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 <[email protected]> and
Barani Muthukumaran <[email protected]>.

Tested-by: Bartosz Golaszewski <[email protected]> # sm8650
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Eric Biggers <[email protected]>

1 files changed, 38 insertions, 6 deletions

diff --git a/fs/crypto/inline_crypt.c b/fs/crypto/inline_crypt.c
index 7fa53d30aec3..1d008c440cb6 100644
--- a/fs/crypto/inline_crypt.c
+++ b/fs/crypto/inline_crypt.c
@@ -89,7 +89,8 @@ static void fscrypt_log_blk_crypto_impl(struct fscrypt_mode *mode,
 }
 
 /* Enable inline encryption for this file if supported. */
-int fscrypt_select_encryption_impl(struct fscrypt_inode_info *ci)
+int fscrypt_select_encryption_impl(struct fscrypt_inode_info *ci,
+				   bool is_hw_wrapped_key)
 {
 	const struct inode *inode = ci->ci_inode;
 	struct super_block *sb = inode->i_sb;
@@ -130,7 +131,8 @@ int fscrypt_select_encryption_impl(struct fscrypt_inode_info *ci)
 	crypto_cfg.crypto_mode = ci->ci_mode->blk_crypto_mode;
 	crypto_cfg.data_unit_size = 1U << ci->ci_data_unit_bits;
 	crypto_cfg.dun_bytes = fscrypt_get_dun_bytes(ci);
-	crypto_cfg.key_type = BLK_CRYPTO_KEY_TYPE_RAW;
+	crypto_cfg.key_type = is_hw_wrapped_key ?
+		BLK_CRYPTO_KEY_TYPE_HW_WRAPPED : BLK_CRYPTO_KEY_TYPE_RAW;
 
 	devs = fscrypt_get_devices(sb, &num_devs);
 	if (IS_ERR(devs))
@@ -151,12 +153,15 @@ out_free_devs:
 }
 
 int fscrypt_prepare_inline_crypt_key(struct fscrypt_prepared_key *prep_key,
-				     const u8 *raw_key,
+				     const u8 *key_bytes, size_t key_size,
+				     bool is_hw_wrapped,
 				     const struct fscrypt_inode_info *ci)
 {
 	const struct inode *inode = ci->ci_inode;
 	struct super_block *sb = inode->i_sb;
 	enum blk_crypto_mode_num crypto_mode = ci->ci_mode->blk_crypto_mode;
+	enum blk_crypto_key_type key_type = is_hw_wrapped ?
+		BLK_CRYPTO_KEY_TYPE_HW_WRAPPED : BLK_CRYPTO_KEY_TYPE_RAW;
 	struct blk_crypto_key *blk_key;
 	struct block_device **devs;
 	unsigned int num_devs;
@@ -167,9 +172,8 @@ int fscrypt_prepare_inline_crypt_key(struct fscrypt_prepared_key *prep_key,
 	if (!blk_key)
 		return -ENOMEM;
 
-	err = blk_crypto_init_key(blk_key, raw_key, ci->ci_mode->keysize,
-				  BLK_CRYPTO_KEY_TYPE_RAW, crypto_mode,
-				  fscrypt_get_dun_bytes(ci),
+	err = blk_crypto_init_key(blk_key, key_bytes, key_size, key_type,
+				  crypto_mode, fscrypt_get_dun_bytes(ci),
 				  1U << ci->ci_data_unit_bits);
 	if (err) {
 		fscrypt_err(inode, "error %d initializing blk-crypto key", err);
@@ -228,6 +232,34 @@ void fscrypt_destroy_inline_crypt_key(struct super_block *sb,
 	kfree_sensitive(blk_key);
 }
 
+/*
+ * Ask the inline encryption hardware to derive the software secret from a
+ * hardware-wrapped key.  Returns -EOPNOTSUPP if hardware-wrapped keys aren't
+ * supported on this filesystem or hardware.
+ */
+int fscrypt_derive_sw_secret(struct super_block *sb,
+			     const u8 *wrapped_key, size_t wrapped_key_size,
+			     u8 sw_secret[BLK_CRYPTO_SW_SECRET_SIZE])
+{
+	int err;
+
+	/* The filesystem must be mounted with -o inlinecrypt. */
+	if (!(sb->s_flags & SB_INLINECRYPT)) {
+		fscrypt_warn(NULL,
+			     "%s: filesystem not mounted with inlinecrypt\n",
+			     sb->s_id);
+		return -EOPNOTSUPP;
+	}
+
+	err = blk_crypto_derive_sw_secret(sb->s_bdev, wrapped_key,
+					  wrapped_key_size, sw_secret);
+	if (err == -EOPNOTSUPP)
+		fscrypt_warn(NULL,
+			     "%s: block device doesn't support hardware-wrapped keys\n",
+			     sb->s_id);
+	return err;
+}
+
 bool __fscrypt_inode_uses_inline_crypto(const struct inode *inode)
 {
 	return inode->i_crypt_info->ci_inlinecrypt;