Re: [PATCH bpf-next v4 3/8] bpf: lsm: provide attachment points for BPF LSM programs

[Casey Schaufler <casey@schaufler-ca.com>]

On 2/21/2020 3:44 AM, KP Singh wrote:
> On 20-Feb 15:49, Casey Schaufler wrote:
>> On 2/20/2020 9:52 AM, KP Singh wrote:
>>> From: KP Singh <kpsingh@google.com>
>> Sorry about the heavy list pruning - the original set
>> blows thunderbird up.
>>
>>> The BPF LSM programs are implemented as fexit trampolines to avoid the
>>> overhead of retpolines. These programs cannot be attached to security_*
>>> wrappers as there are quite a few security_* functions that do more than
>>> just calling the LSM callbacks.
>>>
>>> This was discussed on the lists in:
>>>
>>>   https://lore.kernel.org/bpf/20200123152440.28956-1-kpsingh@chromium.org/T/#m068becce588a0cdf01913f368a97aea4c62d8266
>>>
>>> Adding a NOP callback after all the static LSM callbacks are called has
>>> the following benefits:
>>>
>>> - The BPF programs run at the right stage of the security_* wrappers.
>>> - They run after all the static LSM hooks allowed the operation,
>>>   therefore cannot allow an action that was already denied.
>> I still say that the special call-out to BPF is unnecessary.
>> I remain unconvinced by the arguments. You aren't doing anything
>> so special that the general mechanism won't work.
> The existing mechanism would work functionally, but the cost of an
> indirect call for all the hooks, even those that are completely unused
> is not really acceptable for KRSI’s use cases.

Are you at all familiar with the way LSMs where installed
before the current list infrastructure? Every interface had
a hook that got called, even if the installed module did not
provide one. That was deemed acceptable for a good long time.

Way back in the early days of the stacking effort I seriously
considered implementing a new security module that would do
the stacking, and leave the infrastructure alone. Very much
like what you're proposing for BPF modules. It would have worked,
but the list model works better.

>  It’s easy to avoid and
> I do think that what we’re doing here (with hooks being defined at
> runtime) has significant functional differences from existing LSMs.

KRSI isn't all that different from the other modules.
The way you specify where system policy is restricted
and under which circumstances is different. You're trying
to be extremely general, beyond the Mandatory Access Control
claims of the existing modules. But really, there's nothing
all that special.

I know that you don't want to be making a lot of checks on
empty BPF program lists. You've come up with clever hacks
to avoid doing so. But the cleverer the hack, the more likely
it is to haunt someone else later. It probably won't cause
KRSI any grief, but you can bet someone will take it in the
chin.