On Thu, 20 Dec 2007 23:18:49 -0600 Eric Sandeen <[EMAIL PROTECTED]> wrote:
> Jeff Moyer pointed out that a mount; umount loop of ecryptfs,
> with the same cipher & other mount options, created a new
> ecryptfs_key_tfm_cache item each time, and the cache could
> grow quite large this way.
>
> Looking at this with mhalcrow, we saw that ecryptfs_parse_options()
> unconditionally called ecryptfs_add_new_key_tfm(), which is what
> was adding these items.
>
> Refactor ecryptfs_get_tfm_and_mutex_for_cipher_name() to create a
> new helper function, ecryptfs_tfm_exists(), which checks for the
> cipher on the cached key_tfm_list, and sets a pointer
> to it if it exists. This can then be called from
> ecryptfs_parse_options(), and new key_tfm's can be added only when
> a cached one is not found.
>
This change looks fishy.
> +/**
> + * ecryptfs_tfm_exists - Search for existing tfm for cipher_name.
> + * @cipher_name: the name of the cipher to search for
> + * @key_tfm: set to corresponding tfm if found
> + *
> + * Returns 1 if found, with key_tfm set
> + * Returns 0 if not found, key_tfm set to NULL
> + */
> +int ecryptfs_tfm_exists(char *cipher_name, struct ecryptfs_key_tfm **key_tfm)
> +{
> + struct ecryptfs_key_tfm *tmp_key_tfm;
> +
> + mutex_lock(&key_tfm_list_mutex);
> + list_for_each_entry(tmp_key_tfm, &key_tfm_list, key_tfm_list) {
> + if (strcmp(tmp_key_tfm->cipher_name, cipher_name) == 0) {
> + mutex_unlock(&key_tfm_list_mutex);
> + if (key_tfm)
> + (*key_tfm) = tmp_key_tfm;
Here we return a pointer to an object without holding the lock and without
taking a refcount on it. What prevents it from getting moved/freed/etc
while this thread of control is playing with it?
> + return 1;
> + }
> + }
> + mutex_unlock(&key_tfm_list_mutex);
> + if (key_tfm)
> + (*key_tfm) = NULL;
> + return 0;
> +}
> +
> int ecryptfs_get_tfm_and_mutex_for_cipher_name(struct crypto_blkcipher **tfm,
> struct mutex **tfm_mutex,
> char *cipher_name)
> @@ -1877,22 +1904,15 @@ int ecryptfs_get_tfm_and_mutex_for_ciphe
>
> (*tfm) = NULL;
> (*tfm_mutex) = NULL;
> - mutex_lock(&key_tfm_list_mutex);
> - list_for_each_entry(key_tfm, &key_tfm_list, key_tfm_list) {
> - if (strcmp(key_tfm->cipher_name, cipher_name) == 0) {
> - (*tfm) = key_tfm->key_tfm;
> - (*tfm_mutex) = &key_tfm->key_tfm_mutex;
> - mutex_unlock(&key_tfm_list_mutex);
> +
> + if (!ecryptfs_tfm_exists(cipher_name, &key_tfm)) {
And given that we've just unlocked key_tfm_list_mutex, how do we know that
the return value from ecryptfs_tfm_exists() is still true in this window?
> + 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;
> }
> }
> - mutex_unlock(&key_tfm_list_mutex);
It would all look a lot more solid if this locking was retained and both
ecryptfs_tfm_exists() and ecryptfs_add_new_key_tfm() were designed to be
called under key_tfm_list_mutex.
> @@ -410,9 +410,11 @@ static int ecryptfs_parse_options(struct
> if (!cipher_key_bytes_set) {
> mount_crypt_stat->global_default_cipher_key_size = 0;
> }
> - rc = ecryptfs_add_new_key_tfm(
> - NULL, mount_crypt_stat->global_default_cipher_name,
> - mount_crypt_stat->global_default_cipher_key_size);
> + if (!ecryptfs_tfm_exists(mount_crypt_stat->global_default_cipher_name,
> + NULL))
> + rc = ecryptfs_add_new_key_tfm(
> + NULL, mount_crypt_stat->global_default_cipher_name,
> + mount_crypt_stat->global_default_cipher_key_size);
dittoes.
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