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>

diff --git a/fs/ext4/crypto_fname.c b/fs/ext4/crypto_fname.c
index 7a877e6..fded02f 100644 (file)
--- a/fs/ext4/crypto_fname.c
+++ b/fs/ext4/crypto_fname.c
@@ -66,6 +66,7 @@ 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)
@@ -73,6 +74,7 @@ static int ext4_fname_encrypt(struct ext4_fname_crypto_ctx *ctx,
 
        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
@@ -468,6 +471,7 @@ 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;
@@ -475,6 +479,7 @@ int ext4_fname_crypto_namelen_on_disk(struct ext4_fname_crypto_ctx *ctx,
                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;

diff --git a/fs/ext4/crypto_key.c b/fs/ext4/crypto_key.c
index c8392af..52170d0 100644 (file)
--- a/fs/ext4/crypto_key.c
+++ b/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))

diff --git a/fs/ext4/crypto_policy.c b/fs/ext4/crypto_policy.c
index 30eaf9e..a6d6291 100644 (file)
--- a/fs/ext4/crypto_policy.c
+++ b/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;

diff --git a/fs/ext4/ext4.h b/fs/ext4/ext4.h
index dfb1138..bca1bdc 100644 (file)
--- a/fs/ext4/ext4.h
+++ b/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;

diff --git a/fs/ext4/ext4_crypto.h b/fs/ext4/ext4_crypto.h
index c2ba35a..d75159c 100644 (file)
--- a/fs/ext4/ext4_crypto.h
+++ b/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;
 };