Re: [PATCH v8 13/20] fscrypt: v2 encryption policy support

[Paul Crowley <paulcrowley@xxxxxxxxxx>]

On Mon, 5 Aug 2019 at 09:28, Eric Biggers <ebiggers@xxxxxxxxxx> wrote:
>
> From: Eric Biggers <ebiggers@xxxxxxxxxx>
>
> 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.
>
> Signed-off-by: Eric Biggers <ebiggers@xxxxxxxxxx>

Looks good, feel free to add:

Reviewed-by: Paul Crowley <paulcrowley@xxxxxxxxxx>

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