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

From: Mike Snitzer
Date: Thu Apr 09 2015 - 09:12:26 EST

On Mon, Apr 06 2015 at 9:29am -0400,
Pali Rohár <pali.rohar@xxxxxxxxx> wrote:

> On Monday 06 April 2015 15:00:46 Mike Snitzer wrote:
> > On Sun, Apr 05 2015 at 1:20pm -0400,
> >
> > Pali Rohár <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?
> Hi,
> 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.

Wouldn't it be better to fix freeze_processes() to be tolerant of
processes that are hung as a side-effect of their backing storage being
suspended? A hibernate shouldn't fail simply because a user chose to
suspend a DM device.

Then this entire problem goes away and the key can be wiped from
userspace (like you said above).
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