Warm Boot Attack Defense - RAM Wipe Design Documentation

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ram-wipe logo (development)

ram-wipe wipes the RAM twice during poweroff/reboot.

  • 1. RAM Wipe Pass 1/2: During Linux kernel poweroff/reboot sequences.
  • 2. RAM Wipe Pass 2/2: It kexec's into a new kernel and performs a second RAM wipe pass for the purpose of overwriting the first kernel's memory and any other leftovers in RAM.

Design[edit]

cold-boot-attack-defense[edit]

Implemented by dracut module cold-boot-attack-defensearchive.org iconarchive.today icon (by ram-wipe).

  1. /usr/lib/dracut/modules.d/90crypt/cryptroot-ask.sh runs need_shutdown.
  2. dracut-ng dm-shutdown.sh runs cryptsetup close to release the full disk encryption key during the shutdown process.archive.org iconarchive.today icon
  3. A dracut cleanup hook is declared in /usr/lib/dracut/modules.d/40cold-boot-attack-defense/module-setup.sharchive.org iconarchive.today icon (by ram-wipe): inst_hook cleanup 80 "$moddir/wipe-ram-needshutdown.sh". Priority is 80. TODO
  4. During boot, that dracut cleanup hook /usr/lib/dracut/modules.d/40cold-boot-attack-defense/wipe-ram-needshutdown.sharchive.org iconarchive.today icon (by ram-wipe) is calling dracut API function need_shutdown which results in file /run/initramfs/.need_shutdown being created.
  5. As a result, at shutdown time when /lib/systemd/system/dracut-shutdown.service (by dracut) runs, /usr/lib/dracut/dracut-initramfs-restore (by dracut) will restore the initramfs and pivot into it.
  6. During shutdown, dracut will run its usual cleanup tasks such as unmounting the root (main) drive.
  7. The shutdown module (by dracut) will source and execute other shutdown hooks set up by other dracut modules.
  8. At the time of writing, there were no other dracut modules using the dracut shutdown hook known to the author of this website.
  9. wipe-ram.sharchive.org iconarchive.today icon (by ram-wipe) is the dracut shutdown hook.
  10. An alternative description of the mechanism of dropping back to the initramfs during shutdown can be found under The initrd Interface of systemdarchive.org iconarchive.today icon.
  11. At a very late stage during the shutdown process, when all disks have already been unmounted by dracut, the wipe-ram.sh dracut shutdown hook is executed.
  12. The shutdown hook runs:
  • echo 3 > /proc/sys/vm/drop_caches
    • To ensure any remaining disk cache is erased by Linux's memory poisoning. [1]
  • sdmem -l -l -v: To wipe the RAM using sdmem.
    • The parameters -l -l result in a single pass of RAM wiping using zeros.
      • This is optimized for speed. Otherwise, RAM wiping could take several minutes. However, the longer the shutdown process is delayed, the more likely users are to disable this feature. Also, after wiping the RAM, it could be more important to shut down to aid decay of RAM contents rather than repeatedly wiping RAM for minutes before removing power.
      • When run manually, sdmem with these command-line parameters will show "Wipe mode is insecure (one pass with 0x00)," but no evidence suggests this is actually insecure. The sdmem manpage refers to Peter Gutmann, but the Gutmann method is for hard drives, not RAM. No research indicating how many times RAM must be wiped to be unrecoverable has been found by the author of this website.
    • The parameter -v is for verbose output, which provides only a progress indicator.
    • Any output (default or verbose) is only visible if a serial console is connected. This is because dracut by default hides output of the commands it runs. Redirecting the output of sdmem to /dev/kmsg would result in hundreds or thousands of separate * characters being written to the console, each on its own line, which would not be helpful. For a better progress meter of the RAM wipe process, a buffer mechanism might have to be implemented after user feedback.
  • dmsetup ls --target crypt: To check if all encrypted disks are unmounted.
    • Only if all encrypted disks are unmounted will it be possible for the kernel to wipe the Full Disk Encryption (FDE) key from the kernel.
    • Deletion of the FDE key is considered among the most crucial pieces of information to be wiped from RAM because if the FDE key can be recovered from RAM, then FDE can be compromised.
    • Informs the user if all encrypted disks are unmounted in console output. Otherwise, it shows a warning.

Quote Tails' Memory erasurearchive.org iconarchive.today icon:

First, most memory is erased at the end of a normal shutdown/reboot sequence. This is implemented by the Linux kernel's freed memory poisoning featurearchive.org iconarchive.today icon, more specifically:

  • page_poison
  • passing "P" to slub_debug
  • zeroing heap memory at free time (init_on_free=1)

These kernel parameters are implemented in the security-misc file /etc/default/grub.d/40_kernel_hardening.cfgarchive.org iconarchive.today icon.

The kernel parameter wiperam=skip is available to disable RAM wiping at shutdown, which can be useful to speed up shutdown or in case any issues arise.

For potential limitations, the same limitations described under the "Limitations" chapter of Tails' Memory erasurearchive.org iconarchive.today icon apply.

ram-wipe-exit[edit]

dracut module ram-wipe-exit:

  • The other dracut module cold-boot-attack-defense is independent.
    • The first RAM wiping mechanism is useful regardless of this supplemental kexec-based RAM wipe, which might be more prone to bugs.
    • The cold-boot-attack-defense module, in its main source code file wipe-ram.sh, uses kexec to boot into a new kernel.
    • That new kernel is actually the same kernel image, but thanks to kexec, the old kernel stops and a new kernel runs.
    • kexec is used with the --reuse-cmdline parameter for simplicity and to preserve already existing RAM wipe-related kernel parameters (the Linux kernel's freed memory poisoning feature mentioned above).
    • Additionally, the kernel parameter wiperamexit=1 is appended by the cold-boot-attack-defense module.
  • The kernel parameter wiperamexit=1 in the ram-wipe-exit module results in wiping the RAM and then rebooting or powering off.
  • It does not mount the root image.
    • In other words, ram-wipe-exit runs at a very early boot stage before mounting the root image. This is done using the dracut hook pre-udev (because that hook runs before pre-mount).
    • Therefore, the full disk encryption (FDE) password entry is not required.
    • The RAM wipe is performed during the dracut initramfs stage, before the FDE password is requested or the root disk is mounted.
    • The root image is not mounted at all when the kernel parameter wiperamexit=1 is set.
  • When the kernel parameter wiperamexit=1 is set, after the RAM wipe at the early dracut initramfs stage, the system is rebooted or powered off, depending on the wiperamaction setting, which was set by the previous kernel.
  • A kexec-based wipe cannot rely on systemd with Before=unmount.target because unmounting the root disk and using cryptsetup luksClose to wipe the FDE key from RAM is one of the most critical steps of RAM wipe. It would require After=unmount.target, but at that point, no disks remain mounted with the necessary tools. Hence, dropping back to initramfs at shutdown is the correct design.
  • Executes reboot, poweroff, or halt as instructed by the previous kernel.

Differences of ram-wipe versus Tails Memory Erasure[edit]

Tails memory erasure:

ram-wipe:

  • Based on Linux memory poisoning, execution of sdmem and kexec
  • Requires dracut
  • More generic
  • Should work on any Debian
  • Should be relatively easy to port to any Linux distribution since it is implemented as a dracut module
  • Should work equally for persistent boot from hard drive, live boot from hard drive, or ISO live boot
  • A panic button / panic shutdown / USB kill cord for your laptoparchive.org iconarchive.today icon feature is not mixed with this feature. It should be implemented separately as a standalone feature.

Debugging[edit]

(This file would be shipped out commented by default. Only useful for development / debugging.) 

#!/bin/sh

## Copyright (C) 2022 - 2025 ENCRYPTED SUPPORT LLC <adrelanos@whonix.org>
## See the file COPYING for copying conditions.

## Untested!

set -e

#GRUB_CMDLINE_LINUX="$GRUB_CMDLINE_LINUX wiperamexit=1 wiperamaction=reboot"
#GRUB_CMDLINE_LINUX="$GRUB_CMDLINE_LINUX wiperamexit=1 wiperamaction=poweroff"
#GRUB_CMDLINE_LINUX="$GRUB_CMDLINE_LINUX wiperamexit=1 wiperamaction=halt"

GRUB_CMDLINE_LINUX="$GRUB_CMDLINE_LINUX wiperamexit=1 wiperamaction=debug"

export GRUB_CMDLINE_LINUX

if test -x /etc/grub.d/10_00_linux_dist ; then
	/etc/grub.d/10_00_linux_dist
fi

sudo update-grub

Maybe useful during development:

  • grep -r pre-udev --color /usr/lib/dracut

A panic button / panic shutdown / USB kill cord for your laptoparchive.org iconarchive.today icon feature is not integrated with this feature. It should be implemented separately as a standalone feature.

Status of initramfs-tools Support[edit]

Support for initramfs-tools is not planned by the authors of ram-wipe. No progress on initramfs-tools support should be expected.

The problem with initramfs-tools support is that, in contrast to dracut, while initramfs-tools supports initrd (initial ramdisk), it does not support exitrd (exit ramdisk).

dracut supports both initrd (initial ramdisk at boot time) as well as exitrd (dropping back to the initial ramdisk at shutdown time). A feature request has been posted against the Debian initramfs-toolsarchive.org iconarchive.today icon package: Support restoring initrd on shutdown and pivoting into itarchive.org iconarchive.today icon.

Contributors wishing to add initramfs-tools support to ram-wipe should first add exitrd support to upstream, original initramfs-tools.

As a starting point, Tails has implemented initramfs-restorearchive.org iconarchive.today icon, which might be helpful to examine and use as inspiration when developing exitrd functionality for initramfs-tools. The Tails initramfs-tools exitrd implementation would have to be made generic - meaning not specific to Tails (i.e., no code references to other Tails-specific code) - and made acceptable for the initramfs-tools developers for inclusion into the upstream source code. However, using the Tails implementation as a starting point is not a strict requirement.

Once initramfs-tools gains exitrd support, it might then be straightforward to add initramfs-tools support to ram-wipe.

Development TODO[edit]

ram-wipe Testing inside a VM[edit]

1. Platform-specific notice.

  • Kicksecure: No special notice.
  • Qubes OS: ram-wipe is unavailable for Qubes OS. [2]

2. Install ram-wipe.

ram-wipe is not installed by default in VMs because it is usually not needed there, except for testing.

Install package(s) ram-wipe following these instructions

1 Platform specific notice.

2 Update the package lists and upgrade the system.

sudo apt update && sudo apt full-upgrade

3 Install the ram-wipe package(s).

Using apt command line --no-install-recommends option is in most cases optional.

sudo apt install --no-install-recommends ram-wipe

4 Platform specific notice.

  • Kicksecure: No special notice.
  • Kicksecure-Qubes: Shut down Template and restart App Qubes based on it as per Qubes Template Modification.

5 Done.

The procedure of installing package(s) ram-wipe is complete.

3. Reboot

sudo reboot

4. Set up a virtual serial console.

A virtual serial console helps to read all journal and kernel messages during early boot and shutdown.

Can be set up as per the serial console documentation. Only a read-only serial console was somewhat recently tested and should suffice. An interactive serial console might not be required.

5. Status.

Now a serial console should clearly show the output during boot and shutdown of ram-wipe.

6. Bug dracut should unmount the root encrypted disk `cryptsetup luksClose` during shutdownarchive.org iconarchive.today icon (fixed in dracut-ng, should be fixed in Debian trixie) will not be reproducible because Kicksecure VM images do not use full disk encryption. (The rationale for not using full disk encryption for VM images is documented on the Encrypted VM Images wiki page.)

As a workaround, install Debian bookworm using the Debian DVD (Debian Tips), then install Kicksecure as per the distribution morphing Debian into Kicksecure instructions. Then re-apply the instructions listed here.

ram-wipe Functionality Testing[edit]

TODO:

ram-wipe improvements[edit]

  • sdmem is problematic: it is unmaintained upstream, and there has been at least one case where shutdown was blocked due to a "kernel locked up" error. Writing to RAM until it fills up and sdmem gets OOM-killed is not a robust design. We should either replace sdmem or drop it entirely. It seems feasible to drop it.
  • Review how Tails handles memory erasure: Tails memory erasure designarchive.org iconarchive.today icon. They primarily rely on the kernel parameter init_on_free=1.
  • The second RAM wipe pass might be completely unnecessary. It can cause issues with graphics initialization during the second run due to buggy drivers. This makes reboot or shutdown slower and potentially glitchy.
  • All we might need to do is:
    • Set init_on_free=1 (currently only set in security-misc, but it should be set in ram-wipe to make it standalone).
    • Drop caches (already implemented).
    • Properly unmount all encrypted disks (already done due to fixes in dracut in Debian Trixie).
    • Drop back to initrd (already implemented).
    • Re-test.

Task: Please consider the above, test it, implement necessary changes, and update the source code and wiki accordingly.

Forum Discussion[edit]

https://forums.whonix.org/t/is-ram-wipe-possible-inside-whonix-cold-boot-attack-defense/5596archive.org iconarchive.today icon

See Also[edit]

Footnotes[edit]

  1. ↑ Inspired by Tails. https://gitlab.tails.boum.org/tails/tails/-/blob/master/config/chroot_local-includes/usr/local/lib/initramfs-pre-shutdown-hookarchive.org iconarchive.today icon
  2. ↑ Wipe RAM on shutdown #1562archive.org iconarchive.today icon

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