fscrypt updates for 5.16

Some cleanups for fs/crypto/:
 
 - Allow 256-bit master keys with AES-256-XTS
 
 - Improve documentation and comments
 
 - Remove unneeded field fscrypt_operations::max_namelen
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Merge tag 'fscrypt-for-linus' of git://git.kernel.org/pub/scm/fs/fscrypt/fscrypt

Pull fscrypt updates from Eric Biggers:
 "Some cleanups for fs/crypto/:

   - Allow 256-bit master keys with AES-256-XTS

   - Improve documentation and comments

   - Remove unneeded field fscrypt_operations::max_namelen"

* tag 'fscrypt-for-linus' of git://git.kernel.org/pub/scm/fs/fscrypt/fscrypt:
  fscrypt: improve a few comments
  fscrypt: allow 256-bit master keys with AES-256-XTS
  fscrypt: improve documentation for inline encryption
  fscrypt: clean up comments in bio.c
  fscrypt: remove fscrypt_operations::max_namelen

Documentation/block/inline-encryption.rst

@@ -1,5 +1,7 @@
 .. SPDX-License-Identifier: GPL-2.0
 
+.. _inline_encryption:
+
 =================
 Inline Encryption
 =================

Documentation/filesystems/fscrypt.rst

@@ -77,11 +77,11 @@ Side-channel attacks
 
 fscrypt is only resistant to side-channel attacks, such as timing or
 electromagnetic attacks, to the extent that the underlying Linux
-Cryptographic API algorithms are.  If a vulnerable algorithm is used,
+Cryptographic API algorithms or inline encryption hardware are.  If a
-such as a table-based implementation of AES, it may be possible for an
+vulnerable algorithm is used, such as a table-based implementation of
-attacker to mount a side channel attack against the online system.
+AES, it may be possible for an attacker to mount a side channel attack
-Side channel attacks may also be mounted against applications
+against the online system.  Side channel attacks may also be mounted
-consuming decrypted data.
+against applications consuming decrypted data.
 
 Unauthorized file access
 ~~~~~~~~~~~~~~~~~~~~~~~~
@@ -176,11 +176,11 @@ Master Keys
 
 Each encrypted directory tree is protected by a *master key*.  Master
 keys can be up to 64 bytes long, and must be at least as long as the
-greater of the key length needed by the contents and filenames
+greater of the security strength of the contents and filenames
-encryption modes being used.  For example, if AES-256-XTS is used for
+encryption modes being used.  For example, if any AES-256 mode is
-contents encryption, the master key must be 64 bytes (512 bits).  Note
+used, the master key must be at least 256 bits, i.e. 32 bytes.  A
-that the XTS mode is defined to require a key twice as long as that
+stricter requirement applies if the key is used by a v1 encryption
-required by the underlying block cipher.
+policy and AES-256-XTS is used; such keys must be 64 bytes.
 
 To "unlock" an encrypted directory tree, userspace must provide the
 appropriate master key.  There can be any number of master keys, each
@@ -1135,6 +1135,50 @@ where applications may later write sensitive data.  It is recommended
 that systems implementing a form of "verified boot" take advantage of
 this by validating all top-level encryption policies prior to access.
 
+Inline encryption support
+=========================
+
+By default, fscrypt uses the kernel crypto API for all cryptographic
+operations (other than HKDF, which fscrypt partially implements
+itself).  The kernel crypto API supports hardware crypto accelerators,
+but only ones that work in the traditional way where all inputs and
+outputs (e.g. plaintexts and ciphertexts) are in memory.  fscrypt can
+take advantage of such hardware, but the traditional acceleration
+model isn't particularly efficient and fscrypt hasn't been optimized
+for it.
+
+Instead, many newer systems (especially mobile SoCs) have *inline
+encryption hardware* that can encrypt/decrypt data while it is on its
+way to/from the storage device.  Linux supports inline encryption
+through a set of extensions to the block layer called *blk-crypto*.
+blk-crypto allows filesystems to attach encryption contexts to bios
+(I/O requests) to specify how the data will be encrypted or decrypted
+in-line.  For more information about blk-crypto, see
+:ref:`Documentation/block/inline-encryption.rst <inline_encryption>`.
+
+On supported filesystems (currently ext4 and f2fs), fscrypt can use
+blk-crypto instead of the kernel crypto API to encrypt/decrypt file
+contents.  To enable this, set CONFIG_FS_ENCRYPTION_INLINE_CRYPT=y in
+the kernel configuration, and specify the "inlinecrypt" mount option
+when mounting the filesystem.
+
+Note that the "inlinecrypt" mount option just specifies to use inline
+encryption when possible; it doesn't force its use.  fscrypt will
+still fall back to using the kernel crypto API on files where the
+inline encryption hardware doesn't have the needed crypto capabilities
+(e.g. support for the needed encryption algorithm and data unit size)
+and where blk-crypto-fallback is unusable.  (For blk-crypto-fallback
+to be usable, it must be enabled in the kernel configuration with
+CONFIG_BLK_INLINE_ENCRYPTION_FALLBACK=y.)
+
+Currently fscrypt always uses the filesystem block size (which is
+usually 4096 bytes) as the data unit size.  Therefore, it can only use
+inline encryption hardware that supports that data unit size.
+
+Inline encryption doesn't affect the ciphertext or other aspects of
+the on-disk format, so users may freely switch back and forth between
+using "inlinecrypt" and not using "inlinecrypt".
+
 Implementation details
 ======================
 
@@ -1184,6 +1228,13 @@ keys`_ and `DIRECT_KEY policies`_.
 Data path changes
 -----------------
 
+When inline encryption is used, filesystems just need to associate
+encryption contexts with bios to specify how the block layer or the
+inline encryption hardware will encrypt/decrypt the file contents.
+
+When inline encryption isn't used, filesystems must encrypt/decrypt
+the file contents themselves, as described below:
+
 For the read path (->readpage()) of regular files, filesystems can
 read the ciphertext into the page cache and decrypt it in-place.  The
 page lock must be held until decryption has finished, to prevent the
@@ -1197,18 +1248,6 @@ buffer.  Some filesystems, such as UBIFS, already use temporary
 buffers regardless of encryption.  Other filesystems, such as ext4 and
 F2FS, have to allocate bounce pages specially for encryption.
 
-Fscrypt is also able to use inline encryption hardware instead of the
-kernel crypto API for en/decryption of file contents.  When possible,
-and if directed to do so (by specifying the 'inlinecrypt' mount option
-for an ext4/F2FS filesystem), it adds encryption contexts to bios and
-uses blk-crypto to perform the en/decryption instead of making use of
-the above read/write path changes.  Of course, even if directed to
-make use of inline encryption, fscrypt will only be able to do so if
-either hardware inline encryption support is available for the
-selected encryption algorithm or CONFIG_BLK_INLINE_ENCRYPTION_FALLBACK
-is selected.  If neither is the case, fscrypt will fall back to using
-the above mentioned read/write path changes for en/decryption.
-
 Filename hashing and encoding
 -----------------------------
 

fs/crypto/bio.c

@@ -1,23 +1,10 @@
 // SPDX-License-Identifier: GPL-2.0
 /*
- * This contains encryption functions for per-file encryption.
+ * Utility functions for file contents encryption/decryption on
+ * block device-based filesystems.
  *
  * Copyright (C) 2015, Google, Inc.
  * Copyright (C) 2015, Motorola Mobility
- *
- * Written by Michael Halcrow, 2014.
- *
- * Filename encryption additions
- *	Uday Savagaonkar, 2014
- * Encryption policy handling additions
- *	Ildar Muslukhov, 2014
- * Add fscrypt_pullback_bio_page()
- *	Jaegeuk Kim, 2015.
- *
- * This has not yet undergone a rigorous security audit.
- *
- * The usage of AES-XTS should conform to recommendations in NIST
- * Special Publication 800-38E and IEEE P1619/D16.
  */
 
 #include <linux/pagemap.h>
@@ -26,6 +13,21 @@
 #include <linux/namei.h>
 #include "fscrypt_private.h"
 
+/**
+ * fscrypt_decrypt_bio() - decrypt the contents of a bio
+ * @bio: the bio to decrypt
+ *
+ * Decrypt the contents of a "read" bio following successful completion of the
+ * underlying disk read.  The bio must be reading a whole number of blocks of an
+ * encrypted file directly into the page cache.  If the bio is reading the
+ * ciphertext into bounce pages instead of the page cache (for example, because
+ * the file is also compressed, so decompression is required after decryption),
+ * then this function isn't applicable.  This function may sleep, so it must be
+ * called from a workqueue rather than from the bio's bi_end_io callback.
+ *
+ * This function sets PG_error on any pages that contain any blocks that failed
+ * to be decrypted.  The filesystem must not mark such pages uptodate.
+ */
 void fscrypt_decrypt_bio(struct bio *bio)
 {
 	struct bio_vec *bv;

fs/crypto/fname.c

@@ -429,8 +429,7 @@ int fscrypt_setup_filename(struct inode *dir, const struct qstr *iname,
 
 	if (fscrypt_has_encryption_key(dir)) {
 		if (!fscrypt_fname_encrypted_size(&dir->i_crypt_info->ci_policy,
-						  iname->len,
+						  iname->len, NAME_MAX,
-						  dir->i_sb->s_cop->max_namelen,
 						  &fname->crypto_buf.len))
 			return -ENAMETOOLONG;
 		fname->crypto_buf.name = kmalloc(fname->crypto_buf.len,

fs/crypto/fscrypt_private.h

@@ -20,6 +20,11 @@
 
 #define FSCRYPT_FILE_NONCE_SIZE	16
 
+/*
+ * Minimum size of an fscrypt master key.  Note: a longer key will be required
+ * if ciphers with a 256-bit security strength are used.  This is just the
+ * absolute minimum, which applies when only 128-bit encryption is used.
+ */
 #define FSCRYPT_MIN_KEY_SIZE	16
 
 #define FSCRYPT_CONTEXT_V1	1
@@ -413,7 +418,11 @@ struct fscrypt_master_key_secret {
 	 */
 	struct fscrypt_hkdf	hkdf;
 
-	/* Size of the raw key in bytes.  Set even if ->raw isn't set. */
+	/*
+	 * Size of the raw key in bytes.  This remains set even if ->raw was
+	 * zeroized due to no longer being needed.  I.e. we still remember the
+	 * size of the key even if we don't need to remember the key itself.
+	 */
 	u32			size;
 
 	/* For v1 policy keys: the raw key.  Wiped for v2 policy keys. */
@@ -549,8 +558,9 @@ int __init fscrypt_init_keyring(void);
 struct fscrypt_mode {
 	const char *friendly_name;
 	const char *cipher_str;
-	int keysize;
+	int keysize;		/* key size in bytes */
-	int ivsize;
+	int security_strength;	/* security strength in bytes */
+	int ivsize;		/* IV size in bytes */
 	int logged_impl_name;
 	enum blk_crypto_mode_num blk_crypto_mode;
 };

fs/crypto/hkdf.c

@@ -16,9 +16,14 @@
 
 /*
  * HKDF supports any unkeyed cryptographic hash algorithm, but fscrypt uses
- * SHA-512 because it is reasonably secure and efficient; and since it produces
+ * SHA-512 because it is well-established, secure, and reasonably efficient.
- * a 64-byte digest, deriving an AES-256-XTS key preserves all 64 bytes of
+ *
- * entropy from the master key and requires only one iteration of HKDF-Expand.
+ * HKDF-SHA256 was also considered, as its 256-bit security strength would be
+ * sufficient here.  A 512-bit security strength is "nice to have", though.
+ * Also, on 64-bit CPUs, SHA-512 is usually just as fast as SHA-256.  In the
+ * common case of deriving an AES-256-XTS key (512 bits), that can result in
+ * HKDF-SHA512 being much faster than HKDF-SHA256, as the longer digest size of
+ * SHA-512 causes HKDF-Expand to only need to do one iteration rather than two.
  */
 #define HKDF_HMAC_ALG		"hmac(sha512)"
 #define HKDF_HASHLEN		SHA512_DIGEST_SIZE

fs/crypto/keysetup.c

@@ -19,6 +19,7 @@ struct fscrypt_mode fscrypt_modes[] = {
 		.friendly_name = "AES-256-XTS",
 		.cipher_str = "xts(aes)",
 		.keysize = 64,
+		.security_strength = 32,
 		.ivsize = 16,
 		.blk_crypto_mode = BLK_ENCRYPTION_MODE_AES_256_XTS,
 	},
@@ -26,12 +27,14 @@ struct fscrypt_mode fscrypt_modes[] = {
 		.friendly_name = "AES-256-CTS-CBC",
 		.cipher_str = "cts(cbc(aes))",
 		.keysize = 32,
+		.security_strength = 32,
 		.ivsize = 16,
 	},
 	[FSCRYPT_MODE_AES_128_CBC] = {
 		.friendly_name = "AES-128-CBC-ESSIV",
 		.cipher_str = "essiv(cbc(aes),sha256)",
 		.keysize = 16,
+		.security_strength = 16,
 		.ivsize = 16,
 		.blk_crypto_mode = BLK_ENCRYPTION_MODE_AES_128_CBC_ESSIV,
 	},
@@ -39,12 +42,14 @@ struct fscrypt_mode fscrypt_modes[] = {
 		.friendly_name = "AES-128-CTS-CBC",
 		.cipher_str = "cts(cbc(aes))",
 		.keysize = 16,
+		.security_strength = 16,
 		.ivsize = 16,
 	},
 	[FSCRYPT_MODE_ADIANTUM] = {
 		.friendly_name = "Adiantum",
 		.cipher_str = "adiantum(xchacha12,aes)",
 		.keysize = 32,
+		.security_strength = 32,
 		.ivsize = 32,
 		.blk_crypto_mode = BLK_ENCRYPTION_MODE_ADIANTUM,
 	},
@@ -117,8 +122,9 @@ err_free_tfm:
 
 /*
  * Prepare the crypto transform object or blk-crypto key in @prep_key, given the
- * raw key, encryption mode, and flag indicating which encryption implementation
+ * raw key, encryption mode (@ci->ci_mode), flag indicating which encryption
- * (fs-layer or blk-crypto) will be used.
+ * implementation (fs-layer or blk-crypto) will be used (@ci->ci_inlinecrypt),
+ * and IV generation method (@ci->ci_policy.flags).
  */
 int fscrypt_prepare_key(struct fscrypt_prepared_key *prep_key,
 			const u8 *raw_key, const struct fscrypt_info *ci)
@@ -357,6 +363,45 @@ static int fscrypt_setup_v2_file_key(struct fscrypt_info *ci,
 	return 0;
 }
 
+/*
+ * Check whether the size of the given master key (@mk) is appropriate for the
+ * encryption settings which a particular file will use (@ci).
+ *
+ * If the file uses a v1 encryption policy, then the master key must be at least
+ * as long as the derived key, as this is a requirement of the v1 KDF.
+ *
+ * Otherwise, the KDF can accept any size key, so we enforce a slightly looser
+ * requirement: we require that the size of the master key be at least the
+ * maximum security strength of any algorithm whose key will be derived from it
+ * (but in practice we only need to consider @ci->ci_mode, since any other
+ * possible subkeys such as DIRHASH and INODE_HASH will never increase the
+ * required key size over @ci->ci_mode).  This allows AES-256-XTS keys to be
+ * derived from a 256-bit master key, which is cryptographically sufficient,
+ * rather than requiring a 512-bit master key which is unnecessarily long.  (We
+ * still allow 512-bit master keys if the user chooses to use them, though.)
+ */
+static bool fscrypt_valid_master_key_size(const struct fscrypt_master_key *mk,
+					  const struct fscrypt_info *ci)
+{
+	unsigned int min_keysize;
+
+	if (ci->ci_policy.version == FSCRYPT_POLICY_V1)
+		min_keysize = ci->ci_mode->keysize;
+	else
+		min_keysize = ci->ci_mode->security_strength;
+
+	if (mk->mk_secret.size < min_keysize) {
+		fscrypt_warn(NULL,
+			     "key with %s %*phN is too short (got %u bytes, need %u+ bytes)",
+			     master_key_spec_type(&mk->mk_spec),
+			     master_key_spec_len(&mk->mk_spec),
+			     (u8 *)&mk->mk_spec.u,
+			     mk->mk_secret.size, min_keysize);
+		return false;
+	}
+	return true;
+}
+
 /*
  * Find the master key, then set up the inode's actual encryption key.
  *
@@ -422,18 +467,7 @@ static int setup_file_encryption_key(struct fscrypt_info *ci,
 		goto out_release_key;
 	}
 
-	/*
+	if (!fscrypt_valid_master_key_size(mk, ci)) {
-	 * Require that the master key be at least as long as the derived key.
-	 * Otherwise, the derived key cannot possibly contain as much entropy as
-	 * that required by the encryption mode it will be used for.  For v1
-	 * policies it's also required for the KDF to work at all.
-	 */
-	if (mk->mk_secret.size < ci->ci_mode->keysize) {
-		fscrypt_warn(NULL,
-			     "key with %s %*phN is too short (got %u bytes, need %u+ bytes)",
-			     master_key_spec_type(&mk_spec),
-			     master_key_spec_len(&mk_spec), (u8 *)&mk_spec.u,
-			     mk->mk_secret.size, ci->ci_mode->keysize);
 		err = -ENOKEY;
 		goto out_release_key;
 	}

fs/ext4/super.c

@@ -1572,7 +1572,6 @@ static const struct fscrypt_operations ext4_cryptops = {
 	.set_context		= ext4_set_context,
 	.get_dummy_policy	= ext4_get_dummy_policy,
 	.empty_dir		= ext4_empty_dir,
-	.max_namelen		= EXT4_NAME_LEN,
 	.has_stable_inodes	= ext4_has_stable_inodes,
 	.get_ino_and_lblk_bits	= ext4_get_ino_and_lblk_bits,
 };

fs/f2fs/super.c

@@ -2976,7 +2976,6 @@ static const struct fscrypt_operations f2fs_cryptops = {
 	.set_context		= f2fs_set_context,
 	.get_dummy_policy	= f2fs_get_dummy_policy,
 	.empty_dir		= f2fs_empty_dir,
-	.max_namelen		= F2FS_NAME_LEN,
 	.has_stable_inodes	= f2fs_has_stable_inodes,
 	.get_ino_and_lblk_bits	= f2fs_get_ino_and_lblk_bits,
 	.get_num_devices	= f2fs_get_num_devices,

fs/ubifs/crypto.c

@@ -82,5 +82,4 @@ const struct fscrypt_operations ubifs_crypt_operations = {
 	.get_context		= ubifs_crypt_get_context,
 	.set_context		= ubifs_crypt_set_context,
 	.empty_dir		= ubifs_crypt_empty_dir,
-	.max_namelen		= UBIFS_MAX_NLEN,
 };

include/linux/fscrypt.h

@@ -118,9 +118,6 @@ struct fscrypt_operations {
 	 */
 	bool (*empty_dir)(struct inode *inode);
 
-	/* The filesystem's maximum ciphertext filename length, in bytes */
-	unsigned int max_namelen;
-
 	/*
 	 * Check whether the filesystem's inode numbers and UUID are stable,
 	 * meaning that they will never be changed even by offline operations