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Date:   Mon, 7 Nov 2022 05:31:03 -0800
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Subject: Re: [PATCH 2/2] x86/tdx: Do not allow #VE due to EPT violation on the
 private memory
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To:     Guorui Yu <GuoRui.Yu@linux.alibaba.com>,
        kirill.shutemov@linux.intel.com
Cc:     ak@linux.intel.com, bp@alien8.de, dan.j.williams@intel.com,
        david@redhat.com, elena.reshetova@intel.com, hpa@zytor.com,
        linux-kernel@vger.kernel.org, luto@kernel.org, mingo@redhat.com,
        peterz@infradead.org, sathyanarayanan.kuppuswamy@linux.intel.com,
        seanjc@google.com, tglx@linutronix.de, thomas.lendacky@amd.com,
        x86@kernel.org
References: <20221028141220.29217-3-kirill.shutemov@linux.intel.com>
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 <4bfcd256-b926-9b1c-601c-efcff0d16605@intel.com>
 <c2b60735-84a8-649e-536c-877c790eb101@linux.alibaba.com>
From:   Dave Hansen <dave.hansen@intel.com>
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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.