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    SubjectRe: [PATCH 0/3] dm-crypt: Adds support for wiping key when doing suspend/hibernation
    On Thu, Apr 09 2015 at 10:16am -0400,
    Pali Rohár <pali.rohar@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@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@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.
    > > > >
    > > > > 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.

    > > > 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!

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

    > 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.


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