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Message-ID: <CA+_SqcAWniLLTk4eqWX81ypgMqnh2_9N=0JDKs3JopeV6XX_HA@mail.gmail.com> Date: Tue, 6 Aug 2019 13:44:56 -0700 From: Paul Crowley <paulcrowley@...gle.com> To: Eric Biggers <ebiggers@...nel.org> Cc: linux-fscrypt@...r.kernel.org, linux-ext4@...r.kernel.org, linux-f2fs-devel@...ts.sourceforge.net, linux-mtd@...ts.infradead.org, linux-fsdevel@...r.kernel.org, linux-crypto@...r.kernel.org, keyrings@...r.kernel.org, linux-api@...r.kernel.org, Satya Tangirala <satyat@...gle.com>, "Theodore Ts'o" <tytso@....edu>, Jaegeuk Kim <jaegeuk@...nel.org> Subject: Re: [PATCH v8 13/20] fscrypt: v2 encryption policy support On Mon, 5 Aug 2019 at 09:28, Eric Biggers <ebiggers@...nel.org> wrote: > > From: Eric Biggers <ebiggers@...gle.com> > > 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@...gle.com> Looks good, feel free to add: Reviewed-by: Paul Crowley <paulcrowley@...gle.com>
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