fscrypt: derive dirhash key for casefolded directories

drosen-google · ebiggers · commit aa408f835d02 · 2020-01-22T14:49:55.000-08:00
When we allow indexed directories to use both encryption and casefolding, for the dirhash we can't just hash the ciphertext filenames that are stored on-disk (as is done currently) because the dirhash must be case insensitive, but the stored names are case-preserving. Nor can we hash the plaintext names with an unkeyed hash (or a hash keyed with a value stored on-disk like ext4's s_hash_seed), since that would leak information about the names that encryption is meant to protect. Instead, if we can accept a dirhash that's only computable when the fscrypt key is available, we can hash the plaintext names with a keyed hash using a secret key derived from the directory's fscrypt master key. We'll use SipHash-2-4 for this purpose. Prepare for this by deriving a SipHash key for each casefolded encrypted directory. Make sure to handle deriving the key not only when setting up the directory's fscrypt_info, but also in the case where the casefold flag is enabled after the fscrypt_info was already set up. (We could just always derive the key regardless of casefolding, but that would introduce unnecessary overhead for people not using casefolding.) Signed-off-by: Daniel Rosenberg <drosen@google.com> [EB: improved commit message, updated fscrypt.rst, squashed with change that avoids unnecessarily deriving the key, and many other cleanups] Link: https://lore.kernel.org/r/20200120223201.241390-3-ebiggers@kernel.org Signed-off-by: Eric Biggers <ebiggers@google.com>
diff --git a/Documentation/filesystems/fscrypt.rst b/Documentation/filesystems/fscrypt.rst
@@ -302,6 +302,16 @@ For master keys used for v2 encryption policies, a unique 16-byte "key
 identifier" is also derived using the KDF.  This value is stored in
 the clear, since it is needed to reliably identify the key itself.
 
+Dirhash keys
+------------
+
+For directories that are indexed using a secret-keyed dirhash over the
+plaintext filenames, the KDF is also used to derive a 128-bit
+SipHash-2-4 key per directory in order to hash filenames.  This works
+just like deriving a per-file encryption key, except that a different
+KDF context is used.  Currently, only casefolded ("case-insensitive")
+encrypted directories use this style of hashing.
+
 Encryption modes and usage
 ==========================
 
diff --git a/fs/crypto/fname.c b/fs/crypto/fname.c
@@ -402,6 +402,27 @@ int fscrypt_setup_filename(struct inode *dir, const struct qstr *iname,
 }
 EXPORT_SYMBOL(fscrypt_setup_filename);
 
+/**
+ * fscrypt_fname_siphash() - calculate the SipHash of a filename
+ * @dir: the parent directory
+ * @name: the filename to calculate the SipHash of
+ *
+ * Given a plaintext filename @name and a directory @dir which uses SipHash as
+ * its dirhash method and has had its fscrypt key set up, this function
+ * calculates the SipHash of that name using the directory's secret dirhash key.
+ *
+ * Return: the SipHash of @name using the hash key of @dir
+ */
+u64 fscrypt_fname_siphash(const struct inode *dir, const struct qstr *name)
+{
+	const struct fscrypt_info *ci = dir->i_crypt_info;
+
+	WARN_ON(!ci->ci_dirhash_key_initialized);
+
+	return siphash(name->name, name->len, &ci->ci_dirhash_key);
+}
+EXPORT_SYMBOL_GPL(fscrypt_fname_siphash);
+
 /*
  * Validate dentries in encrypted directories to make sure we aren't potentially
  * caching stale dentries after a key has been added.
diff --git a/fs/crypto/fscrypt_private.h b/fs/crypto/fscrypt_private.h
@@ -12,6 +12,7 @@
 #define _FSCRYPT_PRIVATE_H
 
 #include <linux/fscrypt.h>
+#include <linux/siphash.h>
 #include <crypto/hash.h>
 
 #define CONST_STRLEN(str)	(sizeof(str) - 1)
@@ -188,6 +189,14 @@ struct fscrypt_info {
 	 */
 	struct fscrypt_direct_key *ci_direct_key;
 
+	/*
+	 * This inode's hash key for filenames.  This is a 128-bit SipHash-2-4
+	 * key.  This is only set for directories that use a keyed dirhash over
+	 * the plaintext filenames -- currently just casefolded directories.
+	 */
+	siphash_key_t ci_dirhash_key;
+	bool ci_dirhash_key_initialized;
+
 	/* The encryption policy used by this inode */
 	union fscrypt_policy ci_policy;
 
@@ -263,6 +272,7 @@ extern int fscrypt_init_hkdf(struct fscrypt_hkdf *hkdf, const u8 *master_key,
 #define HKDF_CONTEXT_PER_FILE_KEY	2
 #define HKDF_CONTEXT_DIRECT_KEY		3
 #define HKDF_CONTEXT_IV_INO_LBLK_64_KEY	4
+#define HKDF_CONTEXT_DIRHASH_KEY	5
 
 extern int fscrypt_hkdf_expand(const struct fscrypt_hkdf *hkdf, u8 context,
 			       const u8 *info, unsigned int infolen,
@@ -434,6 +444,9 @@ fscrypt_allocate_skcipher(struct fscrypt_mode *mode, const u8 *raw_key,
 extern int fscrypt_set_derived_key(struct fscrypt_info *ci,
 				   const u8 *derived_key);
 
+extern int fscrypt_derive_dirhash_key(struct fscrypt_info *ci,
+				      const struct fscrypt_master_key *mk);
+
 /* keysetup_v1.c */
 
 extern void fscrypt_put_direct_key(struct fscrypt_direct_key *dk);
diff --git a/fs/crypto/hooks.c b/fs/crypto/hooks.c
@@ -5,6 +5,8 @@
  * Encryption hooks for higher-level filesystem operations.
  */
 
+#include <linux/key.h>
+
 #include "fscrypt_private.h"
 
 /**
@@ -137,15 +139,29 @@ int fscrypt_prepare_setflags(struct inode *inode,
 			     unsigned int oldflags, unsigned int flags)
 {
 	struct fscrypt_info *ci;
+	struct fscrypt_master_key *mk;
 	int err;
 
+	/*
+	 * When the CASEFOLD flag is set on an encrypted directory, we must
+	 * derive the secret key needed for the dirhash.  This is only possible
+	 * if the directory uses a v2 encryption policy.
+	 */
 	if (IS_ENCRYPTED(inode) && (flags & ~oldflags & FS_CASEFOLD_FL)) {
 		err = fscrypt_require_key(inode);
 		if (err)
 			return err;
 		ci = inode->i_crypt_info;
 		if (ci->ci_policy.version != FSCRYPT_POLICY_V2)
 			return -EINVAL;
+		mk = ci->ci_master_key->payload.data[0];
+		down_read(&mk->mk_secret_sem);
+		if (is_master_key_secret_present(&mk->mk_secret))
+			err = fscrypt_derive_dirhash_key(ci, mk);
+		else
+			err = -ENOKEY;
+		up_read(&mk->mk_secret_sem);
+		return err;
 	}
 	return 0;
 }
diff --git a/fs/crypto/keysetup.c b/fs/crypto/keysetup.c
@@ -174,10 +174,24 @@ static int setup_per_mode_key(struct fscrypt_info *ci,
 	return 0;
 }
 
+int fscrypt_derive_dirhash_key(struct fscrypt_info *ci,
+			       const struct fscrypt_master_key *mk)
+{
+	int err;
+
+	err = fscrypt_hkdf_expand(&mk->mk_secret.hkdf, HKDF_CONTEXT_DIRHASH_KEY,
+				  ci->ci_nonce, FS_KEY_DERIVATION_NONCE_SIZE,
+				  (u8 *)&ci->ci_dirhash_key,
+				  sizeof(ci->ci_dirhash_key));
+	if (err)
+		return err;
+	ci->ci_dirhash_key_initialized = true;
+	return 0;
+}
+
 static int fscrypt_setup_v2_file_key(struct fscrypt_info *ci,
 				     struct fscrypt_master_key *mk)
 {
-	u8 derived_key[FSCRYPT_MAX_KEY_SIZE];
 	int err;
 
 	if (ci->ci_policy.v2.flags & FSCRYPT_POLICY_FLAG_DIRECT_KEY) {
@@ -189,8 +203,8 @@ static int fscrypt_setup_v2_file_key(struct fscrypt_info *ci,
 		 * This ensures that the master key is consistently used only
 		 * for HKDF, avoiding key reuse issues.
 		 */
-		return setup_per_mode_key(ci, mk, mk->mk_direct_tfms,
-					  HKDF_CONTEXT_DIRECT_KEY, false);
+		err = setup_per_mode_key(ci, mk, mk->mk_direct_tfms,
+					 HKDF_CONTEXT_DIRECT_KEY, false);
 	} else if (ci->ci_policy.v2.flags &
 		   FSCRYPT_POLICY_FLAG_IV_INO_LBLK_64) {
 		/*
@@ -199,21 +213,33 @@ static int fscrypt_setup_v2_file_key(struct fscrypt_info *ci,
 		 * the IVs.  This format is optimized for use with inline
 		 * encryption hardware compliant with the UFS or eMMC standards.
 		 */
-		return setup_per_mode_key(ci, mk, mk->mk_iv_ino_lblk_64_tfms,
-					  HKDF_CONTEXT_IV_INO_LBLK_64_KEY,
-					  true);
+		err = setup_per_mode_key(ci, mk, mk->mk_iv_ino_lblk_64_tfms,
+					 HKDF_CONTEXT_IV_INO_LBLK_64_KEY, true);
+	} else {
+		u8 derived_key[FSCRYPT_MAX_KEY_SIZE];
+
+		err = fscrypt_hkdf_expand(&mk->mk_secret.hkdf,
+					  HKDF_CONTEXT_PER_FILE_KEY,
+					  ci->ci_nonce,
+					  FS_KEY_DERIVATION_NONCE_SIZE,
+					  derived_key, ci->ci_mode->keysize);
+		if (err)
+			return err;
+
+		err = fscrypt_set_derived_key(ci, derived_key);
+		memzero_explicit(derived_key, ci->ci_mode->keysize);
 	}
-
-	err = fscrypt_hkdf_expand(&mk->mk_secret.hkdf,
-				  HKDF_CONTEXT_PER_FILE_KEY,
-				  ci->ci_nonce, FS_KEY_DERIVATION_NONCE_SIZE,
-				  derived_key, ci->ci_mode->keysize);
 	if (err)
 		return err;
 
-	err = fscrypt_set_derived_key(ci, derived_key);
-	memzero_explicit(derived_key, ci->ci_mode->keysize);
-	return err;
+	/* Derive a secret dirhash key for directories that need it. */
+	if (S_ISDIR(ci->ci_inode->i_mode) && IS_CASEFOLDED(ci->ci_inode)) {
+		err = fscrypt_derive_dirhash_key(ci, mk);
+		if (err)
+			return err;
+	}
+
+	return 0;
 }
 
 /*
diff --git a/include/linux/fscrypt.h b/include/linux/fscrypt.h
@@ -247,6 +247,9 @@ static inline bool fscrypt_match_name(const struct fscrypt_name *fname,
 	return !memcmp(de_name, fname->disk_name.name, fname->disk_name.len);
 }
 
+extern u64 fscrypt_fname_siphash(const struct inode *dir,
+				 const struct qstr *name);
+
 /* bio.c */
 extern void fscrypt_decrypt_bio(struct bio *);
 extern int fscrypt_zeroout_range(const struct inode *, pgoff_t, sector_t,
@@ -479,6 +482,13 @@ static inline bool fscrypt_match_name(const struct fscrypt_name *fname,
 	return !memcmp(de_name, fname->disk_name.name, fname->disk_name.len);
 }
 
+static inline u64 fscrypt_fname_siphash(const struct inode *dir,
+					const struct qstr *name)
+{
+	WARN_ON_ONCE(1);
+	return 0;
+}
+
 /* bio.c */
 static inline void fscrypt_decrypt_bio(struct bio *bio)
 {
