Re: [PATCH v4 00/19] LSM: Module stacking for SARA and Landlock

From: Tetsuo Handa
Date: Mon Sep 24 2018 - 13:53:40 EST

On 2018/09/25 2:16, Casey Schaufler wrote:
>> Not all of LKM-based LSMs use security blobs. And some of LKM-based LSMs
>> might use security blobs for only a few objects. For example, AKARI uses
>> inode security blob for remembering whether source address/port of an
>> accept()ed socket was already checked, only during accept() operation and
>> first socket operation on the accept()ed socket. Thus, there is no need
>> to waste memory by assigning blobs for all inode objects.
> The first question is why use an inode blob? Shouldn't you
> be using a socket blob for this socket based information?

Indeed. AKARI can as well use security_sk_free() using address of
"struct sock" as a key.

> If you only want information part of the time you can declare
> a pointer sized blob and manage what hangs off that as you will.
> I personally think that the added complexity of conditional
> blob management is more pain than it's worth, but if you want
> a really big blob, but only on occasion, I could see doing it.

LKM based LSMs are too late for updating blob_sizes.* fields.
Even if they could, they after all have to somehow check whether
corresponding init hook was called. That's checking for NULL.

>>>> @@ -1202,11 +1183,11 @@ void security_file_free(struct file *file)
>>>> {
>>>> void *blob;
>>>> + call_void_hook(file_free_security, file);
>>>> +
>>>> if (!lsm_file_cache)
>>>> return;
>>>> - call_void_hook(file_free_security, file);
>>>> -
>>> Why does this make sense? If the lsm_file_cache isn't
>>> initialized you can't have allocated any file blobs,
>>> no module can have initialized a file blob, hence there
>>> can be nothing for the module to do.
>> For modules (not limited to LKM-based LSMs) which want to use
>> file blobs for only a few objects and avoid wasting memory by
>> allocating file blobs to all file objects.
>> Infrastructure based blob management fits well for LSM modules
>> which want to assign blobs to all objects (like SELinux). But
>> forcing infrastructure based blob management can become a huge
>> waste of memory for LSM modules which want to assign blobs to
>> only a few objects. Unconditionally calling file_free_security
>> hook (as with other hooks) preserves a room for allowing the
>> latter type of LSM modules without using infrastructure based
>> blob management.
> There is a hypothetical issue here, but that would require abuse
> of the infrastructure. Having a file_free_security hook that doesn't
> free a security blob allocated by file_alloc_security may coincidentaly
> be useful, but that's not the intent of the hook.

The free hook might be used for freeing resources which were not allocated
by alloc hook. Yama is using task_free hook without task_alloc hook.
Someone might want to use file_free hook without file_alloc hook.