eCryptfs: Remove buggy and unnecessary write in file name decode routine

Dmitry Chernenkov used KASAN to discover that eCryptfs writes past the
end of the allocated buffer during encrypted filename decoding. This
fix corrects the issue by getting rid of the unnecessary 0 write when
the current bit offset is 2.

Signed-off-by: Michael Halcrow <mhalcrow@google.com>
Reported-by: Dmitry Chernenkov <dmitryc@google.com>
Suggested-by: Kees Cook <keescook@chromium.org>
Cc: stable@vger.kernel.org # v2.6.29+: 51ca58d eCryptfs: Filename Encryption: Encoding and encryption functions
Signed-off-by: Tyler Hicks <tyhicks@canonical.com>
int ecryptfs_get_tfm_and_mutex_for_cipher_name(struct crypto_blkcipher **tfm,
					       struct mutex **tfm_mutex,
					       char *cipher_name)
{
	struct ecryptfs_key_tfm *key_tfm;
	int rc = 0;

	(*tfm) = NULL;
	(*tfm_mutex) = NULL;

	mutex_lock(&key_tfm_list_mutex);
	if (!ecryptfs_tfm_exists(cipher_name, &key_tfm)) {
		rc = ecryptfs_add_new_key_tfm(&key_tfm, cipher_name, 0);
		if (rc) {
			printk(KERN_ERR "Error adding new key_tfm to list; "
					"rc = [%d]\n", rc);
			goto out;
		}
	}
	(*tfm) = key_tfm->key_tfm;
	(*tfm_mutex) = &key_tfm->key_tfm_mutex;
out:
	mutex_unlock(&key_tfm_list_mutex);
	return rc;
}

int ecryptfs_get_tfm_and_mutex_for_cipher_name(struct crypto_blkcipher **tfm,
					       struct mutex **tfm_mutex,
					       char *cipher_name)
{
	struct ecryptfs_key_tfm *key_tfm;
	int rc = 0;

	(*tfm) = NULL;
	(*tfm_mutex) = NULL;

	mutex_lock(&key_tfm_list_mutex);
	if (!ecryptfs_tfm_exists(cipher_name, &key_tfm)) {
		rc = ecryptfs_add_new_key_tfm(&key_tfm, cipher_name, 0);
		if (rc) {
			printk(KERN_ERR "Error adding new key_tfm to list; "
					"rc = [%d]\n", rc);
			goto out;
		}
	}
	(*tfm) = key_tfm->key_tfm;
	(*tfm_mutex) = &key_tfm->key_tfm_mutex;
out:
	mutex_unlock(&key_tfm_list_mutex);
	return rc;
}
