linux-kernel - Re: [PATCH 05/25] x86/sgx: Introduce runtime protection bits

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Message-ID: <4f50b3b3-2cd5-30a4-5715-3350ef2174d9@kernel.org>
Date:   Fri, 3 Dec 2021 16:38:06 -0800
From:   Andy Lutomirski <luto@...nel.org>
To:     Reinette Chatre <reinette.chatre@...el.com>,
        dave.hansen@...ux.intel.com, jarkko@...nel.org, tglx@...utronix.de,
        bp@...en8.de, mingo@...hat.com, linux-sgx@...r.kernel.org,
        x86@...nel.org
Cc:     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 05/25] x86/sgx: Introduce runtime protection bits

On 12/3/21 14:12, Reinette Chatre wrote:
> Hi Andy,
> 
> On 12/3/2021 11:28 AM, Andy Lutomirski wrote:
>> On 12/1/21 11:23, Reinette Chatre wrote:
>>> Enclave creators declare their paging permission intent at the time
>>> the pages are added to the enclave. These paging permissions are
>>> vetted when pages are added to the enclave and stashed off
>>> (in sgx_encl_page->vm_max_prot_bits) for later comparison with
>>> enclave PTEs.
>>>
>>
>> I'm a bit confused here. ENCLU[EMODPE] allows the enclave to change 
>> the EPCM permission bits however it likes with no oversight from the 
>> kernel.   So we end up with a whole bunch of permission masks:
> 
> Before jumping to the permission masks I would like to step back and 
> just confirm the context. We need to consider the following three 
> permissions:
> 
> EPCM permissions: the enclave page permissions maintained in the SGX 
> hardware. The OS is constrained here in that it cannot query the current 
> EPCM permissions. Even so, the OS needs to ensure PTEs are installed 
> appropriately (we do not want a RW PTE for a read-only enclave page)

Why not?  What's wrong with an RW PTE for a read-only enclave page?

If you convince me that this is actually important, then I'll read all 
the stuff below.