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SubjectRe: [PATCH v2] fscrypt: Add support for AES-128-CBC
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Hi Eric!

Thanks for the feedback!

> On 25 Apr 2017, at 22:10, Eric Biggers <ebiggers3@gmail.com> wrote:
>
> Hi Daniel and David,
>
> On Tue, Apr 25, 2017 at 04:41:00PM +0200, David Gstir wrote:
>> @@ -147,17 +148,28 @@ int fscrypt_do_page_crypto(const struct inode *inode, fscrypt_direction_t rw,
>> {
>> struct {
>> __le64 index;
>> - u8 padding[FS_XTS_TWEAK_SIZE - sizeof(__le64)];
>> - } xts_tweak;
>> + u8 padding[FS_IV_SIZE - sizeof(__le64)];
>> + } blk_desc;
>> struct skcipher_request *req = NULL;
>> DECLARE_FS_COMPLETION_RESULT(ecr);
>> struct scatterlist dst, src;
>> struct fscrypt_info *ci = inode->i_crypt_info;
>> struct crypto_skcipher *tfm = ci->ci_ctfm;
>> int res = 0;
>> + u8 *iv = (u8 *)&blk_desc;
>>
>> BUG_ON(len == 0);
>>
>> + BUILD_BUG_ON(sizeof(blk_desc) != FS_IV_SIZE);
>> + BUILD_BUG_ON(AES_BLOCK_SIZE != FS_IV_SIZE);
>> + blk_desc.index = cpu_to_le64(lblk_num);
>> + memset(blk_desc.padding, 0, sizeof(blk_desc.padding));
>> +
>> + if (ci->ci_essiv_tfm != NULL) {
>> + memset(iv, 0, sizeof(blk_desc));
>> + crypto_cipher_encrypt_one(ci->ci_essiv_tfm, iv, iv);
>> + }
>> +
>> req = skcipher_request_alloc(tfm, gfp_flags);
>> if (!req) {
>> printk_ratelimited(KERN_ERR
>> @@ -170,15 +182,11 @@ int fscrypt_do_page_crypto(const struct inode *inode, fscrypt_direction_t rw,
>> req, CRYPTO_TFM_REQ_MAY_BACKLOG | CRYPTO_TFM_REQ_MAY_SLEEP,
>> page_crypt_complete, &ecr);
>>
>> - BUILD_BUG_ON(sizeof(xts_tweak) != FS_XTS_TWEAK_SIZE);
>> - xts_tweak.index = cpu_to_le64(lblk_num);
>> - memset(xts_tweak.padding, 0, sizeof(xts_tweak.padding));
>> -
>> sg_init_table(&dst, 1);
>> sg_set_page(&dst, dest_page, len, offs);
>> sg_init_table(&src, 1);
>> sg_set_page(&src, src_page, len, offs);
>> - skcipher_request_set_crypt(req, &src, &dst, len, &xts_tweak);
>> + skcipher_request_set_crypt(req, &src, &dst, len, &iv);
>> if (rw == FS_DECRYPT)
>> res = crypto_skcipher_decrypt(req);
>> else
>
> There are two critical bugs here. First the IV is being zeroed before being
> encrypted with the ESSIV tfm, so the resulting IV will always be the same for a
> given file. It should be encrypting the block number instead. Second what's
> actually being passed into the crypto API is '&iv' where IV is a pointer to
> something on the stack... I really doubt that does what's intended :)
>
> Probably the cleanest way to do this correctly is to just have the struct be the
> 'iv', so there's no extra pointer to deal with:
>
> struct {
> __le64 index;
> u8 padding[FS_IV_SIZE - sizeof(__le64)];
> } iv;
>
> [...]
>
> BUILD_BUG_ON(sizeof(iv) != FS_IV_SIZE);
> BUILD_BUG_ON(AES_BLOCK_SIZE != FS_IV_SIZE);
> iv.index = cpu_to_le64(lblk_num);
> memset(iv.padding, 0, sizeof(iv.padding));
>
> if (ci->ci_essiv_tfm != NULL) {
> crypto_cipher_encrypt_one(ci->ci_essiv_tfm, (u8 *)&iv,
> (u8 *)&iv);
> }
>
> [...]
>
> skcipher_request_set_crypt(req, &src, &dst, len, &iv);

You are totally right. Somehow I completely missed that. :-/



>
>> +static int derive_essiv_salt(u8 *raw_key, int keysize, u8 **salt_out,
>> + unsigned int *saltsize)
>> +{
>> + int res;
>> + struct crypto_ahash *hash_tfm;
>> + unsigned int digestsize;
>> + u8 *salt = NULL;
>> + struct scatterlist sg;
>> + struct ahash_request *req = NULL;
>> +
>> + hash_tfm = crypto_alloc_ahash("sha256", 0, 0);
>> + if (IS_ERR(hash_tfm))
>> + return PTR_ERR(hash_tfm);
>> +
>> + digestsize = crypto_ahash_digestsize(hash_tfm);
>> + salt = kzalloc(digestsize, GFP_NOFS);
>> + if (!salt) {
>> + res = -ENOMEM;
>> + goto out;
>> + }
>> +
>> + req = ahash_request_alloc(hash_tfm, GFP_NOFS);
>> + if (!req) {
>> + kfree(salt);
>> + res = -ENOMEM;
>> + goto out;
>> + }
>> +
>> + sg_init_one(&sg, raw_key, keysize);
>> + ahash_request_set_callback(req,
>> + CRYPTO_TFM_REQ_MAY_BACKLOG | CRYPTO_TFM_REQ_MAY_SLEEP,
>> + NULL, NULL);
>> + ahash_request_set_crypt(req, &sg, salt, keysize);
>> +
>> + res = crypto_ahash_digest(req);
>> + if (res) {
>> + kfree(salt);
>> + goto out;
>> + }
>> +
>> + *salt_out = salt;
>> + *saltsize = digestsize;
>> +
>> +out:
>> + crypto_free_ahash(hash_tfm);
>> + ahash_request_free(req);
>> + return res;
>> +}
>> +
>> +static int init_essiv_generator(struct fscrypt_info *ci, u8 *raw_key,
>> + int keysize)
>> +{
>> + int res;
>> + struct crypto_cipher *essiv_tfm;
>> + u8 *salt = NULL;
>> + unsigned int saltsize;
>> +
>> + /* init ESSIV generator */
>> + essiv_tfm = crypto_alloc_cipher("aes", 0, 0);
>> + if (!essiv_tfm || IS_ERR(essiv_tfm)) {
>> + res = essiv_tfm ? PTR_ERR(essiv_tfm) : -ENOMEM;
>> + goto err;
>> + }
>> +
>> + res = derive_essiv_salt(raw_key, keysize, &salt, &saltsize);
>> + if (res)
>> + goto err;
>> +
>> + res = crypto_cipher_setkey(essiv_tfm, salt, saltsize);
>> + if (res)
>> + goto err;
>> +
>> + ci->ci_essiv_tfm = essiv_tfm;
>> +
>> + return 0;
>> +
>> +err:
>> + if (essiv_tfm && !IS_ERR(essiv_tfm))
>> + crypto_free_cipher(essiv_tfm);
>> +
>> + kzfree(salt);
>> + return res;
>> +}
>
> There are a few issues with how the ESSIV generator is initialized:
>
> 1.) It's allocating a possibly asynchronous SHA-256 implementation but then not
> handling it actually being asynchronous. I suggest using the 'shash' API
> which is easier to use.
> 2.) No one is going to change the digest size of SHA-256; it's fixed at 32
> bytes. So just #include <crypto/sha.h> and allocate a 'u8
> salt[SHA256_DIGEST_SIZE];' on the stack. Make sure to do
> memzero_explicit(salt, sizeof(salt)) in all cases.
> 3.) It's always keying the ESSIV transform using a 256-bit AES key. It's still
> secure of course, but I'm not sure it's what you intended, given that it's
> used in combination with AES-128. I really think that someone who's more of
> an expert on ESSIV really should weigh in, but my intuition is that you
> really only need to be using AES-128, using the first 'keysize' bytes of the
> hash.

My intention is to use all 256 bits we get from the hash. Yes, this will then use
AES-256 for the IV generation, but this will still yield just a 128 bit IV for
file contents encryption since the block size of AES is the same. So this is
just a case of using AES with a 256 bit key over a 128 bit one which is then
used for AES-128 computations.

On the other hand, as you pointed out, truncating the hash and using AES-128 *should*
suffice too. Especially since we are using AES-128 everywhere else!

I'm also no export on ESSIV, so I'm not a 100% sure if there is something we're
missing here. If using AES-128 is okay, I'll change it to truncate the hash.


> You also don't really need to handle freeing the essiv_tfm on errors, as long as
> you assign it to the fscrypt_info first. Also crypto_alloc_cipher() returns an
> ERR_PTR(), never NULL.
>
> Also, fs/crypto/Kconfig needs a 'select CRYPTO_SHA256' now.
>
> Here's a revised version to consider, not actually tested though:
>
> static int derive_essiv_salt(const u8 *raw_key, int keysize,
> u8 salt[SHA256_DIGEST_SIZE])
> {
> struct crypto_shash *hash_tfm;
>
> hash_tfm = crypto_alloc_shash("sha256", 0, 0);
> if (IS_ERR(hash_tfm)) {
> pr_warn_ratelimited("fscrypt: error allocating SHA-256 transform: %ld",
> PTR_ERR(hash_tfm));
> return PTR_ERR(hash_tfm);
> } else {
> SHASH_DESC_ON_STACK(desc, hash_tfm);
> int err;
>
> desc->tfm = hash_tfm;
> desc->flags = 0;
>
> BUG_ON(crypto_shash_digestsize(hash_tfm) != SHA256_DIGEST_SIZE);
>
> err = crypto_shash_digest(desc, raw_key, keysize, salt);
> crypto_free_shash(hash_tfm);
> return err;
> }
> }
>
> static int init_essiv_generator(struct fscrypt_info *ci,
> const u8 *raw_key, int keysize)
> {
> struct crypto_cipher *essiv_tfm;
> u8 salt[SHA256_DIGEST_SIZE];
> int err;
>
> if (WARN_ON_ONCE(keysize > sizeof(salt)))
> return -EINVAL;
>
> essiv_tfm = crypto_alloc_cipher("aes", 0, 0);
> if (IS_ERR(essiv_tfm))
> return PTR_ERR(essiv_tfm);
>
> ci->ci_essiv_tfm = essiv_tfm;
>
> err = derive_essiv_salt(raw_key, keysize, salt);
> if (err)
> goto out;
>
> err = crypto_cipher_setkey(essiv_tfm, salt, keysize);
> out:
> memzero_explicit(salt, sizeof(salt));
> return err;
> }

Thanks a lot for that! Using shash and SHA256_DIGEST_SIZE makes this much cleaner.
I'll redo that for v3.

One optimization Richard pointed out is that we should do the
crypto_alloc_shash("sha256", 0, 0) just once and reuse hash_tfm for all sha256 computations.
This will save us some alloc/frees in derive_essiv_salt.

Thanks!
David

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