fscrypt: add support for hardware-wrapped keys

Add support for hardware-wrapped keys to fscrypt.  Such keys are
protected from certain attacks, such as cold boot attacks.  For more
information, see the "Hardware-wrapped keys" section of
Documentation/block/inline-encryption.rst.

To support hardware-wrapped keys in fscrypt, we allow the fscrypt master
keys to be hardware-wrapped.  File contents encryption is done by
passing the wrapped key to the inline encryption hardware via
blk-crypto.  Other fscrypt operations such as filenames encryption
continue to be done by the kernel, using the "software secret" which the
hardware derives.  For more information, see the documentation which
this patch adds to Documentation/filesystems/fscrypt.rst.

Note that this feature doesn't require any filesystem-specific changes.
However it does depend on inline encryption support, and thus currently
it is only applicable to ext4 and f2fs.

The version of this feature introduced by this patch is mostly
equivalent to the version that has existed downstream in the Android
Common Kernels since 2020.  However, a couple fixes are included.
First, the flags field in struct fscrypt_add_key_arg is now placed in
the proper location.  Second, key identifiers for HW-wrapped keys are
now derived using a distinct HKDF context byte; this fixes a bug where a
raw key could have the same identifier as a HW-wrapped key.  Note that
as a result of these fixes, the version of this feature introduced by
this patch is not UAPI or on-disk format compatible with the version in
the Android Common Kernels, though the divergence is limited to just
those specific fixes.  This version should be used going forwards.

This patch has been heavily rewritten from the original version by
Gaurav Kashyap <quic_gaurkash@quicinc.com> and
Barani Muthukumaran <bmuthuku@codeaurora.org>.

Tested-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org> # sm8650
Link: https://lore.kernel.org/r/20250404225859.172344-1-ebiggers@kernel.org
Signed-off-by: Eric Biggers <ebiggers@google.com>

1 files changed, 4 insertions, 2 deletions

diff --git a/include/uapi/linux/fscrypt.h b/include/uapi/linux/fscrypt.h
index 7a8f4c290187..3aff99f2696a 100644
--- a/include/uapi/linux/fscrypt.h
+++ b/include/uapi/linux/fscrypt.h
@@ -119,7 +119,7 @@ struct fscrypt_key_specifier {
  */
 struct fscrypt_provisioning_key_payload {
 	__u32 type;
-	__u32 __reserved;
+	__u32 flags;
 	__u8 raw[];
 };
 
@@ -128,7 +128,9 @@ struct fscrypt_add_key_arg {
 	struct fscrypt_key_specifier key_spec;
 	__u32 raw_size;
 	__u32 key_id;
-	__u32 __reserved[8];
+#define FSCRYPT_ADD_KEY_FLAG_HW_WRAPPED	0x00000001
+	__u32 flags;
+	__u32 __reserved[7];
 	__u8 raw[];
 };