Re: [PATCH v23 12/24] x86/sgx: Linux Enclave Driver

[Stephen Smalley <sds@tycho.nsa.gov>]

On 11/1/19 9:16 AM, Stephen Smalley wrote:
> On 10/31/19 5:17 PM, Jarkko Sakkinen wrote:
>> On Wed, Oct 30, 2019 at 09:45:05AM -0400, Stephen Smalley wrote:
>>> On 10/28/19 5:03 PM, Jarkko Sakkinen wrote:
>>>> Intel Software Guard eXtensions (SGX) is a set of CPU instructions that
>>>> can be used by applications to set aside private regions of code and
>>>> data. The code outside the SGX hosted software entity is disallowed to
>>>> access the memory inside the enclave enforced by the CPU. We call these
>>>> entities as enclaves.
>>>>
>>>> This commit implements a driver that provides an ioctl API to construct
>>>> and run enclaves. Enclaves are constructed from pages residing in
>>>> reserved physical memory areas. The contents of these pages can only be
>>>> accessed when they are mapped as part of an enclave, by a hardware
>>>> thread running inside the enclave.
>>>>
>>>> The starting state of an enclave consists of a fixed measured set of
>>>> pages that are copied to the EPC during the construction process by
>>>> using ENCLS leaf functions and Software Enclave Control Structure 
>>>> (SECS)
>>>> that defines the enclave properties.
>>>>
>>>> Enclave are constructed by using ENCLS leaf functions ECREATE, EADD and
>>>> EINIT. ECREATE initializes SECS, EADD copies pages from system 
>>>> memory to
>>>> the EPC and EINIT check a given signed measurement and moves the 
>>>> enclave
>>>> into a state ready for execution.
>>>>
>>>> An initialized enclave can only be accessed through special Thread 
>>>> Control
>>>> Structure (TCS) pages by using ENCLU (ring-3 only) leaf EENTER.  
>>>> This leaf
>>>> function converts a thread into enclave mode and continues the 
>>>> execution in
>>>> the offset defined by the TCS provided to EENTER. An enclave is exited
>>>> through syscall, exception, interrupts or by explicitly calling another
>>>> ENCLU leaf EEXIT.
>>>>
>>>> The permissions, which enclave page is added will set the limit for 
>>>> maximum
>>>> permissions that can be set for mmap() and mprotect(). This will
>>>> effectively allow to build different security schemes between 
>>>> producers and
>>>> consumers of enclaves. Later on we can increase granularity with LSM 
>>>> hooks
>>>> for page addition (i.e. for producers) and mapping of the enclave 
>>>> (i.e. for
>>>> consumers)
>>>
>>> Where do things stand wrt to ensuring that SGX cannot be used to 
>>> introduce
>>> executable mappings that were never authorized by the LSM (or never 
>>> measured
>>> by IMA)?
>>
>> This was the latest discussion about that subject:
>>
>> https://lore.kernel.org/linux-sgx/CALCETrWDLX68Vi4=9Dicq9ATmJ5mv36bzrc02heNYaHaBeWumQ@mail.gmail.com/ 
>>
> 
> So, IIUC, that means that merging the driver will create a regression 
> with respect to LSM control over executable mappings that will only be 
> rectified at some future point in time if/when someone submits LSM hooks 
> or calls to existing hooks to restore such control.  That doesn't seem 
> like a good idea.  Why can't you include at least that basic level of 
> control now?  It is one thing to defer finer grained control or 
> SGX-specific access controls to the future - that I can understand.  But 
> introducing a regression in the existing controls is not really ok.

Unless you are arguing that the existing checks on mmap/mprotect of 
/dev/sgx/enclave are a coarse-grained approximation (effectively 
requiring WX to the file or execmem for any user of SGX).