Re: [PATCH RFC] x86: Add SGX_IOC_ENCLAVE_AUGMENT_PAGES

Jarkko Sakkinen
2022-03-05T02:33:02Z

On Sat, Mar 05, 2022 at 04:17:53AM +0200, Jarkko Sakkinen wrote:
> On Fri, Mar 04, 2022 at 09:08:20AM -0800, Dave Hansen wrote:
> > On 3/4/22 04:28, Jarkko Sakkinen wrote:
> > > Explicit EAUG ioctl is a better choice than an implicit EAUG from a page
> > > fault handler because it allows to have O(1) number of kernel-enclave round
> > > trips for EAUG-EACCEPT{COPY} process, instead of O(n), as it is in the case
> > > when a page fault handler EAUG single page at a time.
> >
> > So this is basically an optimization? It's MADV_WILLNEED or
> > MAP_POPULATE to the cost of avoid future faults?
>
> Yes.
>
> So the idea would be that based on these the #PF handler would have more
> smartness, and it would do a batch of EAUG's?
>
> That could be possibly acceptable but I also had other concern.
>
> I would like to see this:
>
> 1. Removal of vm_run_prot_bits.
> 2. Use RWX vm_max_prot_bits for EAUG'd pages.
>
> During run-time kernel controls PTE's, and enclave has full control of the
> EPCM (EACCEPT, EACCEPTCOPY, EMODPE). By creating artificial limitations how
> to operate with these, it can limit various optimizations in the user space
> code. E.g. a syscall shim can require clever co-operation between in-enclave
> opcodes and what you do with the kernel in various situations.
>
> RWX sounds provocative yes, but here it means only the limits where kernel
> can set its PTE's and nothing else, not that page table is filled with RWX
> pages, and enclave dictates what is in EPCM, and that's how it actually
> should be (e.g. you can sometimes deliver mmap() without ever going out
> of the enclave with EMODPE).
>
> If MADV_WILLNEED/MAP_POPULATE approach is combined with this what I
> discussed here, then I think we could have solution to write an efficient
> memory management shims.

Do you already have a rough idea what needs to be done? I can take anyway a
look but just in case you had processed this further, please tell what you
have.

BR, Jarkko