On 12/18/24 9:56 AM, Sudhakar Kuppusamy wrote:
From: Daniel Axtens <d...@axtens.net>
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 <d...@axtens.net>
Signed-off-by: Sudhakar Kuppusamy <sudha...@linux.ibm.com>
---
docs/grub.texi | 185
++++++++++++++++++++++++++++++++++++++++++++-----
1 file changed, 167 insertions(+), 18 deletions(-)
diff --git a/docs/grub.texi b/docs/grub.texi
index 6e483298d..f71ce9ffc 100644
--- a/docs/grub.texi
+++ b/docs/grub.texi
@@ -3274,6 +3274,7 @@ These variables have special meaning to GRUB.
@menu
* biosnum::
+* check_appended_signatures::
* check_signatures::
* chosen::
* cmdpath::
@@ -3336,12 +3337,16 @@ this.
For an alternative approach which also changes BIOS drive mappings
for the
chain-loaded system, @pxref{drivemap}.
+@node check_appended_signatures
+@subsection check_appended_signatures
+This variable controls whether GRUB enforces appended signature
validation on
+certain loaded files. @xref{Using appended signatures}.
@node check_signatures
@subsection check_signatures
-This variable controls whether GRUB enforces digital signature
-validation on loaded files. @xref{Using digital signatures}.
+This variable controls whether GRUB enforces GPG-style digital
signature
+validation on loaded files. @xref{Using GPG-style digital
signatures}.
I am not sure whether everybody knows what GPG-style digitiabl
signatures are. Maybe mention here once that those are in a separate
file.
@node chosen
@subsection chosen
@@ -6377,6 +6382,7 @@ you forget a command, you can run the command
@command{help}
* date:: Display or set current date and time
* devicetree:: Load a device tree blob
* distrust:: Remove a pubkey from trusted keys
+* distrust_certificate:: Remove a certificate from the list of
trusted certificates
* drivemap:: Map a drive to another
* echo:: Display a line of text
* efitextmode:: Set/Get text output mode resolution
@@ -6395,6 +6401,7 @@ you forget a command, you can run the command
@command{help}
* hexdump:: Show raw contents of a file or
memory
* insmod:: Insert a module
* keystatus:: Check key modifier status
+* list_certificates:: List trusted certificates
* list_env:: List variables in environment block
* list_trusted:: List trusted public keys
* load_env:: Load variables from environment
block
@@ -6435,8 +6442,10 @@ you forget a command, you can run the command
@command{help}
* tpm2_key_protector_clear:: Clear the TPM2 key protector
* true:: Do nothing, successfully
* trust:: Add public key to list of trusted
keys
+* trust_certificate:: Add an x509 certificate to the list
of trusted certificates
* unset:: Unset an environment variable
@comment * vbeinfo:: List available video modes
+* verify_appended:: Verify appended digital signature
* verify_detached:: Verify detached digital signature
* videoinfo:: List available video modes
* wrmsr:: Write values to model-specific
registers
@@ -6778,7 +6787,24 @@ These keys are used to validate signatures when
environment variable
@code{check_signatures} is set to @code{enforce}
(@pxref{check_signatures}), and by some invocations of
@command{verify_detached} (@pxref{verify_detached}). @xref{Using
-digital signatures}, for more information.
+GPG-style digital signatures}, for more information.
+@end deffn
+
+@node distrust_certificate
+@subsection distrust_certificate
+
+@deffn Command distrust_certificate cert_number
+Remove the x509 certificate numbered @var{cert_number} from GRUB's
keyring of
+trusted x509 certificates for verifying appended signatures.
+
+@var{cert_number} is the certificate number as listed by
+@command{list_certificates} (@pxref{list_certificates}).
+
+These certificates are used to validate appended signatures when
environment
+variable @code{check_appended_signatures} is set to @code{enforce}
+(@pxref{check_appended_signatures}), and by @command{verify_appended}
+(@pxref{verify_appended}). See @xref{Using appended signatures} for
more
+information.
@end deffn
@node drivemap
@@ -7169,6 +7195,19 @@ without any options, the @command{keystatus}
command returns true if and
only if checking key modifier status is supported.
@end deffn
+@node list_certificates
+@subsection list_certificates
+
+@deffn Command list_certificates
+List all x509 certificates trusted by GRUB for validating appended
signatures.
+The output is a numbered list of certificates, showing the
certificate's serial
+number and Common Name.
+
+The certificate number can be used as an argument to
+@command{distrust_certificate} (@pxref{distrust_certificate}).
+
+See @xref{Using appended signatures} for more information.
+@end deffn
@node list_env
@subsection list_env
@@ -7189,7 +7228,7 @@ The output is in GPG's v4 key fingerprint format
(i.e., the output of
@code{gpg --fingerprint}). The least significant four bytes (last
eight hexadecimal digits) can be used as an argument to
@command{distrust} (@pxref{distrust}).
-@xref{Using digital signatures}, for more information about uses for
+@xref{Using GPG-style digital signatures}, for more information about
uses for
these keys.
@end deffn
@@ -7224,8 +7263,11 @@ When used with care, @option{--skip-sig} and
the whitelist enable an
administrator to configure a system to boot only signed
configurations, but to allow the user to select from among multiple
configurations, and to enable ``one-shot'' boot attempts and
-``savedefault'' behavior. @xref{Using digital signatures}, for more
+``savedefault'' behavior. @xref{Using GPG-style digital signatures},
for more
information.
+Extra care should be taken when combining this command with appended
signatures
+(@pxref{Using appended signatures}), as this file is not validated by
an
+appended signature and could set @code{check_appended_signatures=no}.
@end deffn
@@ -7596,7 +7638,7 @@ read. It is possible to modify a digitally
signed environment block
file from within GRUB using this command, such that its signature
will
no longer be valid on subsequent boots. Care should be taken in
such
advanced configurations to avoid rendering the system
-unbootable. @xref{Using digital signatures}, for more information.
+unbootable. @xref{Using GPG-style digital signatures}, for more
information.
@end deffn
@@ -8064,11 +8106,30 @@ signatures when environment variable
@code{check_signatures} is set to
must itself be properly signed. The @option{--skip-sig} option can
be
used to disable signature-checking when reading @var{pubkey_file}
itself. It is expected that @option{--skip-sig} is useful for
testing
-and manual booting. @xref{Using digital signatures}, for more
+and manual booting. @xref{Using GPG-style digital signatures}, for
more
information.
@end deffn
+@node trust_certificate
+@subsection trust_certificate
+
+@deffn Command trust_certificate x509_certificate
+Read a DER-formatted x509 certificate from the file
@var{x509_certificate}
+and add it to GRUB's internal list of trusted x509 certificates.
These
+certificates are used to validate appended signatures when the
environment
+variable @code{check_appended_signatures} is set to @code{enforce}.
+
+Note that if @code{check_appended_signatures} is set to
@code{enforce}
+when @command{trust_certificate} is executed, then
@var{x509_certificate}
+must itself bear an appended signature. (It is not sufficient that
+@var{x509_certificate} be signed by a trusted certificate according
to the
+x509 rules: grub does not include support for validating signatures
within x509
s/grub/GRUB
+certificates themselves.)
+
+See @xref{Using appended signatures} for more information.
+@end deffn
+
@node unset
@subsection unset
@@ -8087,6 +8148,18 @@ only on PC BIOS platforms.
@end deffn
@end ignore
+@node verify_appended
+@subsection verify_appended
+
+@deffn Command verify_appended file
+Verifies an appended signature on @var{file} against the trusted
certificates
+known to GRUB (See @pxref{list_certificates},
@pxref{trust_certificate}, and
+@pxref{distrust_certificate}).
+Exit code @code{$?} is set to 0 if the signature validates
+successfully. If validation fails, it is set to a non-zero value.
+
+See @xref{Using appended signatures}, for more information.
+@end deffn
@node verify_detached
@subsection verify_detached
@@ -8105,7 +8178,7 @@ tried.
Exit code @code{$?} is set to 0 if the signature validates
successfully. If validation fails, it is set to a non-zero value.
-@xref{Using digital signatures}, for more information.
+@xref{Using GPG-style digital signatures}, for more information.
@end deffn
@node videoinfo
@@ -8565,14 +8638,15 @@ environment variables and commands are listed
in the same order.
@chapter Security
@menu
-* Authentication and authorisation:: Users and access control
-* Using digital signatures:: 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 GRUB itself:: Ensuring the integrity of the
GRUB core image
+* 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 GRUB itself:: Ensuring the integrity of the
GRUB core image
@end menu
@node Authentication and authorisation
@@ -8648,8 +8722,8 @@ generating configuration files with
authentication. You can use
adding @kbd{set superusers=} and @kbd{password} or
@kbd{password_pbkdf2}
commands.
-@node Using digital signatures
-@section Using digital signatures in GRUB
+@node Using GPG-style digital signatures
+@section Using GPG-style digital signatures in GRUB
GRUB's @file{core.img} can optionally provide enforcement that all
files
subsequently read from disk are covered by a valid digital
signature.
@@ -8732,6 +8806,81 @@ 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 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 carriage-return character,
@code{0x0a}.
\n is the newline parameter, \r is carriage-return
+
+To enable appended signature verification, load the appendedsig
module and an
+x509 certificate for verification. Building the appendedsig module
into the
+core grub image is recommended.
+
+Certificates can be managed at boot time using the
@pxref{trust_certificate},
+@pxref{distrust_certificate} and @pxref{list_certificates} commands.
+Certificates can also be built in to the core image using the
@code{--x509}
+parameter to @command{grub-install} or @command{grub-mkimage}.
+A file can be explictly verified using the @pxref{verify_appended}
command.
explicitly
+
+Only signatures made with the SHA-256 or SHA-512 hash algorithm are
supported,
+and only RSA signatures 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 x509 certificate containing the
public key:
+
+@example
+sign-file SHA256 signing.key certificate.der vmlinux vmlinux.signed
+@end example
+
+Enforcement of signature verification is controlled by the
+@code{check_appended_signatures} variable. Verification will only
take place
+when files are loaded if the variable is set to @code{enforce}. If a
+certificate is built into the grub core image with the @code{--x509}
parameter,
+the variable will be automatically set to @code{enforce} when the
appendedsig
+module is loaded.
+
+Unlike GPG-style signatures, not all files loaded by GRUB are
required to be
+signed. Once verification is turned on, the following file types must
carry
+appended signatures:
+
+@enumerate
+@item Linux, Multiboot, BSD, XNU and Plan9 kernels
+@item Grub modules, except those built in to the core image
+@item Any new certificate files to be trusted
+@end enumerate
+
+ACPI tables and Device Tree images will not be checked for appended
signatures
+but must be verified by another mechanism such as GPG-style
signatures before
+they will be loaded.
+
+No attempt is made to validate any other file type. In particular,
+chain-loaded binaries are not verified - if your platform supports
+chain-loading and this cannot be disabled, consider an alternative
secure
+boot mechanism.
+
+As with GPG-style appended signatures, signature checking does
@strong{not}
+stop an attacker with console access from dropping manually to the
GRUB
+console and executing:
+
+@example
+set check_appended_signatures=no
+@end example
+
+Refer to the section on password-protecting GRUB
(@pxref{Authentication
+and authorisation}) for more information on preventing this.
+
+Additionally, special care must be taken around the @command{loadenv}
command,
+which can be used to turn off @code{check_appended_signature}.
+
@node UEFI secure boot and shim
@section UEFI secure boot and shim support
With nits fixed:
Reviewed-by: Stefan Berger <stef...@linux.ibm.com>
