linux-kernel - Re: [PATCH 03/25] x86/sgx: Support VMA permissions exceeding enclave permissions

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Message-ID: <YavrSFDJBGqe7K46@iki.fi>
Date:   Sun, 5 Dec 2021 00:27:20 +0200
From:   Jarkko Sakkinen <jarkko@...nel.org>
To:     Reinette Chatre <reinette.chatre@...el.com>
Cc:     dave.hansen@...ux.intel.com, tglx@...utronix.de, bp@...en8.de,
        luto@...nel.org, mingo@...hat.com, linux-sgx@...r.kernel.org,
        x86@...nel.org, seanjc@...gle.com, kai.huang@...el.com,
        cathy.zhang@...el.com, cedric.xing@...el.com,
        haitao.huang@...el.com, mark.shanahan@...el.com, hpa@...or.com,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 03/25] x86/sgx: Support VMA permissions exceeding enclave
 permissions

On Sun, Dec 05, 2021 at 12:25:59AM +0200, Jarkko Sakkinen wrote:
> On Wed, Dec 01, 2021 at 11:23:01AM -0800, Reinette Chatre wrote:
> > === Summary ===
> > 
> > An SGX VMA can only be created if its permissions are the same or
> > weaker than the Enclave Page Cache Map (EPCM) permissions. After VMA
> > creation this rule continues to be enforced by the page fault handler.
> > 
> > With SGX2 the EPCM permissions of a page can change after VMA
> > creation resulting in the VMA exceeding the EPCM permissions and the
> > page fault handler incorrectly blocking access.
> > 
> > Enable the VMA's pages to remain accessible while ensuring that
> > the page table entries are installed to match the EPCM permissions
> > without exceeding the VMA perms issions.
> 
> I don't understand what the short summary means in English, and the
> commit message is way too bloated to make any conclusions. It really
> needs a rewrite.
> 
> These were the questions I could not find answer for:
> 
> 1. Why it would be by any means safe to remove a permission check?
> 2. Why not re-issuing mmap()'s is unfeasible? I.e. close existing
>    VMA's and mmap() new ones.

3. Isn't this an API/ABI break?

/Jarkko