On 2019-07-13 10:07, Jarkko Sakkinen wrote:
> Because the kernel is untrusted, swapping pages in/out of the Enclave
> Page Cache (EPC) has specialized requirements:
> 
> * The kernel cannot directly access EPC memory, i.e. cannot copy data
>    to/from the EPC.
> * To evict a page from the EPC, the kernel must "prove" to hardware that
>    are no valid TLB entries for said page since a stale TLB entry would
>    allow an attacker to bypass SGX access controls.
> * When loading a page back into the EPC, hardware must be able to verify
>    the integrity and freshness of the data.
> * When loading an enclave page, e.g. regular pages and Thread Control
>    Structures (TCS), hardware must be able to associate the page with a
>    Secure Enclave Control Structure (SECS).
> 
> To satisfy the above requirements, the CPU provides dedicated ENCLS
> functions to support paging data in/out of the EPC:
> 
> * EBLOCK:   Mark a page as blocked in the EPC Map (EPCM).  Attempting
>              to access a blocked page that misses the TLB will fault.
> * ETRACK:   Activate blocking tracking.  Hardware verifies that all
>              translations for pages marked as "blocked" have been flushed
> 	    from the TLB.
> * EPA:      Add version array page to the EPC.  As the name suggests, a
>              VA page is an 512-entry array of version numbers that are
> 	    used to uniquely identify pages evicted from the EPC.
> * EWB:      Write back a page from EPC to memory, e.g. RAM.  Software
>              must supply a VA slot, memory to hold the a Paging Crypto
> 	    Metadata (PCMD) of the page and obviously backing for the
> 	    evicted page.
> * ELD{B,U}: Load a page in {un}blocked state from memory to EPC.  The
>              driver only uses the ELDU variant as there is no use case
> 	    for loading a page as "blocked" in a bare metal environment.
> 
> To top things off, all of the above ENCLS functions are subject to
> strict concurrency rules, e.g. many operations will #GP fault if two
> or more operations attempt to access common pages/structures.
> 
> To put it succinctly, paging in/out of the EPC requires coordinating
> with the SGX driver where all of an enclave's tracking resides.  But,
> simply shoving all reclaim logic into the driver is not desirable as
> doing so has unwanted long term implications:
> 
> * Oversubscribing EPC to KVM guests, i.e. virtualizing SGX in KVM and
>    swapping a guest's EPC pages (without the guest's cooperation) needs
>    the same high level flows for reclaim but has painfully different
>    semantics in the details.
> * Accounting EPC, i.e. adding an EPC cgroup controller, is desirable
>    as EPC is effectively a specialized memory type and even more scarce
>    than system memory.  Providing a single touchpoint for EPC accounting
>    regardless of end consumer greatly simplifies the EPC controller.
> * Allowing the userspace-facing driver to be built as a loaded module
>    is desirable, e.g. for debug, testing and development.  The cgroup
>    infrastructure does not support dependencies on loadable modules.
> * Separating EPC swapping from the driver once it has been tightly
>    coupled to the driver is non-trivial (speaking from experience).

Some of these points seem stale now.

--
Jethro Beekman | Fortanix