Re: [PATCH v4] fscrypt: Add support for AES-128-CBC

From: David Gstir <david@sigma-star.at>
To: Eric Biggers <ebiggers3@gmail.com>
Cc: Theodore Ts'o <tytso@mit.edu>,
	Michael Halcrow <mhalcrow@google.com>,
	Jaegeuk Kim <jaegeuk@kernel.org>,
	Richard Weinberger <richard@sigma-star.at>,
	herbert@gondor.apana.org.au, linux-fsdevel@vger.kernel.org,
	linux-kernel@vger.kernel.org, linux-fscrypt@vger.kernel.org,
	Daniel Walter <dwalter@sigma-star.at>
Subject: Re: [PATCH v4] fscrypt: Add support for AES-128-CBC
Date: Fri, 16 Jun 2017 09:39:07 +0200	[thread overview]
Message-ID: <5565BD72-986F-4338-8562-6B5F9FB72110@sigma-star.at> (raw)
In-Reply-To: <20170615204841.GA66403@gmail.com>

> On 15 Jun 2017, at 22:48, Eric Biggers <ebiggers3@gmail.com> 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