Re: [PATCH 0/3] dm-crypt: Adds support for wiping key when doing suspend/hibernation

From: Pali RohÃr
Date: Mon Apr 06 2015 - 09:30:09 EST

On Monday 06 April 2015 15:00:46 Mike Snitzer wrote:
> On Sun, Apr 05 2015 at 1:20pm -0400,
> Pali RohÃr <pali.rohar@xxxxxxxxx> wrote:
> > This patch series increase security of suspend and hibernate
> > actions. It allows user to safely wipe crypto keys before
> > suspend and hibernate actions starts without race
> > conditions on userspace process with heavy I/O.
> >
> > To automatically wipe cryto key for <device> before
> > hibernate action call: $ dmsetup message <device> 0 key
> > wipe_on_hibernation 1
> >
> > To automatically wipe cryto key for <device> before suspend
> > action call: $ dmsetup message <device> 0 key
> > wipe_on_suspend 1
> >
> > (Value 0 after wipe_* string reverts original behaviour - to
> > not wipe key)
> Can you elaborate on the attack vector your changes are meant
> to protect against? The user already authorized access, why
> is it inherently dangerous to _not_ wipe the associated key
> across these events?


yes, I will try to explain current problems with cryptsetup
luksSuspend command and hibernation.

First, sometimes it is needed to put machine into other hands.
You can still watch other person what is doing with machine, but
once if you let machine unlocked (e.g opened luks disk), she/he
can access encrypted data.

If you turn off machine, it could be safe, because luks disk
devices are locked. But if you enter machine into suspend or
hibernate state luks devices are still open. And my patches try
to achieve similar security as when machine is off (= no crypto
keys in RAM or on swap).

When doing hibernate on unencrypted swap it is to prevent leaking
crypto keys to hibernate image (which is stored in swap).

When doing suspend action it is again to prevent leaking crypto
keys. E.g when you suspend laptop and put it off (somebody can
remove RAMs and do some cold boot attack).

The most common situation is:
You have mounted partition from dm-crypt device (e.g. /home/),
some userspace processes access it (e.g opened firefox which
still reads/writes to cache ~/.firefox/) and you want to drop
crypto keys from kernel for some time.

For that operation there is command cryptsetup luksSuspend, which
suspend dm device and then tell kernel to wipe crypto keys. All
I/O operations are then stopped and userspace processes which
want to do some those I/O operations are stopped too (until you
call cryptsetup luksResume and enter correct key).

Now if you want to suspend/hiberate your machine (when some of dm
devices are suspeneded and some processes are stopped due to
pending I/O) it is not possible. Kernel freeze_processes function
will fail because userspace processes are still stopped inside
some I/O syscall (read/write, etc,...).

My patches fixes this problem and do those operations (suspend dm
device, wipe crypto keys, enter suspend/hiberate) in correct
order and without race condition.

dm device is suspended *after* userspace processes are freezed
and after that are crypto keys wiped. And then computer/laptop
enters into suspend/hibernate state.

Pali RohÃr

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