linux-kernel - Re: [PATCH RFC] x86: Add SGX_IOC_ENCLAVE_AUGMENT

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Message-ID: <YiLLrDure3canJYy@iki.fi>
Date:   Sat, 5 Mar 2022 04:32:12 +0200
From:   Jarkko Sakkinen <jarkko@...nel.org>
To:     Dave Hansen <dave.hansen@...el.com>
Cc:     linux-sgx@...r.kernel.org,
        Reinette Chatre <reinette.chatre@...el.com>,
        Nathaniel McCallum <nathaniel@...fian.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        "open list:X86 ARCHITECTURE (32-BIT AND 64-BIT)" 
        <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH RFC] x86: Add SGX_IOC_ENCLAVE_AUGMENT_PAGES

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