[dm-crypt] System comes up very slowly
ross at biostat.ucsf.edu
Sun Sep 28 00:30:59 CEST 2014
On Sat, Sep 27, 2014 at 10:32:58PM +0200, Arno Wagner wrote:
> On Sat, Sep 27, 2014 at 21:39:30 CEST, Ross Boylan wrote:
> > On Sat, Sep 27, 2014 at 12:19:18PM +0200, Arno Wagner wrote:
> > > On Sat, Sep 27, 2014 at 06:01:19 CEST, Ross Boylan wrote:
> > > > When my computer reboots it shows the grub menu and some initial messages
> > > > from the kernel loading and then waits a very long time (minutes) before
> > > > asking for the pass-phrase for the main partition.
> > > >
> > > > I speculate the delay is to gather randomness for the 2 random-encrypted
> > > > swap partitions. However, hitting keys doesn't seem to speed it up.
> > > >
> > > > Is this speculation reasonable?
> > >
> > > It depends. Doing randomness gathering right is difficult. It
> > > always is a trade-off between quelity and speed. If you look
> > > through the mailing-list archives, you find sevveral long
> > > discussions about this.
> > >
> > > That said, current cryptsetup defaults to /dev/urandom, which
> > > gives you randomness even if entropy is low (which may be
> > > insecure). We decided to use the fast option and to warn
> > > people in the man-pages. You can check the defaults with
> > > "cryptsetup --help", at the end it tells you the used
> > > CPRNG.
> > I'm running crypsetup 1.0.6 on Debian Lenny; neither --help nor
> > --version seems to give any info on the random source.
> Aehm, that is so incredible historic that helping you becomes
> very hard. This version is more than 7 years old.
> > I'm asking becaue I'm about to convert the system to wheezy and have
> > the opportunity to change things. I presume this will generate an
> > initrd with the wheezy version, 1.4.3, which uses urandom according to
> > its --help.
> Upgrade and see whether the problem goes away. If not, ask
> again, but it likely will be something else if urandom is the
> > Hmm, the other thing that might be relevant is that there are 7 non-swap
> > encrypted logical volumes. /etc/crypttab lists root first, then the
> > 2 swaps, and then 6 other LV's. For most of them the relevant key is
> > in the 2nd slot. The pass-phrase prompt is for the root device.
> Ah, that may be it. Unlocking a LUKS volume takes checking key-slots
> and, due to hash-iteration, each keyslot check takes a certain time.
> These days, the default is 1 second for each key-slot, and some
> additional time for the master-key. This can be increased on
> creation and at compile-time though. Also, mounting can take
> quite some time depending in volume size. Your 7 volumes can
> easily take the 2 Minutes you observe.
I don't think it's the mounting since the partitions can't be mounted
until they are decrypted, and that can't happen until after I enter
the pass-phrase. Also, I suspect debian does all the decryption
first, and then the mounts.
The wait seems like forever, and could be 5 or 10 minutes.
> What you can do to determine iteration time is to look at
> Key-slot "iteration" parameter in the output of "luksDump"
> and then do a reference test, e.g. with a loop-device as in
> FAQ Item 2.6 and with explicitely specified ---uter-time 1000
> (1 second). That should tell you how long a key-slot needs
> to be checked.
> Unfortunately, there is no way to change iteration count.
> If it is indeed too long, the simplest way is to backup the
> data and re-create the volumes woth a different --iter-time
> or the current 1 sec default. Longer times do not really
> add much security.
Since I'm doing a fresh install and then restore I can change
anything, since I'm building new partitions and file systems.
I was encrypting selected partitions/logical volumes, but I'm probably
going to put the crypt right on top of RAID this time. This means
there will be only one thing to unlock.
One reason I went with the old scheme is that I thought encryption
became less secure the bigger the area encrypted. However, I don't
see anything about that in the FAQ (except for the 2TB limit on some
> > >
> > > > If the delay is from the encrypted swap, is there anything I can do about
> > > > it short of eliminating the swap? Is there any reason to avoid using a
> > > > fixed key for the swap? Fixed keys sound as if they should eliminate the
> > > > need for randomness from the system.
> > >
> > > Do not use fixed-keys! They will be available to an attacker.
I must be misunderstanding, because it seems to me any LUKS device
that is not random encrypted has fixed keys.
> > > The whole point of random keys for swap is that they are
> > > non-predictable and non-recoverable, yet you do not need
> > > to enter them manually. Fixed keys break that.
> > >
> > Why are the fixed keys for swap any more available than fixed keys
> > for other devices/partitions?
> Fixed keys are an exceedingly bad idea in general. Derived keys
I don't know what a derived key is. Like
except not transactional? 6.6 of the FAQ?
> are acceptable, but fixed keys can end up unintentionally
> in all sorts of places. Do not use them. (I gather the fixed keys
> are placed on the encrypted root partition.)
fixed keys, in the sense of --key-file for the other partitions, are in
the encrypted root. I am aware they are sensitive.
So is your concern about fixed keys a concern about keys that are
stored on the disk vs stored in my head?
> > >
> > > > [slightly off-topic]
> > > > Is it still the case that encrypted swap limits the ability to suspend or
> > > > hibernate and resume?
> > >
> > > Depends on the distro, I guess. But using encrypted swap that
> > > way is insecure, as an attacker can easily get access to it,
> > > and so it is not a good idea. For standard attacks (e.g. over
> > > firewire) a machine suspended/hibernated this way is wide open.
> > > Encrypted swap is worthless unless a full power-off is performed,
> > > you cannot have it easy _and_ secure in this case.
> > I think this means, for random-encrypted swap:
> > eencrypted swap + system shutdown and power off is secure
> Yes. Key gone, no way to recover anything from swap.
> > ecnrypted swap + suspend (system alive but low power) is insecure
> Yes. Key is in memory.
I had assumed one would have to reenter the pass-phrase on wakeup.
Though now that I think of it, that's non-trivial since the code to
get the pass-phrase and cryptsetup itself would need to be in memory
that wasn't encrypted.
> > What does it mean for encrypted swap + hibernate (power is off but
> > system state is saved to disk)?
> If you can wake up without giving encryption keys again, the
> key is somehwere on disk.
> > Not sure if I have the suspend/hibernate lingo right. I think those
> > are MS-Windows terms.
> They are. If you want security, it really is better to just
> have faster boot and not suspend or hibernate.
The easiest way to have faster boot is to use SSD, but the FAQ
indicates the security of SSD's is unclear.
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