[Pkg-cryptsetup-devel] Bug#371160: cryptsetup: encrypted swap with variable key fails

[Mika Bostrom]

Package: cryptsetup
Version: 2:1.0.3-2
Severity: normal

  [Repost with reportbug due to PEBKAC]

  (Note: this coinsides with #342709 which deals with the same issues,
although bit differently.)

  Using a variable (boot-time generated and unknown) key for encrypting
swap space triggers a failure. The net effect is that swap space is
never used. Below are included the relevant snippets and/or files and
the annotation for what I believe to be the reason for this behaviour.


% cat /etc/crypttab
# <target name> <source device>         <key file>      <options>
cswap   /dev/hda7       /dev/urandom    swap,cipher=aes-cbc-essiv:sha256
ctmp    /dev/hda8       /dev/urandom    tmp,cipher=aes-cbc-essiv:sha256


% grep mapper /etc/fstab
/dev/mapper/ctmp /tmp           ext2    defaults        0       2
/dev/mapper/cswap none          swap    sw


  With the above setup the swap space is not taken into use when
booting. I believe the reason is this, as seen in changelog.Debian.gz

  * add new check 'vol_id', which uses /lib/udev/vol_id from udev and supports
    checks for any known filesystem type. implement a new option checkargs in 
    cryptdisks for that. suggest udev. closes one half of #370302. thanks to 
    Markus Nass and Darvid HÀrdeman for the suggestion.                    
  * always check for a swap partition before running mkswap

  The sequence during boot goes like this:

  1. /etc/init.d/cryptdisks is run; it in turn uses
      /lib/cryptsetup/cryptdisk.functions          
  2. in cryptdisk.functions:parse_opts option 'swap' causes
      /lib/cryptsetup/checks/vol_id to be used before doing anything to
      swap{space,device,file}                                          
  3. in cryptdisk.functions:do_start the sequence of operations is:
     - parse_opts                                                  
     - [...]     
     - do_luks / do_noluks
     - do_swap            
  4. cryptdisk.functions:do_luks creates the cryptographic mapping layer
      with given key; As this key is read from /dev/{,u}random, the     
      mapping is different each time the operation is run          
  5. The test with /lib/cryptsetup/checks/vol_id naturally fails, since
      the key has changed --> mapping is removed!                      
  6. in cryptdisk.functions:do_swap the mkswap command is now guaranteed
      to fail; since the mapping was removed in previous step, there is 
      no longer /dev/mapper/$dst in place                              
  7. swapon -a naturally fails

  Hence, to get encrypted swap in use after boot, it is required to
manually enter the following commands:

# cryptsetup -c aes-cbc-essiv:sha256 -h sha256 \
  -d /dev/urandom create cswap /dev/hda7        
# mkswap /dev/mapper/cswap
# swapon -a
           
  The fix for this (common) corner-case is to ignore the test result for
'vol_id swap' if keyfile is not static. However, since some mislead
individuals (and/or suspended laptop users) may use a real keyfile for
this purpose, I am not sure how to make this test a proper and robust
one. Perhaps test for keyfile's type for chardevice first?

  A silver lining though, /tmp is recreated and used properly :)

  Thanks.

-- System Information:
Debian Release: testing/unstable
  APT prefers unstable
  APT policy: (990, 'unstable')
Architecture: i386 (i686)
Shell:  /bin/sh linked to /bin/bash
Kernel: Linux 2.6.16.14
Locale: LANG=C, LC_CTYPE=fi_FI at euro (charmap=ISO-8859-15)

Versions of packages cryptsetup depends on:
ii  dmsetup                    2:1.02.05-2.1 The Linux Kernel Device Mapper use
ii  libc6                      2.3.6-13      GNU C Library: Shared libraries
ii  libdevmapper1.02           2:1.02.05-2.1 The Linux Kernel Device Mapper use
ii  libgcrypt11                1.2.2-1       LGPL Crypto library - runtime libr
ii  libgpg-error0              1.2-1         library for common error values an
ii  libpopt0                   1.10-2        lib for parsing cmdline parameters
ii  libuuid1                   1.39-1        universally unique id library

cryptsetup recommends no packages.

-- no debconf information

-- 
 Mika Boström      +358-40-525-7347  \-/  "World peace will be achieved
 Bostik at iki.fi    www.iki.fi/bostik   X    when the last man has killed
 Security freak, and proud of it.    /-\   the second-to-last." -anon?
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