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Date:   Mon, 28 Feb 2022 15:05:26 -0800
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To:     "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
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From:   Dave Hansen <dave.hansen@intel.com>
Subject: Re: [PATCHv4 10/30] x86/tdx: Handle CPUID via #VE
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On 2/28/22 14:53, Kirill A. Shutemov wrote:
> On Mon, Feb 28, 2022 at 08:41:38AM -0800, Dave Hansen wrote:
>>> We realise that this is possible vector of attack and plan to implement
>>> proper filtering. But it is beyon core enabling.
>>>
>>>> Is this better than just returning 0's, for instance?
>>> Plain 0 injection breaks the boot. More complicated solution is need.
>> OK, so we're leaving the kernel open to something that might be an
>> attack vector: we know that we don't know how this might be bad.  It's a
>> "known unknown"[1].
> I looked deeper. The only CPUIDs that actually required are from the
> hypervisor range (the range is reserved and never will be used by CPU, so
> hypervisors adopt it for own use).
> 
> So this filtering makes kernel boot (I didn't test much beyond that).
> 
> 	/*
> 	 * Only allow VMM to control range reserved for hypervisor
> 	 * communication.
> 	 *
> 	 * Return all-zeros for any CPUID outside the range.
> 	 */
> 	if (regs->ax < 0x40000000 || regs->ax > 0x4FFFFFFF) {
> 		regs->ax = regs->bx = regs->cx = regs->dx = 0;
> 		return true;
> 	}
> 
> We may tighten the range further (only few leafs from the range is
> actually used during the boot), but this should be good enough for this
> stage of enabling.

Seems sane to me.  This closes off basically any ability for the VMM to
confuse the guest with CPUID values except for the ones that *must* by
hypervisor-controlled.

Does this, in practice, keep TDX guests from detecting any features that
it supports today?