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

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Date:	Tue, 14 Apr 2015 08:41:48 +0200
From:	Pavel Machek <pavel@....cz>
To:	Mike Snitzer <snitzer@...hat.com>
Cc:	Pali Rohár <pali.rohar@...il.com>,
	Alasdair Kergon <agk@...hat.com>, Neil Brown <neilb@...e.de>,
	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	Len Brown <len.brown@...el.com>, dm-devel@...hat.com,
	linux-raid@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-pm@...r.kernel.org
Subject: Re: [PATCH 0/3] dm-crypt: Adds support for wiping key when doing
 suspend/hibernation

On Thu 2015-04-09 09:12:08, Mike Snitzer wrote:
> On Mon, Apr 06 2015 at  9:29am -0400,
> Pali Rohár <pali.rohar@...il.com> 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@...il.com> 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.

That would be nice, I agree. But that's non-trivial ammount of work
and might be (close to) impossible.
									Pavel
-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html
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