Re: [RFC PATCH v3 3/4] X86/sgx: Introduce EMA as a new LSM module

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

On 7/9/2019 3:13 PM, Xing, Cedric wrote:
> On 7/8/2019 4:53 PM, Casey Schaufler wrote:
>> On 7/8/2019 10:16 AM, Xing, Cedric wrote:
>>> On 7/8/2019 9:26 AM, Casey Schaufler wrote:
>>>> In this scheme you use an ema LSM to manage your ema data.
>>>> A quick sketch looks like:
>>>>
>>>>      sgx_something_in() calls
>>>>          security_enclave_load() calls
>>>>              ema_enclave_load()
>>>>              selinux_enclave_load()
>>>>              otherlsm_enclave_load()
>>>>
>>>> Why is this better than:
>>>>
>>>>      sgx_something_in() calls
>>>>          ema_enclave_load()
>>>>          security_enclave_load() calls
>>>>              selinux_enclave_load()
>>>>              otherlsm_enclave_load()
>>>
>>> Are you talking about moving EMA somewhere outside LSM?
>>
>> Yes. That's what I've been saying all along.
>>
>>> If so, where?
>>
>> I tried to make it obvious. Put the call to your EMA code
>> on the line before you call security_enclave_load().
>
> Sorry but I'm still confused.
>
> EMA code is used by LSMs only. Making it callable from other parts of the kernel IMHO is probably not a good idea. And more importantly I don't understand the motivation behind it. Would you please elaborate?

LSM modules implement additional access control restrictions.
The EMA code does not do that, it provides management of data
that is used by security modules. It is not one itself. VFS
also performs this role, but no one would consider making VFS
a security module.

>>>>> +/**
>>>>> + * ema - Enclave Memory Area structure for LSM modules
>>>>
>>>> LSM modules is redundant. "LSM" or "LSMs" would be better.
>>>
>>> Noted
>>>
>>>>> diff --git a/security/Makefile b/security/Makefile
>>>>> index c598b904938f..b66d03a94853 100644
>>>>> --- a/security/Makefile
>>>>> +++ b/security/Makefile
>>>>> @@ -28,6 +28,7 @@ obj-$(CONFIG_SECURITY_YAMA)        += yama/
>>>>>    obj-$(CONFIG_SECURITY_LOADPIN)        += loadpin/
>>>>>    obj-$(CONFIG_SECURITY_SAFESETID)       += safesetid/
>>>>>    obj-$(CONFIG_CGROUP_DEVICE)        += device_cgroup.o
>>>>> +obj-$(CONFIG_INTEL_SGX)            += commonema.o
>>>>
>>>> The config option and the file name ought to match,
>>>> or at least be closer.
>>>
>>> Just trying to match file names as "capability" uses commoncap.c.
>>
>> Fine, then you should be using CONFIG_SECURITY_EMA.
>>
>>>
>>> Like I said, this feature could potentially be used by TEEs other than SGX. For now, SGX is the only user so it is tied to CONFIG_INTEL_SGX. I can rename it to ema.c or enclave.c. Do you have a preference?
>>
>> Make
>>     CONFIG_SECURITY_EMA
>>     depends on CONFIG_INTEL_SGX
>>
>> When another TEE (maybe MIPS_SSRPQ) comes along you can have
>>
>>     CONFIG_SECURITY_EMA
>>     depends on CONFIG_INTEL_SGX || CONFIG_MIPS_SSRPQ
>
> Your suggestions are reasonable. Given such config change wouldn't affect any code, can we do it later,

That doesn't make the current options any less confusing,
and it will be easier to make the change now than at some
point in the future.

> e.g., when additional TEEs come online and make use of these new hooks? After all, security_enclave_init() will need amendment anyway as one of its current parameters is of type 'struct sgx_sigstruct', which will need to be replaced with something more generic. At the time being, I'd like to keep things intuitive so as not to confuse reviewers.

Reviewers (including me) are already confused by the inconsistency.

>
>>>
>>>>> diff --git a/security/commonema.c b/security/commonema.c
>>>>
>>>> Put this in a subdirectory. Please.
>>>
>>> Then why is commoncap.c located in this directory? I'm just trying to match the existing convention.
>>
>> commoncap is not optional. It is a base part of the
>> security subsystem. ema is optional.
>
> Alright. I'd move it into a sub-folder and rename it to ema.c. Would you be ok with that?

Sounds fine.