187 lines
8.3 KiB
Plaintext
187 lines
8.3 KiB
Plaintext
18 Security
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***********
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18.1 Authentication and authorisation in GRUB
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=============================================
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By default, the boot loader interface is accessible to anyone with
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physical access to the console: anyone can select and edit any menu
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entry, and anyone can get direct access to a GRUB shell prompt. For
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most systems, this is reasonable since anyone with direct physical
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access has a variety of other ways to gain full access, and requiring
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authentication at the boot loader level would only serve to make it
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difficult to recover broken systems.
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However, in some environments, such as kiosks, it may be appropriate
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to lock down the boot loader to require authentication before performing
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certain operations.
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The 'password' (*note password::) and 'password_pbkdf2' (*note
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password_pbkdf2::) commands can be used to define users, each of which
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has an associated password. 'password' sets the password in plain text,
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requiring 'grub.cfg' to be secure; 'password_pbkdf2' sets the password
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hashed using the Password-Based Key Derivation Function (RFC 2898),
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requiring the use of 'grub-mkpasswd-pbkdf2' (*note Invoking
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grub-mkpasswd-pbkdf2::) to generate password hashes.
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In order to enable authentication support, the 'superusers'
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environment variable must be set to a list of usernames, separated by
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any of spaces, commas, semicolons, pipes, or ampersands. Superusers are
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permitted to use the GRUB command line, edit menu entries, and execute
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any menu entry. If 'superusers' is set, then use of the command line
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and editing of menu entries are automatically restricted to superusers.
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Setting 'superusers' to empty string effectively disables both access to
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CLI and editing of menu entries.
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Other users may be allowed to execute specific menu entries by giving
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a list of usernames (as above) using the '--users' option to the
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'menuentry' command (*note menuentry::). If the '--unrestricted' option
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is used for a menu entry, then that entry is unrestricted. If the
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'--users' option is not used for a menu entry, then that only superusers
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are able to use it.
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Putting this together, a typical 'grub.cfg' fragment might look like
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this:
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set superusers="root"
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password_pbkdf2 root grub.pbkdf2.sha512.10000.biglongstring
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password user1 insecure
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menuentry "May be run by any user" --unrestricted {
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set root=(hd0,1)
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linux /vmlinuz
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}
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menuentry "Superusers only" --users "" {
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set root=(hd0,1)
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linux /vmlinuz single
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}
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menuentry "May be run by user1 or a superuser" --users user1 {
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set root=(hd0,2)
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chainloader +1
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}
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The 'grub-mkconfig' program does not yet have built-in support for
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generating configuration files with authentication. You can use
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'/etc/grub.d/40_custom' to add simple superuser authentication, by
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adding 'set superusers=' and 'password' or 'password_pbkdf2' commands.
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18.2 Using digital signatures in GRUB
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=====================================
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GRUB's '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 *not* cover how to ensure that your platform's
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firmware (e.g., Coreboot) validates 'core.img'.
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If environment variable 'check_signatures' (*note check_signatures::)
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is set to 'enforce', then every attempt by the GRUB 'core.img' to load
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another file 'foo' implicitly invokes 'verify_detached foo foo.sig'
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(*note verify_detached::). 'foo.sig' must contain a valid digital
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signature over the contents of 'foo', which can be verified with a
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public key currently trusted by GRUB (*note list_trusted::, *note
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trust::, and *note distrust::). If validation fails, then file 'foo'
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cannot be opened. This failure may halt or otherwise impact the boot
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process.
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GRUB uses GPG-style detached signatures (meaning that a file
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'foo.sig' will be produced when file 'foo' is signed), and currently
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supports the DSA and RSA signing algorithms. A signing key can be
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generated as follows:
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gpg --gen-key
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An individual file can be signed as follows:
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gpg --detach-sign /path/to/file
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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 'grub.cfg'
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file, e.g., by running 'grub-mkconfig' (*note Invoking grub-mkconfig::):
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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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See also: *note check_signatures::, *note verify_detached::, *note
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trust::, *note list_trusted::, *note distrust::, *note load_env::, *note
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save_env::.
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Note that internally signature enforcement is controlled by setting
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the environment variable 'check_signatures' equal to 'enforce'. Passing
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one or more '--pubkey' options to 'grub-mkimage' implicitly defines
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'check_signatures' equal to 'enforce' in 'core.img' prior to processing
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any configuration files.
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Note that signature checking does *not* prevent an attacker with
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(serial, physical, ...) console access from dropping manually to the
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GRUB console and executing:
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set check_signatures=no
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To prevent this, password-protection (*note 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 or
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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 secure
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boot chain.
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18.3 UEFI secure boot and shim support
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======================================
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The GRUB, except the 'chainloader' command, works with the UEFI secure
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boot and the shim. This functionality is provided by the shim_lock
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module. It is recommend to build in this and other required modules
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into the 'core.img'. All modules not stored in the 'core.img' and the
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ACPI tables for the 'acpi' command have to be signed, e.g. using PGP.
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Additionally, the 'iorw', the 'memrw' and the 'wrmsr' commands are
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prohibited if the UEFI secure boot is enabled. This is done due to
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security reasons. All above mentioned requirements are enforced by the
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shim_lock module. And itself it is a persistent module which means that
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it cannot be unloaded if it was loaded into the memory.
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18.4 Measuring boot components
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==============================
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If the tpm module is loaded and the platform has a Trusted Platform
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Module installed, GRUB will log each command executed and each file
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loaded into the TPM event log and extend the PCR values in the TPM
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correspondingly. All events will be logged into the PCR described below
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with a type of EV_IPL and an event description as described below.
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Event type PCR Description
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---------------------------------------------------------------------------
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Command 8 All executed commands (including those
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from configuration files) will be logged
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and measured as entered with a prefix of
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"grub_cmd: "
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Kernel command line 8 Any command line passed to a kernel will
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be logged and measured as entered with a
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prefix of "kernel_cmdline: "
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Module command line 8 Any command line passed to a kernel
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module will be logged and measured as
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entered with a prefix of "module_cmdline:
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"
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Files 9 Any file read by GRUB will be logged and
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measured with a descriptive text
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corresponding to the filename.
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GRUB will not measure its own 'core.img' - it is expected that
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firmware will carry this out. GRUB will also not perform any
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measurements until the tpm module is loaded. As such it is recommended
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that the tpm module be built into 'core.img' in order to avoid a
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potential gap in measurement between 'core.img' being loaded and the tpm
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module being loaded.
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Measured boot is currently only supported on EFI platforms.
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