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

[Pali RohÃr]

On Thursday 09 April 2015 10:08:43 Mike Snitzer wrote:
> On Thu, Apr 09 2015 at  9:28am -0400,
> Pali RohÃr <pali.rohar@xxxxxxxxx> wrote:
> 
> > On Thursday 09 April 2015 09:12:08 Mike Snitzer wrote:
> > > On Mon, Apr 06 2015 at  9:29am -0400,
> > > Pali RohÃr <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 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?
> > > > 
> > > > 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...

> > 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.

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

-- 
Pali RohÃr
pali.rohar@xxxxxxxxx
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