linux-kernel - Re: [PATCH 2/2] x86/tdx: Do not allow #VE due to EPT violation on the private memory

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Message-ID: <0cd8601b-aa74-ee9a-cfb5-bb69445acc4c@linux.alibaba.com>
Date:   Mon, 7 Nov 2022 21:43:29 +0800
From:   Guorui Yu <GuoRui.Yu@...ux.alibaba.com>
To:     Dave Hansen <dave.hansen@...el.com>,
        kirill.shutemov@...ux.intel.com
Cc:     ak@...ux.intel.com, bp@...en8.de, dan.j.williams@...el.com,
        david@...hat.com, elena.reshetova@...el.com, hpa@...or.com,
        linux-kernel@...r.kernel.org, luto@...nel.org, mingo@...hat.com,
        peterz@...radead.org, sathyanarayanan.kuppuswamy@...ux.intel.com,
        seanjc@...gle.com, tglx@...utronix.de, thomas.lendacky@....com,
        x86@...nel.org
Subject: Re: [PATCH 2/2] x86/tdx: Do not allow #VE due to EPT violation on the
 private memory



在 2022/11/7 21:31, Dave Hansen 写道:
> On 11/6/22 21:10, Guorui Yu wrote:
>>> Without ATTR_SEPT_VE_DISABLE, a #VE can occur on basically any
>>> instruction.  We call those kinds of exceptions "paranoid entry" points.
>>>    They need special handling like the NMI or #MC handlers.
>>>
>>> I'd be happy to look at a patch that does the MMIO path check *and*
>>> turns the #VE handler into a robust entry point.
>>>
>>> Bonus points if you can do ~5 lines of C like the approach in this
>>> thread.
>>
>> Yes, there is a fix to satify your requirement and get the bouns points 😄
>>
>> Please refer to
>> https://github.com/intel/tdx/commit/f045b0d52a5f7d8bf66cd4410307d05a90523f10
>>
>> case EXIT_REASON_EPT_VIOLATION:
>> + if (!(ve->gpa & tdx_shared_mask())) {
>> + panic("#VE due to access to unaccepted memory. "
>> + "GPA: %#llx\n", ve->gpa);
>> + }
>> +
>> /* original from Kirill and Kuppuswamy */
>>
>> It's already there, but it just didn't get into the main branch.
> 
> Could you explain how that prevents the #VE from occurring in the
> "syscall gap" or in a place where the kernel is running with the user
> GSBASE value?
> 
Thank you for explaining the "paranoid entry" points with there examples 
to me, now I understand why the SEPT_VE_DISABLE is necessary for TD.

> It doesn't as far as I can tell.  You need the SEPT_VE_DISABLE check for
> that.