Re: how to (really) cleanly shutdown the system when root is on multiplestacked block devices

From: Milan Broz
Date: Sat Jun 26 2010 - 15:17:52 EST

On 06/26/2010 06:44 PM, Christoph Anton Mitterer wrote:
On Sat, 2010-06-26 at 18:17 +0200, Milan Broz wrote:

With recent kernel and flush (issuing barrier internally) device-mapper
properly propagates barrier request.
a) What is recent? ;)
b) The barrier thingy,... does it have to be supported by the thing
(e.g. filesystem, LV, etc.) on top? Or is this something generically
implemented for flushing?

IIRC barriers are fully supported for DM devices since 2.6.31
but every fs must properly flush IO on umount or remount RO
(using whatever method is possible).
But note that you are running shutdown scripts from device itself
if it is root-fs script itself produces reads to the device.
Uhm what exactly do you mean?

Just that every command run will issue some reads when it need to load
libraries etc.

But from the security point of view dm-crypt encryption key remains in memory
because you cannot properly remove LUKS device thus wipe the key.

Anyone with proper boot image can recover such key from RAM memory using
so called cold-boot attack.
How long is this about staying in the RAM (after poweroff)?
And after reboot.... isn't everything set to 0x0? Otherwise,... booting
e.g. another OS or a compromised Linux could leak the key...


You have several options how to solve this, but I am afraid all require
some kind of ramdisk, where are the basic tools are copied before unmounting
root-fs and unmapping devices and reboot.
I've already feared that... so we need de-initramfs? ;)

(For non-root devices it is easy, you can even call luksSuspend to wipe
key on still active device as workaround before reboot.

(just FYI: luksSuspend is simple wrapper for dm-crypt wipe key functionality;
it was written to provide tool to safely wipe all dm-crypt encryption keys
in memory without need to close device, it is de-initialising cryptoAPI
modules etc.
It was intended to provide something which can help "suspend to RAM" to be
at least partially immune to cold boot attack on RAM-suspended laptop.)

I guess non-root devices should be cleanly closed, with luksClose, or

yes. But for security reasons there still should be some fallback if it
fails - so at least key is properly wiped if all other attempts to unmout fails.
After luksSuspend
device is frozen - until the key is provided back using luksResume.
So only some e.g. page cache leaks of plaintext data are possible -
but not encryption key itself.)
Isn't it possible to patch the kernel,.. that always when halting or
rebooting,.. it "simply" wipes _ALL_ dm-cryptkeys available,...
And why/how are plaintext leaks possible?

That will solve only one problem. Then you will have some some device which
need ioctl to wipe its state (including possible sensitive data in
its buffers) etc. It is just an example, hack to wipe memory on reboot is
not proper solution.

I think this should be solved systematically - clean shutdown should keep
machine in safe state.

I mean something like this on shutdown:

- create ramdisk containing basic utilities
(mount, sync, lvm, cryptsetup, halt, etc)
- remount device read-only, iow sync and flush write IO
- switch to ramdisk, all command now must run from there
- try to cleanly unmout root-fs, deactivate underlying LV, deactivate LUKS
- if deactivation fails, fallback to wipe LUKS device key in memory
using luksSuspend
(more options here, like trying to dmsetup remove -f do remap to error target,
which disconnects underlying devices and allows deactivate them,
but it is quite dangerous)
- reboot

(sounds like we need shutdownramfs but initramfs can be probably reused here:-)
Already thought about that before,... but it seems impossible to me,...
to convice distros to do that...

It is always amusing these discussions which super-hyper encryption mode
to use and then we are not able to properly shutdown it, keeping the root-fs
encryption key in memory.
Seriously, this is problem and must be solved if we are serious with
full disk encryption.

And it's quite complex I guess,... given the fact that there are
basically arbitrary ways to stack your block devices...

It is exactly the same complexity like in initramfs, just steps are reversed.

But I think that solution which 1) flush page cache 2) remount read-only
and 3) wipe all encryption keys for remaining devices is enough.

Right now when I shutdown,... I get errors for lvm/dm-crypt/md,... as
they all can't close there devices,... as the root-fs is just ro-mounted
(ok the Debian cryptsetup package seems to not display that error,.. but
it's probably there).

cryptsetup should print error when device is busy on luksClose,
report it to upstream (currently probably me) if not:-)
(It prints "Device <dev> is busy." in that situtation and fails.)

Anyway, I am sure that other distro maintainers must solved similar problem...

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