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

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Date:   Sun, 5 Dec 2021 00:25:55 +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 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.

/Jarkko