Annotation of wikisrc/security/cgdroot.mdwn, revision 1.6

1.1       khorben     1: Root filesystem encryption
                      2: ==========================
1.3       leot        4: It is possible to run NetBSD with [complete root filesystem encryption][1], thanks to the `cgdroot.kmod` kernel module. It really is a memory disk (also knows as RAM disk) that is expected to be loaded in the kernel while booting.
1.1       khorben     5: 
1.5       khorben     6: Full disk encryption makes it more difficult for an attacker to modify the unencrypted part of the disk to plant a backdoor. With only partial encryption, the original [[!template id=man name="cgdconfig" section="8"]] binary may be modifiedto send the passphrase away, allowing an attacker with a disk dump to recover the data.
                      8: The mechanism described here still requires one unencrypted partition to boot from (typically `wd0a`).
1.1       khorben     9: 
                     10: The boot process
                     11: ----------------
1.5       khorben    13: Instead of booting normally the GENERIC kernel and using the root filesystem, a kernel module is loaded at boot-time containing a memory disk. This minimal filesystem image is then considered the actual root filesystem.
1.1       khorben    14: 
                     15: The boot partition on disk only needs to contain:
1.4       leot       16: 
1.3       leot       17: * [[!template id=man name="boot" section="8"]], the second-stage bootloader
                     18: * [[!template id=man name="boot.cfg" section="5"]], the configuration file for the bootloader (optional)
1.1       khorben    19: * a GENERIC kernel
                     20: * the `cgdroot.kmod` kernel module
                     21: * configuration and encryption key for the encrypted volume to start from (`cgd.conf`)
1.3       leot       23: Once loaded the memory disk mounts the `wd0a` partition onto `/etc/cgd`, and asks for the encryption passphrase as usual (with [[!template id=man name="cgdconfig" section="8"]]). If successful, the `cgd0a` volume configured is mounted on `/altroot`, and [[!template id=man name="init" section="8"]] is told via [[!template id=man name="sysctl" section="7"]] to chroot into this volume before actually booting. The system then starts normally.
1.1       khorben    24: 
                     25: In practice the memory disk remains the real root, and the regular system is
                     26: really ran from a chroot in `/altroot`.
                     28: Obtaining the kernel module
                     29: ---------------------------
                     31: The `cgdroot.kmod` kernel module is part of the regular NetBSD releases since NetBSD 7.0. It can be found in the `<arch>/installation/miniroot` folder from the release. For instance, for the amd64 architecture on the German mirror for the 7.0.1 release, download it at (
                     33: Configuring the kernel module
                     34: -----------------------------
                     36: The kernel module needs to be available in the boot partition, alongside the desired kernel. The bootloader configuration in `/boot.cfg` should be modified to load the module, as in this example:
1.3       leot       37: 
                     38: [[!template id=filecontent name="/boot.cfg" text="""
1.1       khorben    39: menu=Boot normally:rndseed /etc/entropy-file;load /cgdroot.kmod;boot /netbsd.gz -z
1.3       leot       40: """]]
1.1       khorben    41: 
                     42: Building the kernel module
                     43: --------------------------
                     45: The kernel module can be compiled in two steps from within the source tree for the NetBSD base system, once the distribution has been built. Change to the `distrib/<arch>/ramdisks/ramdisk-cgdroot` and use `nbmake-<arch>` to build:
1.3       leot       47: [[!template id=programlisting text="""
1.2       khorben    48: src/distrib/amd64/ramdisks/ramdisk-cgdroot$ /path/to/tooldir/bin/nbmake-amd64
1.1       khorben    49: [...]
                     50:      create  ramdisk-cgdroot/ramdisk-cgdroot.fs
                     51: Calculated size of `ramdisk-cgdroot.fs.tmp': 5120000 bytes, 85 inodes
                     52: Extent size set to 4096
                     53: ramdisk-cgdroot.fs.tmp: 4.9MB (10000 sectors) block size 4096, fragment size 512
                     54:         using 1 cylinder groups of 4.88MB, 1250 blks, 96 inodes.
                     55: super-block backups (for fsck -b #) at:
                     56:  32,
                     57: Populating `ramdisk-cgdroot.fs.tmp'
                     58: Image `ramdisk-cgdroot.fs.tmp' complete
1.3       leot       59: """]]
1.1       khorben    60: 
                     61: Then the kernel module can be built:
1.3       leot       63: [[!template id=programlisting text="""
1.2       khorben    64: src/distrib/amd64/kmod-cgdroot$ /path/to/tooldir/bin/nbmake-amd64
1.3       leot       65: """]]
1.1       khorben    66: 
                     67: Caveats
                     68: -------
1.5       khorben    70: The biggest (known) issue with this setup occurs when firmware needs to be loaded early in the boot process (such as graphics drivers for the console). At the moment they need to be provided as part of the memory disk. Some network interfaces, of which some wireless devices in particular, also require loading firmware to work properly.
1.1       khorben    71: 
1.5       khorben    72: This setup is not entirely safe against physical attacks. An attacker can modify the boot process to store the passphrase for later retrieval, or insert a backdoor while booting. To defend against such attacks, the bootloader, kernel and ramdisk all need to be signed and their integrity checked before booting (eg with [[!template id=man name="tpm" section="4"]]). Alternatively, it is possible to boot from a removable medium (eg USB stick), which can be protected against tampering attacks (eg secure storage, read-only volume...).
1.1       khorben    73: 
1.6     ! khorben    74: It is also possible to boot a Xen DOM0 system with root filesystem encryption. However, Xen-enabled NetBSD kernels currently do not support loading modules at boot-time. The memory disk has to be placed directly inside the kernel instead (with [[!template id=man name="mdconfig" section="8"]] or a new kernel configuration).
        !            75: 
1.1       khorben    76: References
                     77: ----------
1.4       leot       79: * [Full Disk Encryption with cgd (well, almost)][1]
1.3       leot       81: [1]: "Full Disk Encryption with cgd (well, almost)"

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