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

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

On 2/25/2020 9:15 PM, KP Singh wrote:
> On 25-Feb 16:30, Casey Schaufler wrote:
>> On 2/24/2020 9:41 PM, Alexei Starovoitov wrote:
>>> On Mon, Feb 24, 2020 at 01:41:19PM -0800, Kees Cook wrote:
>>>> But the LSM subsystem doesn't want special cases (Casey has worked very
>>>> hard to generalize everything there for stacking). It is really hard to
>>>> accept adding a new special case when there are still special cases yet
>>>> to be worked out even in the LSM code itself[2].
>>>> [2] Casey's work to generalize the LSM interfaces continues and it quite
>>>> complex:
>>>> https://lore.kernel.org/linux-security-module/20200214234203.7086-1-casey@schaufler-ca.com/
>>> I think the key mistake we made is that we classified KRSI as LSM.
>>> LSM stacking, lsmblobs that the above set is trying to do are not necessary for KRSI.
>>> I don't see anything in LSM infra that KRSI can reuse.
>>> The only thing BPF needs is a function to attach to.
>>> It can be a nop function or any other.
>>> security_*() functions are interesting from that angle only.
>>> Hence I propose to reconsider what I was suggesting earlier.
>>> No changes to secruity/ directory.
>>> Attach to security_*() funcs via bpf trampoline.
>>> The key observation vs what I was saying earlier is KRSI and LSM are wrong names.
>>> I think "security" is also loaded word that should be avoided.
>> No argument there.
>>
>>> I'm proposing to rename BPF_PROG_TYPE_LSM into BPF_PROG_TYPE_OVERRIDE_RETURN.
>>>
>>>> So, unless James is going to take this over Casey's objections, the path
>>>> forward I see here is:
>>>>
>>>> - land a "slow" KRSI (i.e. one that hooks every hook with a stub).
>>>> - optimize calling for all LSMs
>>> I'm very much surprised how 'slow' KRSI is an option at all.
>>> 'slow' KRSI means that CONFIG_SECURITY_KRSI=y adds indirect calls to nop
>>> functions for every place in the kernel that calls security_*().
>>> This is not an acceptable overhead. Even w/o retpoline
>>> this is not something datacenter servers can use.
>> In the universe I live in data centers will disable hyper-threading,
>> reducing performance substantially, in the face of hypothetical security
>> exploits. That's a massively greater performance impact than the handful
>> of instructions required to do indirect calls. Not to mention the impact
> Indirect calls have worse performance implications than just a few
> instructions and are especially not suitable for hotpaths.
>
> There have been multiple efforts to reduce their usage e.g.:
>
>   - https://lwn.net/Articles/774743/
>   - https://lwn.net/Articles/773985/
>
>> of the BPF programs that have been included. Have you ever looked at what
>   BPF programs are JIT'ed and optimized to native code.

Doesn't mean people won't write slow code.


>> happens to system performance when polkitd is enabled?
> However, let's discuss all this separately when we follow-up with
> performance improvements after submitting the initial patch-set.

Think performance up front. Don't ignore issues.

>>> Another option is to do this:
>>> diff --git a/include/linux/security.h b/include/linux/security.h
>>> index 64b19f050343..7887ce636fb1 100644
>>> --- a/include/linux/security.h
>>> +++ b/include/linux/security.h
>>> @@ -240,7 +240,7 @@ static inline const char *kernel_load_data_id_str(enum kernel_load_data_id id)
>>>         return kernel_load_data_str[id];
>>>  }
>>>
>>> -#ifdef CONFIG_SECURITY
>>> +#if defined(CONFIG_SECURITY) || defined(CONFIG_BPF_OVERRIDE_RETURN)
>>>
>>> Single line change to security.h and new file kernel/bpf/override_security.c
>>> that will look like:
>>> int security_binder_set_context_mgr(struct task_struct *mgr)
>>> {
>>>         return 0;
>>> }
>>>
>>> int security_binder_transaction(struct task_struct *from,
>>>                                 struct task_struct *to)
>>> {
>>>         return 0;
>>> }
>>> Essentially it will provide BPF side with a set of nop functions.
>>> CONFIG_SECURITY is off. It may seem as a downside that it will force a choice
>>> on kernel users. Either they build the kernel with CONFIG_SECURITY and their
>>> choice of LSMs or build the kernel with CONFIG_BPF_OVERRIDE_RETURN and use
>>> BPF_PROG_TYPE_OVERRIDE_RETURN programs to enforce any kind of policy. I think
>>> it's a pro not a con.
>> Err, no. All distros use an LSM or two. Unless you can re-implement SELinux
> The users mentioned here in this context are (I would assume) the more
> performance sensitive users who would, potentially, disable
> CONFIG_SECURITY because of the current performance characteristics.

You assume that the most performance sensitive people would allow
a mechanism to arbitrarily add overhead that is out of their control?
How does that make sense?

> We can also discuss this separately and only if we find that we need
> it for the BPF_OVERRIDE_RET type attachment.
>
> - KP
>
>> in BPF (good luck with state transitions) you've built a warp drive without
>> ever having mined dilithium crystals.
>>
>>