blk-crypto: add ioctls to create and prepare hardware-wrapped keys

Until this point, the kernel can use hardware-wrapped keys to do
encryption if userspace provides one -- specifically a key in
ephemerally-wrapped form.  However, no generic way has been provided for
userspace to get such a key in the first place.

Getting such a key is a two-step process.  First, the key needs to be
imported from a raw key or generated by the hardware, producing a key in
long-term wrapped form.  This happens once in the whole lifetime of the
key.  Second, the long-term wrapped key needs to be converted into
ephemerally-wrapped form.  This happens each time the key is "unlocked".

In Android, these operations are supported in a generic way through
KeyMint, a userspace abstraction layer.  However, that method is
Android-specific and can't be used on other Linux systems, may rely on
proprietary libraries, and also misleads people into supporting KeyMint
features like rollback resistance that make sense for other KeyMint keys
but don't make sense for hardware-wrapped inline encryption keys.

Therefore, this patch provides a generic kernel interface for these
operations by introducing new block device ioctls:

- BLKCRYPTOIMPORTKEY: convert a raw key to long-term wrapped form.

- BLKCRYPTOGENERATEKEY: have the hardware generate a new key, then
  return it in long-term wrapped form.

- BLKCRYPTOPREPAREKEY: convert a key from long-term wrapped form to
  ephemerally-wrapped form.

These ioctls are implemented using new operations in blk_crypto_ll_ops.

Signed-off-by: Eric Biggers <ebiggers@google.com>
Tested-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org> # sm8650
Link: https://lore.kernel.org/r/20250204060041.409950-4-ebiggers@kernel.org
Signed-off-by: Jens Axboe <axboe@kernel.dk>

1 files changed, 36 insertions, 0 deletions

diff --git a/Documentation/block/inline-encryption.rst b/Documentation/block/inline-encryption.rst
index f03bd5b090d8..6380e6ab492b 100644
--- a/Documentation/block/inline-encryption.rst
+++ b/Documentation/block/inline-encryption.rst
@@ -492,6 +492,42 @@ when hardware support is available.  This works in the following way:
 blk-crypto-fallback doesn't support hardware-wrapped keys.  Therefore,
 hardware-wrapped keys can only be used with actual inline encryption hardware.
 
+All the above deals with hardware-wrapped keys in ephemerally-wrapped form only.
+To get such keys in the first place, new block device ioctls have been added to
+provide a generic interface to creating and preparing such keys:
+
+- ``BLKCRYPTOIMPORTKEY`` converts a raw key to long-term wrapped form.  It takes
+  in a pointer to a ``struct blk_crypto_import_key_arg``.  The caller must set
+  ``raw_key_ptr`` and ``raw_key_size`` to the pointer and size (in bytes) of the
+  raw key to import.  On success, ``BLKCRYPTOIMPORTKEY`` returns 0 and writes
+  the resulting long-term wrapped key blob to the buffer pointed to by
+  ``lt_key_ptr``, which is of maximum size ``lt_key_size``.  It also updates
+  ``lt_key_size`` to be the actual size of the key.  On failure, it returns -1
+  and sets errno.  An errno of ``EOPNOTSUPP`` indicates that the block device
+  does not support hardware-wrapped keys.  An errno of ``EOVERFLOW`` indicates
+  that the output buffer did not have enough space for the key blob.
+
+- ``BLKCRYPTOGENERATEKEY`` is like ``BLKCRYPTOIMPORTKEY``, but it has the
+  hardware generate the key instead of importing one.  It takes in a pointer to
+  a ``struct blk_crypto_generate_key_arg``.
+
+- ``BLKCRYPTOPREPAREKEY`` converts a key from long-term wrapped form to
+  ephemerally-wrapped form.  It takes in a pointer to a ``struct
+  blk_crypto_prepare_key_arg``.  The caller must set ``lt_key_ptr`` and
+  ``lt_key_size`` to the pointer and size (in bytes) of the long-term wrapped
+  key blob to convert.  On success, ``BLKCRYPTOPREPAREKEY`` returns 0 and writes
+  the resulting ephemerally-wrapped key blob to the buffer pointed to by
+  ``eph_key_ptr``, which is of maximum size ``eph_key_size``.  It also updates
+  ``eph_key_size`` to be the actual size of the key.  On failure, it returns -1
+  and sets errno.  Errno values of ``EOPNOTSUPP`` and ``EOVERFLOW`` mean the
+  same as they do for ``BLKCRYPTOIMPORTKEY``.  An errno of ``EBADMSG`` indicates
+  that the long-term wrapped key is invalid.
+
+Userspace needs to use either ``BLKCRYPTOIMPORTKEY`` or ``BLKCRYPTOGENERATEKEY``
+once to create a key, and then ``BLKCRYPTOPREPAREKEY`` each time the key is
+unlocked and added to the kernel.  Note that these ioctls have no relevance for
+raw keys; they are only for hardware-wrapped keys.
+
 Testability
 -----------