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SubjectRe: [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@gmail.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@gmail.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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