linux-stable.git/fs/crypto/crypto.c, branch linux-5.4.y

fscrypt: v2 encryption policy support

2019-08-13T02:18:50+00:00

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: Paul Crowley 
Reviewed-by: Theodore Ts'o 
Signed-off-by: Eric Biggers

fscrypt: add FS_IOC_ADD_ENCRYPTION_KEY ioctl

2019-08-13T02:06:13+00:00

Add a new fscrypt ioctl, FS_IOC_ADD_ENCRYPTION_KEY.  This ioctl adds an
encryption key to the filesystem's fscrypt keyring ->s_master_keys,
making any files encrypted with that key appear "unlocked".

Why we need this
~~~~~~~~~~~~~~~~

The main problem is that the "locked/unlocked" (ciphertext/plaintext)
status of encrypted files is global, but the fscrypt keys are not.
fscrypt only looks for keys in the keyring(s) the process accessing the
filesystem is subscribed to: the thread keyring, process keyring, and
session keyring, where the session keyring may contain the user keyring.

Therefore, userspace has to put fscrypt keys in the keyrings for
individual users or sessions.  But this means that when a process with a
different keyring tries to access encrypted files, whether they appear
"unlocked" or not is nondeterministic.  This is because it depends on
whether the files are currently present in the inode cache.

Fixing this by consistently providing each process its own view of the
filesystem depending on whether it has the key or not isn't feasible due
to how the VFS caches work.  Furthermore, while sometimes users expect
this behavior, it is misguided for two reasons.  First, it would be an
OS-level access control mechanism largely redundant with existing access
control mechanisms such as UNIX file permissions, ACLs, LSMs, etc.
Encryption is actually for protecting the data at rest.

Second, almost all users of fscrypt actually do need the keys to be
global.  The largest users of fscrypt, Android and Chromium OS, achieve
this by having PID 1 create a "session keyring" that is inherited by
every process.  This works, but it isn't scalable because it prevents
session keyrings from being used for any other purpose.

On general-purpose Linux distros, the 'fscrypt' userspace tool [1] can't
similarly abuse the session keyring, so to make 'sudo' work on all
systems it has to link all the user keyrings into root's user keyring
[2].  This is ugly and raises security concerns.  Moreover it can't make
the keys available to system services, such as sshd trying to access the
user's '~/.ssh' directory (see [3], [4]) or NetworkManager trying to
read certificates from the user's home directory (see [5]); or to Docker
containers (see [6], [7]).

By having an API to add a key to the *filesystem* we'll be able to fix
the above bugs, remove userspace workarounds, and clearly express the
intended semantics: the locked/unlocked status of an encrypted directory
is global, and encryption is orthogonal to OS-level access control.

Why not use the add_key() syscall
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

We use an ioctl for this API rather than the existing add_key() system
call because the ioctl gives us the flexibility needed to implement
fscrypt-specific semantics that will be introduced in later patches:

- Supporting key removal with the semantics such that the secret is
  removed immediately and any unused inodes using the key are evicted;
  also, the eviction of any in-use inodes can be retried.

- Calculating a key-dependent cryptographic identifier and returning it
  to userspace.

- Allowing keys to be added and removed by non-root users, but only keys
  for v2 encryption policies; and to prevent denial-of-service attacks,
  users can only remove keys they themselves have added, and a key is
  only really removed after all users who added it have removed it.

Trying to shoehorn these semantics into the keyrings syscalls would be
very difficult, whereas the ioctls make things much easier.

However, to reuse code the implementation still uses the keyrings
service internally.  Thus we get lockless RCU-mode key lookups without
having to re-implement it, and the keys automatically show up in
/proc/keys for debugging purposes.

References:

    [1] https://github.com/google/fscrypt
    [2] https://goo.gl/55cCrI#heading=h.vf09isp98isb
    [3] https://github.com/google/fscrypt/issues/111#issuecomment-444347939
    [4] https://github.com/google/fscrypt/issues/116
    [5] https://bugs.launchpad.net/ubuntu/+source/fscrypt/+bug/1770715
    [6] https://github.com/google/fscrypt/issues/128
    [7] https://askubuntu.com/questions/1130306/cannot-run-docker-on-an-encrypted-filesystem

Reviewed-by: Theodore Ts'o 
Signed-off-by: Eric Biggers

fscrypt: use FSCRYPT_* definitions, not FS_*

2019-08-13T02:05:19+00:00

Update fs/crypto/ to use the new names for the UAPI constants rather
than the old names, then make the old definitions conditional on
!__KERNEL__.

Reviewed-by: Theodore Ts'o 
Signed-off-by: Eric Biggers

fscrypt: make fscrypt_msg() take inode instead of super_block

2019-08-13T02:04:44+00:00

Most of the warning and error messages in fs/crypto/ are for situations
related to a specific inode, not merely to a super_block.  So to make
things easier, make fscrypt_msg() take an inode rather than a
super_block, and make it print the inode number.

Note: This is the same approach I'm taking for fsverity_msg().

Signed-off-by: Eric Biggers

fscrypt: remove loadable module related code

2019-08-13T02:04:41+00:00

Since commit 643fa9612bf1 ("fscrypt: remove filesystem specific build
config option"), fs/crypto/ can no longer be built as a loadable module.
Thus it no longer needs a module_exit function, nor a MODULE_LICENSE.
So remove them, and change module_init to late_initcall.

Reviewed-by: Chandan Rajendra 
Signed-off-by: Eric Biggers

fscrypt: support decrypting multiple filesystem blocks per page

2019-05-28T17:27:53+00:00

Rename fscrypt_decrypt_page() to fscrypt_decrypt_pagecache_blocks() and
redefine its behavior to decrypt all filesystem blocks in the given
region of the given page, rather than assuming that the region consists
of just one filesystem block.  Also remove the 'inode' and 'lblk_num'
parameters, since they can be retrieved from the page as it's already
assumed to be a pagecache page.

This is in preparation for allowing encryption on ext4 filesystems with
blocksize != PAGE_SIZE.

This is based on work by Chandan Rajendra.

Reviewed-by: Chandan Rajendra 
Signed-off-by: Eric Biggers

fscrypt: introduce fscrypt_decrypt_block_inplace()

2019-05-28T17:27:53+00:00

Currently fscrypt_decrypt_page() does one of two logically distinct
things depending on whether FS_CFLG_OWN_PAGES is set in the filesystem's
fscrypt_operations: decrypt a pagecache page in-place, or decrypt a
filesystem block in-place in any page.  Currently these happen to share
the same implementation, but this conflates the notion of blocks and
pages.  It also makes it so that all callers have to provide inode and
lblk_num, when fscrypt could determine these itself for pagecache pages.

Therefore, move the FS_CFLG_OWN_PAGES behavior into a new function
fscrypt_decrypt_block_inplace().  This mirrors
fscrypt_encrypt_block_inplace().

This is in preparation for allowing encryption on ext4 filesystems with
blocksize != PAGE_SIZE.

Reviewed-by: Chandan Rajendra 
Signed-off-by: Eric Biggers

fscrypt: support encrypting multiple filesystem blocks per page

2019-05-28T17:27:53+00:00

Rename fscrypt_encrypt_page() to fscrypt_encrypt_pagecache_blocks() and
redefine its behavior to encrypt all filesystem blocks from the given
region of the given page, rather than assuming that the region consists
of just one filesystem block.  Also remove the 'inode' and 'lblk_num'
parameters, since they can be retrieved from the page as it's already
assumed to be a pagecache page.

This is in preparation for allowing encryption on ext4 filesystems with
blocksize != PAGE_SIZE.

This is based on work by Chandan Rajendra.

Reviewed-by: Chandan Rajendra 
Signed-off-by: Eric Biggers

fscrypt: introduce fscrypt_encrypt_block_inplace()

2019-05-28T17:27:52+00:00

fscrypt_encrypt_page() behaves very differently depending on whether the
filesystem set FS_CFLG_OWN_PAGES in its fscrypt_operations.  This makes
the function difficult to understand and document.  It also makes it so
that all callers have to provide inode and lblk_num, when fscrypt could
determine these itself for pagecache pages.

Therefore, move the FS_CFLG_OWN_PAGES behavior into a new function
fscrypt_encrypt_block_inplace().

This is in preparation for allowing encryption on ext4 filesystems with
blocksize != PAGE_SIZE.

Reviewed-by: Chandan Rajendra 
Signed-off-by: Eric Biggers

fscrypt: clean up some BUG_ON()s in block encryption/decryption

2019-05-28T17:27:52+00:00

Replace some BUG_ON()s with WARN_ON_ONCE() and returning an error code,
and move the check for len divisible by FS_CRYPTO_BLOCK_SIZE into
fscrypt_crypt_block() so that it's done for both encryption and
decryption, not just encryption.

Reviewed-by: Chandan Rajendra 
Signed-off-by: Eric Biggers