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

Thu Apr 9 14:38:27 UTC 2015

On Thursday 09 April 2015 10:26:58 Mike Snitzer wrote:
> On Thu, Apr 09 2015 at 10:16am -0400,
> Pali Rohár <pali.rohar at gmail.com> wrote:
> 
> > On Thursday 09 April 2015 10:08:43 Mike Snitzer wrote:
> > > On Thu, Apr 09 2015 at  9:28am -0400,
> > > Pali Rohár <pali.rohar at gmail.com> wrote:
> > > 
> > > > On Thursday 09 April 2015 09:12:08 Mike Snitzer wrote:
> > > > > On Mon, Apr 06 2015 at  9:29am -0400,
> > > > > Pali Rohár <pali.rohar at 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 at 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.
> > > > > 
> > > > > Then this entire problem goes away and the key can be wiped from
> > > > > userspace (like you said above).
> > > > 
> > > > Still there will be race condition. Before hibernation (and device
> > > > poweroff) we should have synced disks and filesystems to prevent data
> > > > lose (or other damage) as more as we can. And if there will be some
> > > > application which using lot of I/O (e.g normal firefox) then there
> > > > always will be race condtion.
> > > 
> > > The DM suspend will take care of flushing any pending I/O.  So I don't
> > > see where the supposed race is...
> > > 
> > 
> > Any I/O operation after DM suspend is race condition and could cause
> > data lost.
> > 
> > > Anything else that is trapped in userspace memory will be there when the
> > > machine resumes.
> > > 
> > 
> > You are expecting that machine resumes always at 100% and correctly. But
> > this is not truth in real world. There are planty of users who reported
> > lot of random problems with suspend or hibernate...
> 
> But the system was left in a crash consistent state.  Properly written
> apps will wait for I/O to ensure data loss (in the event of a failed
> resume) isn't a problem.
>  

I think you are too optimistic about ideal world...
"Properly written apps" "ensure data loss"

> > > > So proper way is to wipe luks crypto keys *after* userspace processes
> > > > are freezed.
> > > 
> > > I know you believe that I'm just not accepting that at face value.
> > 
> > If disks are synced before any DM suspend operation then we have higher
> > chance of preventing data corruption.
> 
> disks are already synced as part of the DM suspend operation!
> 

Yes, but part of hibernate operation is also sync call.

> But you're saying that all user processes are frozen (and associated
> I/O flushed) before the DM suspend, that is different:
> 

Yes, I want to ensure that. So processes wont be able to do any other
I/O.

> > I still think that correct order is only:
> > 
> > * freeze processes (which doing continous I/O)
> > * fs & disk sync
> > * DM suspend
> > * wipe crypto keys
> > * enter hibernate
> 
> I just don't think that extreme is _required_ to have a hibernate/resume
> that incorporates dm-crypt key wiping.

Ok, and what other developers think?

I'm saying that if I want to wipe luks keys before suspend/hibernate and
have system in consistant state as much as possible, keys must be wiped
*after* userspace processes are freezed. Or do you have relevant or
functional argument why not? Or is there any problem in my thinking?

-- 
Pali Rohár
pali.rohar at gmail.com