* docs/grub.texi: Document new signatures possibility.
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Jon McCune
Vladimir 'phcoder' Serbinenko
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2 changed files with 186 additions and 2 deletions
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@ -1,3 +1,7 @@
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2013-10-14 Jon McCune <jonmccune@google.com>
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* docs/grub.texi: Document new signatures possibility.
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2013-10-14 Vladimir Serbinenko <phcoder@gmail.com>
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Define GRUB_UTIL_FD_O_* and always use them with grub_util_fd_open.
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184
docs/grub.texi
184
docs/grub.texi
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@ -99,6 +99,7 @@ This edition documents version @value{VERSION}.
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* Commands:: The list of available builtin commands
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* Internationalisation:: Topics relating to language support
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* Security:: Authentication and authorisation
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* Security and signatures:: Verifying digital signatures in GRUB
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* Platform limitations:: The list of platform-specific limitations
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* Platform-specific operations:: Platform-specific operations
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* Supported kernels:: The list of supported kernels
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@ -2882,6 +2883,7 @@ These variables have special meaning to GRUB.
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@menu
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* biosnum::
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* check_signatures::
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* chosen::
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* color_highlight::
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* color_normal::
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@ -2935,6 +2937,25 @@ 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_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 (@pxref{Security and signatures}) on all loaded files. If
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@code{check_signatures=enforce}, then every attempt by the GRUB
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@file{core.img} to load another file @file{foo} (e.g., a loadable
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module, a configuration file, or a Linux kernel) implicitly invokes
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@code{verify_detached foo foo.sig} (@pxref{verify_detached}).
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@code{foo.sig} must contain a valid digital signature over the
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contents of @code{foo}, which can be verified with a public key
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currently trusted by GRUB (@pxref{list_trusted}, @pxref{trust}, and
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@pxref{distrust}). If validation fails, then file @file{foo} cannot
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be opened. This failure may halt or otherwise impact the boot
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process. An initial trusted public key can be embedded within the
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GRUB @file{core.img} using the @code{--pubkey} option to
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@command{grub-mkimage} (@pxref{Invoking grub-install}).
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@node chosen
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@subsection chosen
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@ -3582,6 +3603,7 @@ you forget a command, you can run the command @command{help}
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* cryptomount:: Mount a crypto device
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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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* 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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@ -3599,6 +3621,7 @@ you forget a command, you can run the command @command{help}
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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_env:: List variables in environment block
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* list_trusted:: List trusted public keys
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* loadfont:: Load font files
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* load_env:: Load variables from environment block
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* loopback:: Make a device from a filesystem image
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@ -3630,9 +3653,11 @@ you forget a command, you can run the command @command{help}
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* source:: Read a configuration file in same context
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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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* unset:: Unset an environment variable
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* uppermem:: Set the upper memory size
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@comment * vbeinfo:: List available video modes
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* verify_detached:: Verify detached digital signature
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* videoinfo:: List available video modes
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@end menu
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@ -3903,6 +3928,16 @@ but rather replaces it completely.
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@ref{GNU/Linux}.
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@end deffn
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@node distrust
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@subsection distrust
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@deffn Command distrust pubkey_id
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Remove public key @var{pubkey_id} from GRUB's keyring of trusted keys.
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These keys are used to validate signatures when
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@code{check_signatures=enforce} (@pxref{check_signatures}), and by some
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invocations of @command{verify_detached} (@pxref{verify_detached}).
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@xref{Security and signatures} for more information.
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@end deffn
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@node drivemap
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@subsection drivemap
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@ -4061,7 +4096,8 @@ list of @var{hash name} pairs in the same format as used by UNIX
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@command{md5sum} command. Option @option{--prefix}
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may be used to give directory where files are located. Hash verification
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stops after the first mismatch was found unless option @option{--keep-going}
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was given.
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was given. The exit code @code{$?} is set to 0 if hash verification
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is successful. If it fails, @code{$?} is set to a nonzero value.
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@end deffn
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@ -4170,16 +4206,50 @@ The @option{-f} option overrides the default location of the environment
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block.
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@end deffn
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@node list_trusted
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@subsection list_trusted
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@deffn Command list_trusted
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List all public keys trusted by GRUB for validating signatures. These
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public keys are used implicitly when environment variable
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@code{check_signatures=enforce} (@pxref{check_signatures}), and by some
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invocations of @command{verify_detached}. @xref{Security and
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signatures} for more information.
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@end deffn
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@node load_env
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@subsection load_env
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@deffn Command load_env [@option{-f} file]
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@deffn Command load_env [@option{-f} file] [@option{-s}] [whitelisted_variable_name] @dots{}
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Load all variables from the environment block file into the environment.
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@xref{Environment block}.
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The @option{-f} option overrides the default location of the environment
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block.
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The @option{-s} (long form @option{--skip-sig}) option skips signature
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checking even when the value of @code{check_signatures=enforce}
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(@pxref{check_signatures}).
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If one or more variable names are provided as arguments, they are
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interpreted as a whitelist of variables to load from the environment
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block file. Variables set in the file but not present in the
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whitelist are ignored.
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The @option{-s} option should be used with care, and should always be
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used in concert with a whitelist of acceptable variables whose values
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should be set. Failure to employ a carefully constructed whitelist
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could result in reading a malicious value of critical environment
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variables from the file, such as setting @code{check_signatures=no},
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modifying @code{prefix} to boot from an unexpected location or not at
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all, etc.
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When used with care, @option{-s} 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{Security and signatures} for more
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information.
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@end deffn
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@ -4429,6 +4499,16 @@ Save the named variables from the environment to the environment block file.
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The @option{-f} option overrides the default location of the environment
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block.
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This command will operate successfully even when
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@code{check_signatures=enforce} (@pxref{check_signatures}), since it
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writes to disk and does not alter the behavior of GRUB based on any
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contents of disk that have been read. It is possible to modify a
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digitally signed environment block file from within GRUB using this
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command, such that its signature will no longer be valid on subsequent
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boots. Care should be taken in such advanced configurations to avoid
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rendering the system unbootable. @xref{Security and signatures} for
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more information.
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@end deffn
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@ -4738,6 +4818,16 @@ Do nothing, successfully. This is mainly useful in control constructs such
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as @code{if} and @code{while} (@pxref{Shell-like scripting}).
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@end deffn
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@node trust
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@subsection trust
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@deffn Command trust pubkey_file
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Read public key from @var{pubkey_file} and add it to GRUB's internal
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list of trusted public keys. These keys are used to validate digital
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signatures when @code{check_signatures=enforce}.
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@xref{Security and signatures} for more information.
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@end deffn
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@node unset
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@subsection unset
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@ -4764,6 +4854,25 @@ only on PC BIOS platforms.
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@end ignore
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@node verify_detached
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@subsection verify_detached
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@deffn Command verify_detached file signature_file [pubkey_file]
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Verifies a GPG-style detached signature, where the signed file is
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@var{file}, and the signature itself is in file @var{signature_file}.
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Optionally, a specific public key to use can be specified using
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@var{pubkey_file}. Otherwise, public keys from GRUB's trusted keys
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(@pxref{list_trusted}, @pxref{trust}, and @pxref{distrust}) are
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tried. Note that, when @code{check_signatures=enforce}, an explicitly
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identified @var{pubkey_file} must itself be signed by an
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already-trusted key.
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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{Security and signatures} for more information.
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@end deffn
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@node videoinfo
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@subsection videoinfo
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@ -5127,6 +5236,77 @@ 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 Security and signatures
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@chapter Security considerations when using digital signatures
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GRUB's @file{core.img} can optionally provide enforcement that all
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files subsequently read from disk are covered by a valid digital
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signature. This includes GRUB configuration files, the GRUB
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environment block, GRUB loadable modules and their dependency files,
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and loaded operating system files such as a Linux kernel. This
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document does @strong{not} cover how to ensure that your platform's
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firmware (e.g., Coreboot) validates
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@file{core.img}.
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GRUB uses GPG-style detached signatures (meaning that a file
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@file{foo.sig} will be produced when file @file{foo} is signed), and
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currently supports the DSA signing algorithm. Both 2048-bit and
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3072-bit keys are supported. A signing key can be generated as
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follows:
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@example
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gpg --gen-key
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@end example
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An individual file can be signed as follows:
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@example
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gpg --detach-sign /path/to/file
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@end example
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For successful validation of all of GRUB's subcomponents and the
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loaded OS kernel, they must all be signed. One way to accomplish this
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is the following (after having already produced the desired
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@file{grub.cfg} file, e.g., by running @command{grub-mkconfig}
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(@pxref{Invoking grub-mkconfig}):
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@example
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@group
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# Edit /dev/shm/passphrase.txt to contain your signing key's passphrase
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for i in `find /boot -name "*.cfg" -or -name "*.lst" -or \
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-name "*.mod" -or -name "vmlinuz*" -or -name "initrd*" -or \
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-name "grubenv"`;
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do
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gpg --batch --detach-sign --passphrase-fd 0 $i < \
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/dev/shm/passphrase.txt
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done
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shred /dev/shm/passphrase.txt
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@end group
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@end example
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See also: @ref{check_signatures}, @ref{verify_detached}, @ref{trust},
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@ref{list_trusted}, @ref{distrust}, @ref{load_env}, @ref{save_env}.
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Note that internally signature enforcement is controlled by setting
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the environment variable @code{check_signatures=enforce}. Passing one
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or more @code{--pubkey} options to @command{grub-mkimage} implicitly
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sets @code{check_signatures=enforce} in @file{core.img} prior to
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processing any configuration files.
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Note that signature checking does @strong{not} prevent an attacker
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with (serial, physical, ...) console access from dropping manually to
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the GRUB console and executing:
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@example
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set check_signatures=no
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@end example
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To prevent this, password-protection (@pxref{Security}) is essential.
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Note that even with GRUB password protection, GRUB itself cannot
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prevent someone with physical access to the machine from altering that
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machine's firmware (e.g., Coreboot or BIOS) configuration to cause
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the machine to boot from a different (attacker-controlled) device.
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GRUB is at best only one link in a secure boot chain.
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@node Platform limitations
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@chapter Platform limitations
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