• Eric Biggers's avatar
    fscrypt: v2 encryption policy support · 5dae460c
    Eric Biggers authored
    Add a new fscrypt policy version, "v2".  It has the following changes
    from the original policy version, which we call "v1" (*):
    
    - Master keys (the user-provided encryption keys) are only ever used as
      input to HKDF-SHA512.  This is more flexible and less error-prone, and
      it avoids the quirks and limitations of the AES-128-ECB based KDF.
      Three classes of cryptographically isolated subkeys are defined:
    
        - Per-file keys, like used in v1 policies except for the new KDF.
    
        - Per-mode keys.  These implement the semantics of the DIRECT_KEY
          flag, which for v1 policies made the master key be used directly.
          These are also planned to be used for inline encryption when
          support for it is added.
    
        - Key identifiers (see below).
    
    - Each master key is identified by a 16-byte master_key_identifier,
      which is derived from the key itself using HKDF-SHA512.  This prevents
      users from associating the wrong key with an encrypted file or
      directory.  This was easily possible with v1 policies, which
      identified the key by an arbitrary 8-byte master_key_descriptor.
    
    - The key must be provided in the filesystem-level keyring, not in a
      process-subscribed keyring.
    
    The following UAPI additions are made:
    
    - The existing ioctl FS_IOC_SET_ENCRYPTION_POLICY can now be passed a
      fscrypt_policy_v2 to set a v2 encryption policy.  It's disambiguated
      from fscrypt_policy/fscrypt_policy_v1 by the version code prefix.
    
    - A new ioctl FS_IOC_GET_ENCRYPTION_POLICY_EX is added.  It allows
      getting the v1 or v2 encryption policy of an encrypted file or
      directory.  The existing FS_IOC_GET_ENCRYPTION_POLICY ioctl could not
      be used because it did not have a way for userspace to indicate which
      policy structure is expected.  The new ioctl includes a size field, so
      it is extensible to future fscrypt policy versions.
    
    - The ioctls FS_IOC_ADD_ENCRYPTION_KEY, FS_IOC_REMOVE_ENCRYPTION_KEY,
      and FS_IOC_GET_ENCRYPTION_KEY_STATUS now support managing keys for v2
      encryption policies.  Such keys are kept logically separate from keys
      for v1 encryption policies, and are identified by 'identifier' rather
      than by 'descriptor'.  The 'identifier' need not be provided when
      adding a key, since the kernel will calculate it anyway.
    
    This patch temporarily keeps adding/removing v2 policy keys behind the
    same permission check done for adding/removing v1 policy keys:
    capable(CAP_SYS_ADMIN).  However, the next patch will carefully take
    advantage of the cryptographically secure master_key_identifier to allow
    non-root users to add/remove v2 policy keys, thus providing a full
    replacement for v1 policies.
    
    (*) Actually, in the API fscrypt_policy::version is 0 while on-disk
        fscrypt_context::format is 1.  But I believe it makes the most sense
        to advance both to '2' to have them be in sync, and to consider the
        numbering to start at 1 except for the API quirk.
    Reviewed-by: default avatarPaul Crowley <paulcrowley@google.com>
    Reviewed-by: default avatarTheodore Ts'o <tytso@mit.edu>
    Signed-off-by: default avatarEric Biggers <ebiggers@google.com>
    5dae460c
crypto.c 15.7 KB