Documentation cleanup in response to ML discussion.
[v0] Accepted with modifications by phcoder@ [v1] Introduce subsections within Security [v1] Correct errors regarding public key files not being automatically signature-checked in trust and verify_detached [v1] Replace check_signatures=enforce with check_signatures set to enforce [v1] Move detailed discussion of using signatures out of check_signatures environment variable description [v1] Use long form for option flags to security-relevant commands [v2] Explain the key fingerprint format for distrust and list_trusted. [v2] Eliminates references to grub-mkimage and UEFI Secure Boot. [v3] Updates in response to addition of --skip-sig to trust and verify_detached [ ] Restore @xref for cross-references at the start of sentences
This commit is contained in:
Jon McCune
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2 changed files with 115 additions and 84 deletions
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@ -1,3 +1,7 @@
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2013-10-25 Jon McCune <jonmccune@google.com>
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* docs/grub.texi: Cleanup security documentation around signatures.
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2013-10-25 Vladimir Serbinenko <phcoder@gmail.com>
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* conf/Makefile.common (CPPFLAGS_KERNEL): Add -DGRUB_KERNEL=1.
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195
docs/grub.texi
195
docs/grub.texi
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@ -98,8 +98,7 @@ This edition documents version @value{VERSION}.
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* Environment:: GRUB environment variables
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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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* Security:: Authentication, authorisation, and signatures
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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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@ -3006,20 +3005,7 @@ chain-loaded system, @pxref{drivemap}.
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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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validation on loaded files. @xref{Using digital signatures}.
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@node chosen
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@subsection chosen
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@ -3998,10 +3984,15 @@ but rather replaces it completely.
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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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@pxref{Security and signatures} for more information.
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@var{pubkey_id} is the last four bytes (eight hexadecimal digits) of
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the GPG v4 key id, which is also the output of @command{list_trusted}
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(@pxref{list_trusted}). Outside of GRUB, the key id can be obtained
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using @code{gpg --fingerprint}).
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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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@end deffn
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@node drivemap
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@ -4264,56 +4255,58 @@ This command is only available on x86 systems.
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@node list_env
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@subsection list_env
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@deffn Command list_env [@option{-f} file]
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@deffn Command list_env [@option{--file} file]
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List all variables in the environment block file. @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{--file} option overrides the default location of the
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environment 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}. @pxref{Security and
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signatures} for more information.
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List all public keys trusted by GRUB for validating signatures.
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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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these keys.
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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] [@option{-s}] [whitelisted_variable_name] @dots{}
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@deffn Command load_env [@option{--file} file] [@option{--skip-sig}] [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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The @option{--file} 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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The @option{--skip-sig} option skips signature checking even when the
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value of environment variable @code{check_signatures} is set to
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@code{enforce} (@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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The @option{--skip-sig} option should be used with care, and should
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always be used in concert with a whitelist of acceptable variables
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whose values should be set. Failure to employ a carefully constructed
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whitelist could result in reading a malicious value into critical
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environment variables from the file, such as setting
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@code{check_signatures=no}, modifying @code{prefix} to boot from an
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unexpected location or not at all, etc.
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When used with care, @option{-s} and the whitelist enable an
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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. @pxref{Security and signatures} for more
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``savedefault'' behavior. @xref{Using digital signatures}, for more
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information.
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@end deffn
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@ -4558,22 +4551,22 @@ Remove a loaded @var{module}.
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@node save_env
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@subsection save_env
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@deffn Command save_env [@option{-f} file] var @dots{}
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@deffn Command save_env [@option{--file} file] var @dots{}
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Save the named variables from the environment to the environment block file.
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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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The @option{--file} 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. @pxref{Security and signatures} for
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more information.
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This command will operate successfully even when environment variable
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@code{check_signatures} is set to @code{enforce}
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(@pxref{check_signatures}), since it writes to disk and does not alter
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the behavior of GRUB based on any contents of disk that have been
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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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@end deffn
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@ -4886,11 +4879,17 @@ as @code{if} and @code{while} (@pxref{Shell-like scripting}).
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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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@deffn Command trust [@option{--skip-sig}] 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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@pxref{Security and signatures} for more information.
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signatures when environment variable @code{check_signatures} is set to
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@code{enforce}. Note that if @code{check_signatures} is set to
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@code{enforce} when @command{trust} executes, then @var{pubkey_file}
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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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information.
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@end deffn
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@ -4922,20 +4921,21 @@ only on PC BIOS platforms.
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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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@deffn Command verify_detached [@option{--skip-sig}] 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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@var{pubkey_file}. When environment variable @code{check_signatures}
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is set to @code{enforce}, then @var{pubkey_file} must itself be
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properly signed by an already-trusted key. An unsigned
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@var{pubkey_file} can be loaded by specifying @option{--skip-sig}.
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If @var{pubkey_file} is omitted, then 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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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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@pxref{Security and signatures} for more information.
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@xref{Using digital signatures}, for more information.
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@end deffn
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@node videoinfo
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@ -5234,7 +5234,15 @@ test from coreutils.
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environment variables and commands are listed in the same order.
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@node Security
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@chapter Authentication and authorisation
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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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@end menu
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@node Authentication and authorisation
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@section Authentication and authorisation in GRUB
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By default, the boot loader interface is accessible to anyone with physical
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access to the console: anyone can select and edit any menu entry, and anyone
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@ -5301,17 +5309,34 @@ 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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@node Using digital signatures
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@section Using digital signatures in GRUB
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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'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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This document does @strong{not} cover how to ensure that your
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platform's firmware (e.g., Coreboot) validates @file{core.img}.
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If environment variable @code{check_signatures}
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(@pxref{check_signatures}) is set to @code{enforce}, then every
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attempt by the GRUB @file{core.img} to load another file @file{foo}
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implicitly invokes @code{verify_detached foo foo.sig}
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(@pxref{verify_detached}). @code{foo.sig} must contain a valid
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digital signature over the contents of @code{foo}, which can be
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verified with a public key currently trusted by GRUB
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(@pxref{list_trusted}, @pxref{trust}, and @pxref{distrust}). If
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validation fails, then file @file{foo} cannot be opened. This failure
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may halt or otherwise impact the boot process.
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@comment Unfortunately --pubkey is not yet supported by grub-install,
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@comment but we should not bring up internal detail grub-mkimage here
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@comment in the user guide (as opposed to developer's manual).
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@comment An initial trusted public key can be embedded within the GRUB
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@comment @file{core.img} using the @code{--pubkey} option to
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@comment @command{grub-mkimage} (@pxref{Invoking grub-install}). Presently it
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@comment is necessary to write a custom wrapper around @command{grub-mkimage}
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@comment using the @code{--grub-mkimage} flag to @command{grub-install}.
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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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@ -5353,10 +5378,11 @@ 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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the environment variable @code{check_signatures} equal to
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@code{enforce}. Passing one or more @code{--pubkey} options to
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@command{grub-mkimage} implicitly defines @code{check_signatures}
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equal to @code{enforce} in @file{core.img} prior to processing any
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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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@ -5366,12 +5392,13 @@ the GRUB console and executing:
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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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To prevent this, password-protection (@pxref{Authentication and
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authorisation}) is essential. Note that even with GRUB password
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protection, GRUB itself cannot prevent someone with physical access to
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the machine from altering that machine's firmware (e.g., Coreboot
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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 Platform limitations
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@chapter Platform limitations
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Reference in a new issue