.texi
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+++ b/docs/grub.texi
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@@ -3214,6 +3214,7 @@ These variables have special meaning to GRUB.
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@menu
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* biosnum::
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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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@@ -3273,11 +3274,18 @@ For an alternative approach which also changes BIOS drive mappings for the
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chain-loaded system, @pxref{drivemap}.
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+@node check_appended_signatures
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+@subsection check_appended_signatures
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+
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+This variable controls whether GRUB enforces appended signature validation on
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+certain loaded files. @xref{Using appended signatures}.
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+
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+
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@node check_signatures
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@subsection check_signatures
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-This variable controls whether GRUB enforces digital signature
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-validation on loaded files. @xref{Using digital signatures}.
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+This variable controls whether GRUB enforces GPG-style digital signature
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+validation on loaded files. @xref{Using GPG-style digital signatures}.
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@node chosen
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@subsection chosen
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@@ -3994,6 +4002,7 @@ you forget a command, you can run the command @command{help}
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* date:: Display or set current date and time
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* devicetree:: Load a device tree blob
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* distrust:: Remove a pubkey from trusted keys
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+* distrust_certificate:: Remove a certificate from the list of trusted certificates
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* drivemap:: Map a drive to another
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* echo:: Display a line of text
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* eval:: Evaluate agruments as GRUB commands
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@@ -4010,6 +4019,7 @@ you forget a command, you can run the command @command{help}
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* keystatus:: Check key modifier status
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* linux:: Load a Linux kernel
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* linux16:: Load a Linux kernel (16-bit mode)
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+* list_certificates:: List trusted certificates
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* list_env:: List variables in environment block
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* list_trusted:: List trusted public keys
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* load_env:: Load variables from environment block
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@@ -4047,8 +4057,10 @@ you forget a command, you can run the command @command{help}
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* test:: Check file types and compare values
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* true:: Do nothing, successfully
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* trust:: Add public key to list of trusted keys
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+* trust_certificate:: Add an x509 certificate to the list of trusted certificates
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* unset:: Unset an environment variable
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@comment * vbeinfo:: List available video modes
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+* verify_appended:: Verify appended digital signature
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* verify_detached:: Verify detached digital signature
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* videoinfo:: List available video modes
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@comment * xen_*:: Xen boot commands for AArch64
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@@ -4376,9 +4388,28 @@ These keys are used to validate signatures when environment variable
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@code{check_signatures} is set to @code{enforce}
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(@pxref{check_signatures}), and by some invocations of
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@command{verify_detached} (@pxref{verify_detached}). @xref{Using
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-digital signatures}, for more information.
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+GPG-style digital signatures}, for more information.
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@end deffn
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+
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+@node distrust_certificate
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+@subsection distrust_certificate
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+
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+@deffn Command distrust_certificate cert_number
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+Remove the x509 certificate numbered @var{cert_number} from GRUB's keyring of
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+trusted x509 certificates for verifying appended signatures.
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+
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+@var{cert_number} is the certificate number as listed by
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+@command{list_certificates} (@pxref{list_certificates}).
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+
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+These certificates are used to validate appended signatures when environment
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+variable @code{check_appended_signatures} is set to @code{enforce} or
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+@code{forced} (@pxref{check_appended_signatures}), and by
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+@command{verify_appended} (@pxref{verify_appended}). See
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+@xref{Using appended signatures} for more information.
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+@end deffn
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+
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+
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@node drivemap
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@subsection drivemap
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@@ -4636,6 +4667,21 @@ This command is only available on x86 systems.
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@end deffn
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+@node list_certificates
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+@subsection list_certificates
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+
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+@deffn Command list_certificates
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+List all x509 certificates trusted by GRUB for validating appended signatures.
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+The output is a numbered list of certificates, showing the certificate's serial
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+number and Common Name.
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+
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+The certificate number can be used as an argument to
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+@command{distrust_certificate} (@pxref{distrust_certificate}).
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+
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+See @xref{Using appended signatures} for more information.
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+@end deffn
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+
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+
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@node list_env
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@subsection list_env
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@@ -4655,7 +4701,7 @@ The output is in GPG's v4 key fingerprint format (i.e., the output of
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@code{gpg --fingerprint}). The least significant four bytes (last
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eight hexadecimal digits) can be used as an argument to
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@command{distrust} (@pxref{distrust}).
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-@xref{Using digital signatures}, for more information about uses for
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+@xref{Using GPG-style digital signatures}, for more information about uses for
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these keys.
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@end deffn
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@@ -4690,8 +4736,13 @@ When used with care, @option{--skip-sig} and the whitelist enable an
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administrator to configure a system to boot only signed
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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 digital signatures}, for more
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+``savedefault'' behavior. @xref{Using GPG-style digital signatures}, for more
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information.
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+
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+Extra care should be taken when combining this command with appended signatures
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+(@pxref{Using appended signatures}), as this file is not validated by an
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+appended signature and could set @code{check_appended_signatures=no} if GRUB is
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+not in @pxref{Lockdown} mode.
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@end deffn
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@@ -4987,7 +5038,7 @@ read. It is possible to modify a digitally signed environment block
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file from within GRUB using this command, such that its signature will
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no longer be valid on subsequent boots. Care should be taken in such
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advanced configurations to avoid rendering the system
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-unbootable. @xref{Using digital signatures}, for more information.
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+unbootable. @xref{Using GPG-style digital signatures}, for more information.
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@end deffn
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@@ -5387,11 +5438,32 @@ signatures when environment variable @code{check_signatures} is set to
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must itself be properly signed. The @option{--skip-sig} option can be
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used to disable signature-checking when reading @var{pubkey_file}
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itself. It is expected that @option{--skip-sig} is useful for testing
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-and manual booting. @xref{Using digital signatures}, for more
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+and manual booting. @xref{Using GPG-style digital signatures}, for more
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information.
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@end deffn
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+@node trust_certificate
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+@subsection trust_certificate
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+
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+@deffn Command trust_certificate x509_certificate
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+Read an DER-formatted x509 certificate from the file @var{x509_certificate}
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+and add it to GRUB's internal list of trusted x509 certificates. These
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+certificates are used to validate appended signatures when the environment
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+variable @code{check_appended_signatures} is set to @code{enforce} or
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+@code{forced}.
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+
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+Note that if @code{check_appended_signatures} is set to @code{enforce} or
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+@code{forced} when @command{trust_certificate} is executed, then
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+@var{x509_certificate} must itself bear an appended signature. (It is not
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+sufficient that @var{x509_certificate} be signed by a trusted certificate
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+according to the x509 rules: grub does not include support for validating
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+signatures within x509 certificates themselves.)
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+
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+See @xref{Using appended signatures} for more information.
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+@end deffn
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+
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+
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@node unset
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@subsection unset
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@@ -5410,6 +5482,18 @@ only on PC BIOS platforms.
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@end deffn
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@end ignore
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+@node verify_appended
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+@subsection verify_appended
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+
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+@deffn Command verify_appended file
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+Verifies an appended signature on @var{file} against the trusted certificates
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+known to GRUB (See @pxref{list_certificates}, @pxref{trust_certificate}, and
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+@pxref{distrust_certificate}).
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+
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+Exit code @code{$?} is set to 0 if the signature validates
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+successfully. If validation fails, it is set to a non-zero value.
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+See @xref{Using appended signatures}, for more information.
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+@end deffn
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@node verify_detached
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@subsection verify_detached
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@@ -5428,7 +5512,7 @@ tried.
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Exit code @code{$?} is set to 0 if the signature validates
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successfully. If validation fails, it is set to a non-zero value.
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-@xref{Using digital signatures}, for more information.
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+@xref{Using GPG-style digital signatures}, for more information.
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@end deffn
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@node videoinfo
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@@ -5811,13 +5895,14 @@ environment variables and commands are listed in the same order.
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@chapter Security
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@menu
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-* Authentication and authorisation:: Users and access control
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-* Using digital signatures:: 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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-* Signing GRUB itself:: Ensuring the integrity of the GRUB core image
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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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+* Signing GRUB itself:: Ensuring the integrity of the GRUB core image
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@end menu
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@node Authentication and authorisation
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@@ -5891,8 +5976,8 @@ generating configuration files with authentication. You can use
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adding @kbd{set superusers=} and @kbd{password} or @kbd{password_pbkdf2}
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commands.
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-@node Using digital signatures
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-@section Using digital signatures in GRUB
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+@node Using GPG-style digital signatures
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+@section Using GPG-style digital signatures in GRUB
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GRUB's @file{core.img} can optionally provide enforcement that all files
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subsequently read from disk are covered by a valid digital signature.
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@@ -5985,6 +6070,86 @@ 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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+
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+GRUB supports verifying Linux-style 'appended signatures' for secure boot.
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+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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+
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+@example
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+~Module signature appended~\n
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+@end example
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+
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+where @code{\n} represents the line-feed character, @code{0x0a}.
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+
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+Certificates can be managed at boot time using the @pxref{trust_certificate},
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+@pxref{distrust_certificate} and @pxref{list_certificates} commands.
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+Certificates can also be built in to the core image using the @code{--x509}
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+parameter to @command{grub-install} or @command{grub-mkimage}.
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+
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+A file can be explictly verified using the @pxref{verify_appended} command.
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+
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+Only signatures made with the SHA-256 or SHA-512 hash algorithm are supported,
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+and only RSA signatures are supported.
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+
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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 x509 certificate containing the public key:
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+
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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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+
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+Enforcement of signature verification is controlled by the
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+@code{check_appended_signatures} variable.
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+
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+@itemize
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+@item @samp{no}: no verification is performed. This is the default when GRUB
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+ is not in @pxref{Lockdown} mode.
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+@item @samp{enforce}: verification is performed. Verification can be disabled
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+ by setting the variable back to @samp{no}.
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+@item @samp{forced}: verification is performed and cannot be disabled. This is
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+ set when GRUB is in Lockdown when the appendedsig module is loaded.
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+@end itemize
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+
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+Unlike GPG-style signatures, not all files loaded by GRUB are required to be
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+signed. Once verification is turned on, the following file types will have
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+appended signatures verified:
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+
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+@itemize
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+@item Linux kernels
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+@item GRUB modules, except those built into the core image
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+@item Any new certificate files to be trusted
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+@end itemize
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+
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+ACPI tables and Device Tree images will not be checked for appended signatures
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+but must be verified by another mechanism such as GPG-style signatures before
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+they will be loaded.
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+
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+Unless lockdown mode is enabled, signature checking does @strong{not}
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+stop an attacker with console access from dropping manually to the GRUB
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+console and executing:
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+
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+@example
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+set check_appended_signatures=no
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+@end example
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+
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+Refer to the section on password-protecting GRUB (@pxref{Authentication
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+and authorisation}) for more information on preventing this.
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+
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+Additionally, unless lockdown mode is enabled:
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+
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+@itemize
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+@item Special care must be taken around the @command{loadenv} command, which
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+ can be used to turn off @code{check_appended_signature}.
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+
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+@item If the grub configuration file is loaded from the disk, anyone who can
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+ modify the file on disk can turn off @code{check_appended_signature}.
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+ Consider embedding the configuration into the core grub image.
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+@end itemize
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+
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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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