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

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Date:   Mon, 6 Dec 2021 13:16:17 -0800
From:   Reinette Chatre <reinette.chatre@...el.com>
To:     Jarkko Sakkinen <jarkko@...nel.org>
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

Hi Jarkko,

On 12/4/2021 2:27 PM, Jarkko Sakkinen wrote:
> 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?

The permission check is redundant for SGX1 and incorrect for SGX2.

In the current SGX1 implementation the permission check in 
sgx_encl_load_page() is redundant because an SGX VMA can only be created 
if its permissions are the same or weaker than the EPCM permissions.

In SGX2 a user is able to change EPCM permissions during runtime (while 
VMA has the memory mapped). A RW VMA may thus originally have mapped an 
enclave page with RW EPCM permissions but since then the enclave page 
may have its permissions changed to read-only. The VMA should still be 
able to read those enclave pages but the check in sgx_encl_load_page() 
will prevent that.

>> 2. Why not re-issuing mmap()'s is unfeasible? I.e. close existing
>>     VMA's and mmap() new ones.

User is not prevented from closing existing VMAs and creating new ones.

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

Could you please elaborate where you see the API/ABI break? The rule 
that new VMAs cannot exceed EPCM permissions is untouched.

Reinette