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

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Date:   Mon, 7 Nov 2022 05:31:03 -0800
From:   Dave Hansen <dave.hansen@...el.com>
To:     Guorui Yu <GuoRui.Yu@...ux.alibaba.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

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?

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