fscrypt: add FS_IOC_ADD_ENCRYPTION_KEY ioctl

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 <tytso@mit.edu>
Signed-off-by: Eric Biggers <ebiggers@google.com>

1 files changed, 39 insertions, 10 deletions

diff --git a/include/uapi/linux/fscrypt.h b/include/uapi/linux/fscrypt.h
index 29a945d165de..6aeca3cb0a2d 100644
--- a/include/uapi/linux/fscrypt.h
+++ b/include/uapi/linux/fscrypt.h
@@ -36,22 +36,51 @@ struct fscrypt_policy {
 	__u8 master_key_descriptor[FSCRYPT_KEY_DESCRIPTOR_SIZE];
 };
 
-#define FS_IOC_SET_ENCRYPTION_POLICY	_IOR('f', 19, struct fscrypt_policy)
-#define FS_IOC_GET_ENCRYPTION_PWSALT	_IOW('f', 20, __u8[16])
-#define FS_IOC_GET_ENCRYPTION_POLICY	_IOW('f', 21, struct fscrypt_policy)
-
-/* Parameters for passing an encryption key into the kernel keyring */
+/*
+ * Process-subscribed "logon" key description prefix and payload format.
+ * Deprecated; prefer FS_IOC_ADD_ENCRYPTION_KEY instead.
+ */
 #define FSCRYPT_KEY_DESC_PREFIX		"fscrypt:"
-#define FSCRYPT_KEY_DESC_PREFIX_SIZE		8
-
-/* Structure that userspace passes to the kernel keyring */
-#define FSCRYPT_MAX_KEY_SIZE			64
-
+#define FSCRYPT_KEY_DESC_PREFIX_SIZE	8
+#define FSCRYPT_MAX_KEY_SIZE		64
 struct fscrypt_key {
 	__u32 mode;
 	__u8 raw[FSCRYPT_MAX_KEY_SIZE];
 	__u32 size;
 };
+
+/*
+ * Keys are specified by an arbitrary 8-byte key "descriptor",
+ * matching fscrypt_policy::master_key_descriptor.
+ */
+#define FSCRYPT_KEY_SPEC_TYPE_DESCRIPTOR	1
+
+/*
+ * Specifies a key.  This doesn't contain the actual key itself; this is just
+ * the "name" of the key.
+ */
+struct fscrypt_key_specifier {
+	__u32 type;	/* one of FSCRYPT_KEY_SPEC_TYPE_* */
+	__u32 __reserved;
+	union {
+		__u8 __reserved[32]; /* reserve some extra space */
+		__u8 descriptor[FSCRYPT_KEY_DESCRIPTOR_SIZE];
+	} u;
+};
+
+/* Struct passed to FS_IOC_ADD_ENCRYPTION_KEY */
+struct fscrypt_add_key_arg {
+	struct fscrypt_key_specifier key_spec;
+	__u32 raw_size;
+	__u32 __reserved[9];
+	__u8 raw[];
+};
+
+#define FS_IOC_SET_ENCRYPTION_POLICY		_IOR('f', 19, struct fscrypt_policy)
+#define FS_IOC_GET_ENCRYPTION_PWSALT		_IOW('f', 20, __u8[16])
+#define FS_IOC_GET_ENCRYPTION_POLICY		_IOW('f', 21, struct fscrypt_policy)
+#define FS_IOC_ADD_ENCRYPTION_KEY		_IOWR('f', 23, struct fscrypt_add_key_arg)
+
 /**********************************************************************/
 
 /* old names; don't add anything new here! */