blk-crypto: add basic hardware-wrapped key support

To prevent keys from being compromised if an attacker acquires read
access to kernel memory, some inline encryption hardware can accept keys
which are wrapped by a per-boot hardware-internal key.  This avoids
needing to keep the raw keys in kernel memory, without limiting the
number of keys that can be used.  Such hardware also supports deriving a
"software secret" for cryptographic tasks that can't be handled by
inline encryption; this is needed for fscrypt to work properly.

To support this hardware, allow struct blk_crypto_key to represent a
hardware-wrapped key as an alternative to a raw key, and make drivers
set flags in struct blk_crypto_profile to indicate which types of keys
they support.  Also add the ->derive_sw_secret() low-level operation,
which drivers supporting wrapped keys must implement.

For more information, see the detailed documentation which this patch
adds to Documentation/block/inline-encryption.rst.

Signed-off-by: Eric Biggers <ebiggers@google.com>
Tested-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org> # sm8650
Link: https://lore.kernel.org/r/20250204060041.409950-2-ebiggers@kernel.org
Signed-off-by: Jens Axboe <axboe@kernel.dk>

1 files changed, 46 insertions, 0 deletions

diff --git a/block/blk-crypto-profile.c b/block/blk-crypto-profile.c
index 7fabc883e39f..a990d9026c83 100644
--- a/block/blk-crypto-profile.c
+++ b/block/blk-crypto-profile.c
@@ -352,6 +352,8 @@ bool __blk_crypto_cfg_supported(struct blk_crypto_profile *profile,
 		return false;
 	if (profile->max_dun_bytes_supported < cfg->dun_bytes)
 		return false;
+	if (!(profile->key_types_supported & cfg->key_type))
+		return false;
 	return true;
 }
 
@@ -463,6 +465,44 @@ bool blk_crypto_register(struct blk_crypto_profile *profile,
 EXPORT_SYMBOL_GPL(blk_crypto_register);
 
 /**
+ * blk_crypto_derive_sw_secret() - Derive software secret from wrapped key
+ * @bdev: a block device that supports hardware-wrapped keys
+ * @eph_key: a hardware-wrapped key in ephemerally-wrapped form
+ * @eph_key_size: size of @eph_key in bytes
+ * @sw_secret: (output) the software secret
+ *
+ * Given a hardware-wrapped key in ephemerally-wrapped form (the same form that
+ * it is used for I/O), ask the hardware to derive the secret which software can
+ * use for cryptographic tasks other than inline encryption.  This secret is
+ * guaranteed to be cryptographically isolated from the inline encryption key,
+ * i.e. derived with a different KDF context.
+ *
+ * Return: 0 on success, -EOPNOTSUPP if the block device doesn't support
+ *	   hardware-wrapped keys, -EBADMSG if the key isn't a valid
+ *	   ephemerally-wrapped key, or another -errno code.
+ */
+int blk_crypto_derive_sw_secret(struct block_device *bdev,
+				const u8 *eph_key, size_t eph_key_size,
+				u8 sw_secret[BLK_CRYPTO_SW_SECRET_SIZE])
+{
+	struct blk_crypto_profile *profile =
+		bdev_get_queue(bdev)->crypto_profile;
+	int err;
+
+	if (!profile)
+		return -EOPNOTSUPP;
+	if (!(profile->key_types_supported & BLK_CRYPTO_KEY_TYPE_HW_WRAPPED))
+		return -EOPNOTSUPP;
+	if (!profile->ll_ops.derive_sw_secret)
+		return -EOPNOTSUPP;
+	blk_crypto_hw_enter(profile);
+	err = profile->ll_ops.derive_sw_secret(profile, eph_key, eph_key_size,
+					       sw_secret);
+	blk_crypto_hw_exit(profile);
+	return err;
+}
+
+/**
  * blk_crypto_intersect_capabilities() - restrict supported crypto capabilities
  *					 by child device
  * @parent: the crypto profile for the parent device
@@ -485,10 +525,12 @@ void blk_crypto_intersect_capabilities(struct blk_crypto_profile *parent,
 			    child->max_dun_bytes_supported);
 		for (i = 0; i < ARRAY_SIZE(child->modes_supported); i++)
 			parent->modes_supported[i] &= child->modes_supported[i];
+		parent->key_types_supported &= child->key_types_supported;
 	} else {
 		parent->max_dun_bytes_supported = 0;
 		memset(parent->modes_supported, 0,
 		       sizeof(parent->modes_supported));
+		parent->key_types_supported = 0;
 	}
 }
 EXPORT_SYMBOL_GPL(blk_crypto_intersect_capabilities);
@@ -521,6 +563,9 @@ bool blk_crypto_has_capabilities(const struct blk_crypto_profile *target,
 	    target->max_dun_bytes_supported)
 		return false;
 
+	if (reference->key_types_supported & ~target->key_types_supported)
+		return false;
+
 	return true;
 }
 EXPORT_SYMBOL_GPL(blk_crypto_has_capabilities);
@@ -555,5 +600,6 @@ void blk_crypto_update_capabilities(struct blk_crypto_profile *dst,
 	       sizeof(dst->modes_supported));
 
 	dst->max_dun_bytes_supported = src->max_dun_bytes_supported;
+	dst->key_types_supported = src->key_types_supported;
 }
 EXPORT_SYMBOL_GPL(blk_crypto_update_capabilities);