Re: Are the keys of mounted encrypted disks secured during reboot?

On 12/26/22 21:10, John Smith wrote:
> I'm asking about when a regular shutdown/reboot/suspend-to-disk is
> issued. In most cases the rootfs volume is not unmounted because that
> would require a pivot_root to a special initrd/ramdisk-rootfs just for
> powering off.
> So that would mean the device isn't deactivated via cryptsetup -
> because that would fail.

Cryptsetup (libcryptsetup) is just library that performs the job,
it does not run itself any actions.
Once it is called, keys should be wiped.

The deactivation/reboot/suspend is handled by init scripts, usually systemd
units these days - so check there. It really depends on system configuration.

However, last time we played with memory scan for keys after reboot,
cold boot is no longer such a problem as memory is wiped during reboot
on modern hw (but virtual machines images is a different problem).

m.

> In that case, when the kernel is preparing to reboot/poweroff, will it
> wipe the keys or just leave them there vulnerable to cold boot or
> forensics?
> 
> On Mon, Dec 26, 2022 at 7:42 PM Milan Broz <[email protected]> wrote:
>>
>> On 12/26/22 20:13, John Smith wrote:
>>> During the course of proper shutdown/reboot/suspend-to-disk, does the
>>> kernel securely erase (or at least free memory of - as it pertains to
>>> init_on_free=1) encryption keys of all dmcrypt/luks systems which
>>> remained mounted?
>>
>> If the device is properly deactivated, then keys are always wiped.
>>
>> (Actually, with LUKS2, dm-crypt no longer keeps own copy of the key, it
>> is stored in kernel keyring only for activation and then, obviously, in kernel
>> crypto where is is directly used for encryption. So if it is not
>> deactivated, it is responsibility of these subsystems to wipe it on reboot.)
>>
>> For suspend to ram, it is more complicated - there is a way how to wipe key
>> and freeze device temporarily (see luksSuspend), but I do no think many systems
>> actually use it. Debian has cryptsetup-suspend that can do this AFAIK.
>>
>> For suspend to disk, the memory should be written to encrypted device.
>> (And RAM contents disappears after some short time with no power anyway.)
>>
>> Milan