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Vulnerability in encrypted loop device for linux


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Date: Wed, 2 Jan 2002 14:57:54 -0500
From: Jerome Etienne <[email protected]>
To: [email protected]
Subject: Vulnerability in encrypted loop device for linux

Hello,

The following text describes a security hole in the encrypted loop 
device for linux. Because of it, an attacker is able to modify the 
content of the encrypted device without being detected. This text 
proposes to fix the hole by authenticating the device.

comments are welcome

ps: version in html, pdf and ps can be found in http://www.off.net/~jme


                Vulnerability in encrypted loop device for Linux

                           Jerome Etienne [email protected]

Abstract

   This text describes a security hole i found in encrypted loop device for
   Linux. An attacker is able to modify the content of the encrypted device
   without being detected (see section 2). This text proposes to fix the hole
   by authenticating the device (see section 3).

1  Threat model

   Encrypting a disk device aims to protect against a off-line attacker who
   would be able to access the disk between 2 legitimate mounts.

   It isn't against an attacker who has access to the running computer when
   the encrypted device is mounted as either (i) the attacker is root and it
   can access the encrypted device anyway or (ii) he is an unprivileged user
   and can be stopped with Unix's right management (i.e. user/group).

2  Attack description

   The vulnerability of encrypted loop device is due to its lack of
   authentication. The aim of encryption is to make the data unreadable for
   anybody who doesn't know the key. It doesn't prevent an attacker from
   modifying the data. People assume that an attacker won't do it because the
   attacker wouldn't be able to choose the resulting clear text. But this
   section shows that the attacker can choose the resulting clear text to
   some extends and that modifying the cypher text data may be interesting
   even if the attacker ignores the result.

   This attack is only applicable to device storing data which are reused
   across mounts: most file-system (e.g. ext2, reiserfs, ext3) but not swap.
   In some systems, encrypted devices are stored in the same location than
   the encrypted disk containing the operating system. For those systems the
   attacker who can access the encrypted device, can easily modify the OS to
   gain access (e.g. kernel) independtly of the encrypted device.

  2.1  To insert random data

   If the attacker modifies the cipher text without choosing the resulting
   clear text, it will likely produce random data. The legitimate user won't
   detect the modification and will use them as if they were valid. As they
   likely appears random, it will result of a Denial of Service (aka DoS).

  2.2  To insert chosen data

   The encryption mode used by encrypted loop device is CBC[oST81,sec 5.3].
   CBC allows cut/past attacks i.e. the attacker can cut encrypted data from
   one part of the device and paste them in another location. As both data
   sections have been encrypted by the same key, the clear text won't be
   completely random data.

   This lack of authentication isn't a CBC flaw. Authentication isn't
   considered a aim of the encryption mode, so most modes (e.g. ECB, CFB,
   OFB) doesn't authenticate the data. To use another mode would be flawed in
   the same way except if they explicitly protect against forgery. Recently
   some modes including authentication popped up to speed up the encryption /
   authentication couple but as far as i know they are all patented.

   In very short, encrypting with CBC is Cn=Enc(Cn-1 xor Pn) where Enc(x) is
   encrypting x, Pn is the nth block of plain text and Cn the nth block of
   cipher text. For the first block, Cn-1 is an Initial vector (aka IV) which
   may be public and must be unique for a given key. The decryption is Pn =
   Dec(Cn) xor Cn-1. See [oST81,sec 5.3] for a longer description of CBC.

   If the attacker copies s blocks from the location m to n (aka
   [Cn,...,Cn+s-1] == [Cm,...,Cm+s-1]), Pn+1 up to Pn+s-1 will the same as
   Pm+1 to Pm+s-1 and Pn will likely appears random. Cn (i.e. Cm) will be
   decrypted as Pn = Dec(Cm) xor Cn-1 but Cm-1 and Cn-1 are different so Pn
   will likely appears random. Nevertheless Pn+1 = Dec(Cn+1) xor Cn =
   Dec(Cm+1) xor Cm = Pm+1, so Pn+1=Pm+1. So if the attacker has an idea of
   the content of a group of blocks in the device, he can copy them to the
   Nth block, thus it can choose the content of it without being detected.

   As an file-system isn't designed to appears random, its content may be
   predictable to some extents (e.g. common directories and files, inode,
   superblock). The attacker may use such informations to guess the contents
   and do a knowledgeable cut/past. For example, an attacker knowing the
   location of a password file may replace a password by another one which is
   already known.

3  Proposed fixes

   We propose 2 types of fixes: one which authenticate at mount time (see
   section 3.1) and the other which authenticates at the cluster level (see
   section 3.2). The choice between the two (see section 3.4) is a user
   matter as it mostly depends on the access pattern on the encrypted device.

   In the proposed fixes, the authentication is a MAC computed over the
   encrypted device. The MAC is HMAC[KBC97] combined with a configured hash
   function, preferably a well studied one such as SHA1[oST95] or MD5[Riv92].
   The MAC secret key is derived from the pass-phrase via PKCS-5 key
   derivations ([Kal00,sec 5.1]).

  3.1  Authenticating at mount time

   As we need to authenticate the device across mounts and not while it is
   mounted (see section 1), it is sufficient to authenticate the whole device
   during mount operations. It slows down mount operations but they are
   rather infrequent so we consider the trade-off delay/security acceptable.
   The MAC is verified during mount operations and generated during unmount
   operations. It isn't supposed to be valid while the device is mounted.

   The MAC generation is done when unmounting the device. The MAC is computed
   over all the sectors of the device and the result is appended in the
   device file after all the sectors.

   The MAC verification is done when mounting the device. The MAC is computed
   over all the sectors of the device. If the result is equal to the MAC
   appended to the block device, the verification succeed, else it failed.
   The verification may fail (i) if an attacker attempted to modify the
   device during 2 legitimates mounts or (ii) if the device hasn't been
   cleanly unmounted (e.g. computer crash). It is impossible to automatically
   distinguish both cases with certainty. So if the verification fails, the
   user is notified and the mount operation may be stopped depending on
   configuration.

  3.2  Authenticating at cluster level

   To authenticate the whole device at mount time, may be considered
   prohibitive by some users, so this section describe an alternative which
   authenticate the device at the cluster level. A cluster is a group of one
   or more sectors, the exact number depends on configuration. In this case,
   the MAC is verified each time a cluster is read from the disk and
   generated at each write.

   If the device isn't cleanly unmounted, the authentication of one or more
   cluster may fail (e.g. the super block). This case will be detected at
   mount time. But if an attacker forges data in the device, it will be
   detected only when the user read the modified data. The kernel will read
   the forged cluster and the authentication will fail. It may report it with
   a printk with a rate limitor, it isn't clean but i don't see any better
   way.

  3.3  MAC location

   Currently the encrypted loop file-system is stored in a regular file of a
   hosting file-system. Its size is a multiple of a sector size (i.e. 512
   byte). The MAC could be stored in a separate file or included in the
   regular file. To store the MAC in a separate file, generates problems
   while managing the loop device file (e.g. copy, backup). The administrator
   must not forget to copy the MAC file when he copies the device file, else
   the copied device won't be usable anymore. To store the MAC in the same
   file as the clusters doesn't has this disadvantage.

  3.4  Comparison

   To authenticate at the cluster level will increase the access time of each
   cluster but won't affect mount operation. The exact increase depends on
   the MAC and encryption algorithms. As a rule of thumb, MAC algorithms are
   typically 3 times faster than encryption ones so the time dedicated to
   cryptography for each block will increase by around 30%. To authenticate
   at mount time will largely slow down the mount operations but won't affect
   every access once mounted.

   The authentication at mount time will detect forgery at mount time,
   whereas the alternative detects it only when the forged cluster is read,
   possibly a long time after the modification. Users may consider that it is
   easier to diagnose who forged it if they have a better idea of when the
   attack occurred.

   To authenticate the whole device at mount time requires a single MAC per
   device, so the space overhead (typically 16 byte) is negligible compared
   to the device's size. To authenticate at the cluster level requires a MAC
   per cluster, it is significantly more but some people may consider it
   still negligible, especially with cheap disks.

   The choice between the two mostly depends on the access pattern on the
   encrypted device. If the device is used for interactive purpose, the
   increased access latency may be unsuitable. If the access latency is
   important or if every block is frequently modified, to authenticate only
   once at mount time may be more interesting. If the user can't stand long
   mount operations, to authenticate at cluster level will be more suitable.
   As only the final user knows the type access made on his encrypted device,
   he should be the one able to choose between the two.

4  Acknowledgments

   Thanks to Andy Kleen and Phil Swan for their useful comments.

5  Conclusion

   This text described an vulnerability in encrypted loop device which allows
   an attacker to modify the encrypted device without being detected (see
   section 2). We propose a fix which authenticate the whole device during
   mount operation (see section 3.1). This fix slows down mount operations
   but we consider the trade-off longer delay vs additional security very
   reasonable as mount operations are rather infrequent. We propose another
   fix which authenticate at cluster level for people who can't stand long
   mount operation. The choice between the two is a final user matter.

   The authentication may be optionally disabled thus if an user considers
   the trade-off delay/security not in favor of security, he may choose to be
   vulnerable to this attack and disable it. Nevertheless the author thinks
   encrypted loop device must be secure by default.

References

   [Kal00]
           B. Kaliski. Pkcs 5: Password-based cryptography specification
           version 2.0. Request For Comment (Informational) RFC2898,
           September 2000.

   [KBC97]
           H. Krawczyk, M. Bellare, and R. Canetti. Hmac: Keyed-hashing for
           message authentication. Request For Comment (Informational)
           RFC2104, February 1997.

   [oST81]
           National Institute of Standards and Technology. implementing and
           using the nbs data encryption standard. Federal information
           processing standards fips74, April 1981.

   [oST95]
           National Institute of Standards and Technology. Secure hash
           standard. Federal information processing standards fips180-1,
           April 1995.

   [Riv92]
           R. Rivest. The md5 message-digest algorithm. Request For Comment
           (Informational) RFC1321, April 1992.

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