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ext4 crypto: add padding to filenames before encrypting

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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>
This commit is contained in:
Theodore Ts'o
2015-05-01 16:56:50 -04:00
parent 5de0b4d0cd
commit a44cd7a054
5 changed files with 31 additions and 8 deletions
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12
fs/ext4/crypto_fname.c
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@@ -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;