ext4 crypto: add padding to filenames before encrypting

This obscures the length of the filenames, to decrease the amount of
information leakage.  By default, we pad the filenames to the next 4
byte boundaries.  This costs nothing, since the directory entries are
aligned to 4 byte boundaries anyway.  Filenames can also be padded to
8, 16, or 32 bytes, which will consume more directory space.

Change-Id: Ibb7a0fb76d2c48e2061240a709358ff40b14f322
Signed-off-by: Theodore Ts'o <tytso@mit.edu>

fs/ext4/crypto_fname.c

@@ -66,13 +66,15 @@ static int ext4_fname_encrypt(struct ext4_fname_crypto_ctx *ctx,
 	int res = 0;
 	char iv[EXT4_CRYPTO_BLOCK_SIZE];
 	struct scatterlist sg[1];
+	int padding = 4 << (ctx->flags & EXT4_POLICY_FLAGS_PAD_MASK);
 	char *workbuf;
 
 	if (iname->len <= 0 || iname->len > ctx->lim)
 		return -EIO;
 
 	ciphertext_len = (iname->len < EXT4_CRYPTO_BLOCK_SIZE) ?
 		EXT4_CRYPTO_BLOCK_SIZE : iname->len;
+	ciphertext_len = ext4_fname_crypto_round_up(ciphertext_len, padding);
 	ciphertext_len = (ciphertext_len > ctx->lim)
 			? ctx->lim : ciphertext_len;
 
@@ -101,7 +103,7 @@ static int ext4_fname_encrypt(struct ext4_fname_crypto_ctx *ctx,
 	/* Create encryption request */
 	sg_init_table(sg, 1);
 	sg_set_page(sg, ctx->workpage, PAGE_SIZE, 0);
-	ablkcipher_request_set_crypt(req, sg, sg, iname->len, iv);
+	ablkcipher_request_set_crypt(req, sg, sg, ciphertext_len, iv);
 	res = crypto_ablkcipher_encrypt(req);
 	if (res == -EINPROGRESS || res == -EBUSY) {
 		BUG_ON(req->base.data != &ecr);
@@ -356,6 +358,7 @@ struct ext4_fname_crypto_ctx *ext4_get_fname_crypto_ctx(
 	if (IS_ERR(ctx))
 		return ctx;
 
+	ctx->flags = ei->i_crypt_policy_flags;
 	if (ctx->has_valid_key) {
 		if (ctx->key.mode != EXT4_ENCRYPTION_MODE_AES_256_CTS) {
 			printk_once(KERN_WARNING
@@ -468,13 +471,15 @@ int ext4_fname_crypto_namelen_on_disk(struct ext4_fname_crypto_ctx *ctx,
 				      u32 namelen)
 {
 	u32 ciphertext_len;
+	int padding = 4 << (ctx->flags & EXT4_POLICY_FLAGS_PAD_MASK);
 
 	if (ctx == NULL)
 		return -EIO;
 	if (!(ctx->has_valid_key))
 		return -EACCES;
 	ciphertext_len = (namelen < EXT4_CRYPTO_BLOCK_SIZE) ?
 		EXT4_CRYPTO_BLOCK_SIZE : namelen;
+	ciphertext_len = ext4_fname_crypto_round_up(ciphertext_len, padding);
 	ciphertext_len = (ciphertext_len > ctx->lim)
 			? ctx->lim : ciphertext_len;
 	return (int) ciphertext_len;
@@ -490,10 +495,13 @@ int ext4_fname_crypto_alloc_buffer(struct ext4_fname_crypto_ctx *ctx,
 				   u32 ilen, struct ext4_str *crypto_str)
 {
 	unsigned int olen;
+	int padding = 4 << (ctx->flags & EXT4_POLICY_FLAGS_PAD_MASK);
 
 	if (!ctx)
 		return -EIO;
-	olen = ext4_fname_crypto_round_up(ilen, EXT4_CRYPTO_BLOCK_SIZE);
+	if (padding < EXT4_CRYPTO_BLOCK_SIZE)
+		padding = EXT4_CRYPTO_BLOCK_SIZE;
+	olen = ext4_fname_crypto_round_up(ilen, padding);
 	crypto_str->len = olen;
 	if (olen < EXT4_FNAME_CRYPTO_DIGEST_SIZE*2)
 		olen = EXT4_FNAME_CRYPTO_DIGEST_SIZE*2;

fs/ext4/crypto_key.c

@@ -110,6 +110,7 @@ int ext4_generate_encryption_key(struct inode *inode)
 	}
 	res = 0;
 
+	ei->i_crypt_policy_flags = ctx.flags;
 	if (S_ISREG(inode->i_mode))
 		crypt_key->mode = ctx.contents_encryption_mode;
 	else if (S_ISDIR(inode->i_mode) || S_ISLNK(inode->i_mode))

fs/ext4/crypto_policy.c

@@ -37,6 +37,8 @@ static int ext4_is_encryption_context_consistent_with_policy(
 		return 0;
 	return (memcmp(ctx.master_key_descriptor, policy->master_key_descriptor,
 			EXT4_KEY_DESCRIPTOR_SIZE) == 0 &&
+		(ctx.flags ==
+		 policy->flags) &&
 		(ctx.contents_encryption_mode ==
 		 policy->contents_encryption_mode) &&
 		(ctx.filenames_encryption_mode ==
@@ -56,25 +58,25 @@ static int ext4_create_encryption_context_from_policy(
 		printk(KERN_WARNING
 		       "%s: Invalid contents encryption mode %d\n", __func__,
 			policy->contents_encryption_mode);
-		res = -EINVAL;
-		goto out;
+		return -EINVAL;
 	}
 	if (!ext4_valid_filenames_enc_mode(policy->filenames_encryption_mode)) {
 		printk(KERN_WARNING
 		       "%s: Invalid filenames encryption mode %d\n", __func__,
 			policy->filenames_encryption_mode);
-		res = -EINVAL;
-		goto out;
+		return -EINVAL;
 	}
+	if (policy->flags & ~EXT4_POLICY_FLAGS_VALID)
+		return -EINVAL;
 	ctx.contents_encryption_mode = policy->contents_encryption_mode;
 	ctx.filenames_encryption_mode = policy->filenames_encryption_mode;
+	ctx.flags = policy->flags;
 	BUILD_BUG_ON(sizeof(ctx.nonce) != EXT4_KEY_DERIVATION_NONCE_SIZE);
 	get_random_bytes(ctx.nonce, EXT4_KEY_DERIVATION_NONCE_SIZE);
 
 	res = ext4_xattr_set(inode, EXT4_XATTR_INDEX_ENCRYPTION,
 			     EXT4_XATTR_NAME_ENCRYPTION_CONTEXT, &ctx,
 			     sizeof(ctx), 0);
-out:
 	if (!res)
 		ext4_set_inode_flag(inode, EXT4_INODE_ENCRYPT);
 	return res;
@@ -115,6 +117,7 @@ int ext4_get_policy(struct inode *inode, struct ext4_encryption_policy *policy)
 	policy->version = 0;
 	policy->contents_encryption_mode = ctx.contents_encryption_mode;
 	policy->filenames_encryption_mode = ctx.filenames_encryption_mode;
+	policy->flags = ctx.flags;
 	memcpy(&policy->master_key_descriptor, ctx.master_key_descriptor,
 	       EXT4_KEY_DESCRIPTOR_SIZE);
 	return 0;
@@ -176,6 +179,7 @@ int ext4_inherit_context(struct inode *parent, struct inode *child)
 				EXT4_ENCRYPTION_MODE_AES_256_XTS;
 			ctx.filenames_encryption_mode =
 				EXT4_ENCRYPTION_MODE_AES_256_CTS;
+			ctx.flags = 0;
 			memset(ctx.master_key_descriptor, 0x42,
 			       EXT4_KEY_DESCRIPTOR_SIZE);
 			res = 0;

fs/ext4/ext4.h

@@ -911,6 +911,7 @@ struct ext4_inode_info {
 
 	/* on-disk additional length */
 	__u16 i_extra_isize;
+	char i_crypt_policy_flags;
 
 	/* Indicate the inline data space. */
 	u16 i_inline_off;

fs/ext4/ext4_crypto.h

@@ -20,12 +20,20 @@ struct ext4_encryption_policy {
 	char version;
 	char contents_encryption_mode;
 	char filenames_encryption_mode;
+	char flags;
 	char master_key_descriptor[EXT4_KEY_DESCRIPTOR_SIZE];
 } __attribute__((__packed__));
 
 #define EXT4_ENCRYPTION_CONTEXT_FORMAT_V1 1
 #define EXT4_KEY_DERIVATION_NONCE_SIZE 16
 
+#define EXT4_POLICY_FLAGS_PAD_4		0x00
+#define EXT4_POLICY_FLAGS_PAD_8		0x01
+#define EXT4_POLICY_FLAGS_PAD_16	0x02
+#define EXT4_POLICY_FLAGS_PAD_32	0x03
+#define EXT4_POLICY_FLAGS_PAD_MASK	0x03
+#define EXT4_POLICY_FLAGS_VALID		0x03
+
 /**
  * Encryption context for inode
  *
@@ -41,7 +49,7 @@ struct ext4_encryption_context {
 	char format;
 	char contents_encryption_mode;
 	char filenames_encryption_mode;
-	char reserved;
+	char flags;
 	char master_key_descriptor[EXT4_KEY_DESCRIPTOR_SIZE];
 	char nonce[EXT4_KEY_DERIVATION_NONCE_SIZE];
 } __attribute__((__packed__));
@@ -120,6 +128,7 @@ struct ext4_fname_crypto_ctx {
 	struct crypto_hash *htfm;
 	struct page *workpage;
 	struct ext4_encryption_key key;
+	unsigned flags : 8;
 	unsigned has_valid_key : 1;
 	unsigned ctfm_key_is_ready : 1;
 };