| Message ID | 20190520172552.217253-1-ebiggers@kernel.org (mailing list archive) |
|---|---|
| Headers | show |
| Series | fscrypt: key management improvements | expand |
On Mon, May 20, 2019 at 10:25:36AM -0700, Eric Biggers wrote: > > This patchset makes major improvements to how keys are added, removed, > and derived in fscrypt, aka ext4/f2fs/ubifs encryption. It does this by > adding new ioctls that add and remove encryption keys directly to/from > the filesystem, and by adding a new encryption policy version ("v2") > where the user-provided keys are only used as input to HKDF-SHA512 and > are identified by their cryptographic hash. Do you have userspace programs which use these new ioctl's? What's are testing strategy for these new ioctls? Thanks, - Ted
On Mon, May 20, 2019 at 08:16:36PM -0400, Theodore Ts'o wrote: > On Mon, May 20, 2019 at 10:25:36AM -0700, Eric Biggers wrote: > > > > This patchset makes major improvements to how keys are added, removed, > > and derived in fscrypt, aka ext4/f2fs/ubifs encryption. It does this by > > adding new ioctls that add and remove encryption keys directly to/from > > the filesystem, and by adding a new encryption policy version ("v2") > > where the user-provided keys are only used as input to HKDF-SHA512 and > > are identified by their cryptographic hash. > > Do you have userspace programs which use these new ioctl's? What's > are testing strategy for these new ioctls? > > Thanks, > > - Ted This was answered in the cover letter, quoted below: I've written xfstests for the new APIs. They test the APIs themselves as well as verify the correctness of the ciphertext stored on-disk for v2 encryption policies. The tests can be found at: Repository: https://git.kernel.org/pub/scm/linux/kernel/git/ebiggers/xfstests-dev.git Branch: fscrypt-key-mgmt-improvements The xfstests depend on new xfs_io commands which can be found at: Repository: https://git.kernel.org/pub/scm/linux/kernel/git/ebiggers/xfsprogs-dev.git Branch: fscrypt-key-mgmt-improvements I've also made proof-of-concept changes to the 'fscrypt' userspace program (https://github.com/google/fscrypt) to make it support v2 encryption policies. You can find these changes in git at: Repository: https://github.com/ebiggers/fscrypt.git Branch: fscrypt-key-mgmt-improvements To make the 'fscrypt' userspace program experimentally use v2 encryption policies on new encrypted directories, add the following to /etc/fscrypt.conf within the "options" section: "policy_version": "2" Finally, it's also planned for Android and Chromium OS to switch to the new ioctls and eventually to v2 encryption policies. Work-in-progress, proof-of-concept changes by Satya Tangirala for AOSP can be found at https://android-review.googlesource.com/q/topic:fscrypt-key-mgmt-improvements - Eric
On Mon, May 20, 2019 at 05:41:20PM -0700, Eric Biggers wrote: > > This was answered in the cover letter, quoted below: Oops, thanks. My bad, I stopped reading when I saw the changelog, and missed that part of the description. - Ted
Hello, [Note: I'd like to apply this for v5.3. Additional review is greatly appreciated, especially of the API before it's set in stone. Thanks!] This patchset makes major improvements to how keys are added, removed, and derived in fscrypt, aka ext4/f2fs/ubifs encryption. It does this by adding new ioctls that add and remove encryption keys directly to/from the filesystem, and by adding a new encryption policy version ("v2") where the user-provided keys are only used as input to HKDF-SHA512 and are identified by their cryptographic hash. All new APIs and all cryptosystem changes are documented in Documentation/filesystems/fscrypt.rst. Userspace can use the new key management ioctls with existing encrypted directories, but migrating to v2 encryption policies is needed for the full benefits. These changes solve four interrelated problems: (1) Providing fscrypt keys via process-subscribed keyrings is abusing encryption as an OS-level access control mechanism, causing many bugs where processes don't get access to the keys they need -- e.g., when a 'sudo' command or a system service needs to access encrypted files. It's also inconsistent with the filesystem/VFS "view" of encrypted files which is global, so sometimes things randomly happen to work anyway due to caching. Regardless, currently almost all fscrypt users actually do need global keys, so they're having to use workarounds that heavily abuse the session or user keyrings, e.g. Android and Chromium OS both use a systemwide "session keyring" and the 'fscrypt' tool links all user keyrings into root's user keyring. (2) Currently there's no way to securely and efficiently remove a fscrypt key such that not only is the original key wiped, but also all files and directories protected by that key are "locked" and their per-file keys wiped. Many users want this and are using 'echo 2 > /proc/sys/vm/drop_caches' as a workaround, but this is root-only, and also is overkill so can be a performance disaster. (3) The key derivation function (KDF) that fscrypt uses to derive per-file keys is nonstandard, inflexible, and has some weaknesses such as being reversible and not evenly distributing the entropy from the user-provided keys. (4) fscrypt doesn't check that the correct key was supplied. This can be a security vulnerability, since it allows malicious local users to associate the wrong key with files to which they have read-only access, causing other users' processes to read/write the wrong data. Ultimately, the solutions to these problems all tie into each other. By adding a filesystem-level encryption keyring with ioctls to add/remove keys to/from it, the keys are made usable filesystem-wide (solves problem #1). It also becomes easy to track the inodes that were "unlocked" with each key, so they can be evicted when the key is removed (solves problem #2). Moreover, the filesystem-level keyring is a natural place to store an HMAC transform keyed by each key, thus making it easy and efficient to switch the KDF to HKDF (solves problem #3). Finally, to check that the correct key was supplied, I use HKDF to derive a cryptographically secure key_identifier for each key (solves problem #4). This in combination with key quotas and other careful precautions also makes it safe to allow non-root users to add and remove keys to/from the filesystem-level keyring. Thus, all problems are solved without having to restrict the fscrypt API to root only. The patchset is organized as follows: - Patches 1-8 add new ioctls FS_IOC_ADD_ENCRYPTION_KEY, FS_IOC_REMOVE_ENCRYPTION_KEY, and FS_IOC_GET_ENCRYPTION_KEY_STATUS. Adding a key logically "unlocks" all files on the filesystem that are protected by that key; removing a key "locks" them again. - Patches 9-12 add support for v2 encryption policies. - Patches 13-15 wire up the new ioctls to ext4, f2fs, and ubifs. - Patch 16 updates the fscrypt documentation for all the changes. Changes v5 => v6: - Change HKDF to use the specification-defined default salt rather than a custom fixed salt, and prepend the string "fscrypt" to 'info' instead. This is arguably needed to match how RFC 5869 and SP 800-56C are worded. Both ways are secure in this context, so prefer the "boring" way that clearly matches the standards. - Rebase onto v5.2-rc1. - A few small cleanups. Changes v4 => v5: - Simplify shrink_dcache_inode(), as suggested by Al Viro; also move it into fs/crypto/. - Fix a build error on some architectures by calling copy_from_user() rather than get_user() with a __u64 pointer. Changes v3 => v4: - Introduce fscrypt_sb_free() to avoid an extra #ifdef. - Fix UBIFS's ->drop_inode(). - Add 'version' to union fscrypt_policy and union fscrypt_context. Changes v2 => v3: - Use ->drop_inode() to trigger the inode eviction during/after FS_IOC_REMOVE_ENCRYPTION_KEY, as suggested by Dave Chinner. - A few small cleanups. v1 of this patchset was sent in October 2017 with title "fscrypt: filesystem-level keyring and v2 policy support". This revived version follows the same basic design but incorporates numerous improvements, such as splitting keyinfo.c into multiple files for much better understandability, and introducing "per-mode" encryption keys to implement the semantics of the DIRECT_KEY encryption policy flag. This patchset applies to v5.2-rc1. You can also get it from git at: Repository: https://git.kernel.org/pub/scm/linux/kernel/git/ebiggers/linux.git Branch: fscrypt-key-mgmt-improvements-v6 I've written xfstests for the new APIs. They test the APIs themselves as well as verify the correctness of the ciphertext stored on-disk for v2 encryption policies. The tests can be found at: Repository: https://git.kernel.org/pub/scm/linux/kernel/git/ebiggers/xfstests-dev.git Branch: fscrypt-key-mgmt-improvements The xfstests depend on new xfs_io commands which can be found at: Repository: https://git.kernel.org/pub/scm/linux/kernel/git/ebiggers/xfsprogs-dev.git Branch: fscrypt-key-mgmt-improvements I've also made proof-of-concept changes to the 'fscrypt' userspace program (https://github.com/google/fscrypt) to make it support v2 encryption policies. You can find these changes in git at: Repository: https://github.com/ebiggers/fscrypt.git Branch: fscrypt-key-mgmt-improvements To make the 'fscrypt' userspace program experimentally use v2 encryption policies on new encrypted directories, add the following to /etc/fscrypt.conf within the "options" section: "policy_version": "2" Finally, it's also planned for Android and Chromium OS to switch to the new ioctls and eventually to v2 encryption policies. Work-in-progress, proof-of-concept changes by Satya Tangirala for AOSP can be found at https://android-review.googlesource.com/q/topic:fscrypt-key-mgmt-improvements Eric Biggers (16): fs, fscrypt: move uapi definitions to new header <linux/fscrypt.h> fscrypt: use FSCRYPT_ prefix for uapi constants fscrypt: use FSCRYPT_* definitions, not FS_* fscrypt: add ->ci_inode to fscrypt_info fscrypt: refactor v1 policy key setup into keysetup_legacy.c fscrypt: add FS_IOC_ADD_ENCRYPTION_KEY ioctl fscrypt: add FS_IOC_REMOVE_ENCRYPTION_KEY ioctl fscrypt: add FS_IOC_GET_ENCRYPTION_KEY_STATUS ioctl fscrypt: add an HKDF-SHA512 implementation fscrypt: v2 encryption policy support fscrypt: allow unprivileged users to add/remove keys for v2 policies fscrypt: require that key be added when setting a v2 encryption policy ext4: wire up new fscrypt ioctls f2fs: wire up new fscrypt ioctls ubifs: wire up new fscrypt ioctls fscrypt: document the new ioctls and policy version Documentation/filesystems/fscrypt.rst | 670 ++++++++++++++---- MAINTAINERS | 1 + fs/crypto/Kconfig | 2 + fs/crypto/Makefile | 10 +- fs/crypto/crypto.c | 14 +- fs/crypto/fname.c | 5 +- fs/crypto/fscrypt_private.h | 366 +++++++++- fs/crypto/hkdf.c | 178 +++++ fs/crypto/keyinfo.c | 612 ---------------- fs/crypto/keyring.c | 963 ++++++++++++++++++++++++++ fs/crypto/keysetup.c | 560 +++++++++++++++ fs/crypto/keysetup_legacy.c | 340 +++++++++ fs/crypto/policy.c | 392 ++++++++--- fs/ext4/ioctl.c | 24 + fs/ext4/super.c | 3 + fs/f2fs/file.c | 46 ++ fs/f2fs/super.c | 2 + fs/super.c | 2 + fs/ubifs/ioctl.c | 16 + fs/ubifs/super.c | 11 + include/linux/fs.h | 1 + include/linux/fscrypt.h | 48 +- include/uapi/linux/fs.h | 54 +- include/uapi/linux/fscrypt.h | 163 +++++ 24 files changed, 3541 insertions(+), 942 deletions(-) create mode 100644 fs/crypto/hkdf.c delete mode 100644 fs/crypto/keyinfo.c create mode 100644 fs/crypto/keyring.c create mode 100644 fs/crypto/keysetup.c create mode 100644 fs/crypto/keysetup_legacy.c create mode 100644 include/uapi/linux/fscrypt.h