> On 15 Jun 2017, at 22:48, Eric Biggers <ebiggers3@xxxxxxxxx> wrote:
>
> On Thu, Jun 15, 2017 at 01:41:29PM -0700, Michael Halcrow wrote:
>>> static int validate_user_key(struct fscrypt_info *crypt_info,
>>> struct fscrypt_context *ctx, u8 *raw_key,
>>> - const char *prefix)
>>> + const char *prefix, int min_keysize)
>>> {
>>> char *description;
>>> struct key *keyring_key;
>>> @@ -111,50 +116,60 @@ static int validate_user_key(struct fscrypt_info *crypt_info,
>>> master_key = (struct fscrypt_key *)ukp->data;
>>> BUILD_BUG_ON(FS_AES_128_ECB_KEY_SIZE != FS_KEY_DERIVATION_NONCE_SIZE);
>>>
>>> - if (master_key->size != FS_AES_256_XTS_KEY_SIZE) {
>>> + if (master_key->size < min_keysize || master_key->size > FS_MAX_KEY_SIZE
>>> + || master_key->size % AES_BLOCK_SIZE != 0) {
>>
>> I suggest validating the provided key size directly against the mode.
>> Else, it looks to me that this code will accept a 128-bit key for
>> AES-256.
>>
>
> It's doing that already; min_keysize depends on the mode.
We are a bit more forgiving than the code was before: In case AES-128-CBC is
selected, we accept a longer key and use the first 128 bits of the derived key.
(see fscrypt_get_encryption_info())
The alternative is to make this check as strict as it was and just check for
master_key->size != min_keysize.
IMO the current check is okay. I will however add a comment that documents this.
We could also add a pr_warn_once(), but I don't think this is really necessary.
David
