Re: [swsusp] encrypt suspend data for easy wiping

From: Andreas Steinmetz
Date: Wed Jul 27 2005 - 09:24:03 EST

Pavel Machek wrote:
> Hi!
>>>>2) An attacker breaks into your machine remotely while you're using
>>>>it. He has access to all your RAM, which if you're actually using it,
>>>>very likely including the same IPSEC, dm_crypt, and ssh-agent keys as
>>>>are saved on suspend. Further, he can trivially capture your
>>>>keystrokes and thus capture any keys you use from that point forward.
>>>>This patch gets us very close to nothing.
>>>>3) An attacker steals your unsuspended laptop. He has access to all
>>>>your RAM, which in all likelihood includes your IPSEC, dm_crypt, and
>>>>ssh-agent keys. Odds are good that he invokes swsusp by closing the
>>>>laptop. This patch gets us very close to nothing.
>>>>4) You suspend your laptop between typing your GPG key password and
>>>>hitting enter, thus leaving your password in memory when it would
>>>>otherwise be cleared. Then you resume your laptop and hit enter, thus
>>>>clearing the password from RAM but leaving it on the suspend
>>>>partition. Then an attacker steals your machine (without re-suspending
>>>>it!) and manages to recover the swsusp image which contains the
>>>Why without resuspending it? Position of critical data in swap is
>>>pretty much random.
>>Typical swap partition sizes are about the same as RAM sizes. So the
>>odds of any given thing in a previous suspend getting overwritten by
>>the next one are high.
> Well, if you suspend with 100MB of RAM used, then keep suspending half
> a year with only 50MB of RAM used, you'll have that half-year-old data
> in there.
>>>What I'm worried is: attacker steals your laptop after you were using
>>>swsusp for half a year. Now your swap partition contains random pieces
>>>of GPG keys you were using for last half a year. That's bad.
>>And it's incredibly unlikely. Suspending while a supposedly
>>short-lived key is in RAM should be rare. Surviving on disk after half
>>a year of swapping and suspending should be negligible probability.
> Disagreed.
>>It's not worth even thinking about when we have real suspended laptops
>>getting stolen every day in REAL LIFE. Anyone who cares about your
>>highly contrived case also cares about 1000 times more about the real
>>life case of the stolen laptop. Otherwise they're fooling themselves.
>>This code is bad. It attacks a very rare problem, gives its users (and
>>apparently its authors) a false sense of security, reimplements
>>dm_crypt functionality apparently without much attention to the
>>subtleties of block device encryption and without serious review, and
>>it stands in the way of doing things right.
> Think about current code as really quick disk wiping method. Now, if
> you feel that we are giving false sense of security... we should
> not. Perhaps option should be renamed to CONFIG_SWSUSP_WIPE. Patches
> would certainly be accepted and I believe Andreas is going to cook
> some when he gets back online :-). I'd like to keep it there, because
> it enables us to do it properly in future. Contrary to what you think,
> I believe this is going to be part of a solution.

this is really supposed to do wiping, nothing else. Maybe the reference
to crypto may confuse a bit, but then crypto is the best way to wipe.
The documentation I did send to Pavel some time ago for swsusp.txt does
contain a big fat warning about that and starts with:

Q: What is this 'Encrypt suspend image' for?

A: First of all: it is not a replacement for dm-crypt encrypted swap.
It cannot protect your computer while it is suspended. Instead it does
protect from leaking sensitive data after resume from suspend.

I'm still dreaming of using dm-crypt for encrypted swap on the one hand
and wiping on resume from suspend on the other working together which
would mean:

You have an initrd that either prompts for a key or retrieves a key from
an attached hardware device (pcmcia, usb, ...). The key is used for
dmsetup to enable the encrypted swap partition. swsusp then resumes from
the encrypted swap partition and wipes the suspend image.

Unfortunately, as of 2.6.13rc3 I don't see any chance for this to work
(excerpt from init/main.c):


if (sys_access((const char __user *) "/init", 0) == 0)
execute_command = "/init";

do_basic_setup calls do_initcalls and swsusp is using late_initcall().
prepare_namespace calls initrd_load and friends. So there is currently
no chance to use an initrd to setup any crypto key.

This is really the missing link here.
Andreas Steinmetz SPAMmers use robotrap@xxxxxxxx
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