Re: [PATCH v2 3/3] KVM: TDX: Reject KVM_TDX_INIT_VM if indexed CPUID functions don't match

[Xiaoyao Li <xiaoyao.li@intel.com>]

On 10/23/2026 5:01 AM, Sean Christopherson wrote:
> Reject KVM_TDX_INIT_VM if userspace doesn't configure the canonical CPUID
> entries correctly with respect to KVM_CPUID_FLAG_SIGNIFCANT_INDEX to fix a
> bug where userspace could bypass the TSX and WAITPKG checks by simply not
> marking a CPUID.0x7 entry as being indexed.  This also sets the stage for
> more in-depth validation of the incoming CPUID configuration.
> 
> This is obviously an ABI change and diverges from KVM's general approach of
> "let userspace do what it wants".  In practice, no "real" VMM that supports
> TDX misconfigures a TD's CPUID entries in this way, i.e. the probability of
> actually breaking userspace is basically nil.  And unlike normal VMs, where
> KVM has full control and thus can deal with bizarre CPUID input without too
> much fuss, TDX VMs require extra care due to the TDX Module also reacting
> to the CPUID configuration, i.e. KVM_TDX_INIT_VM needs to be more strict
> than KVM_SET_CPUID{,2} to avoid creating potentially dangerous-to-the-host
> setups.
> 
> Opportunistically rename the helper to make it more obvious that it's
> checking if the passed in @entry is unsupported, as opposed to returning a
> set of unsupported entries or something.
> 
> Fixes: 6d415778f106 ("KVM: TDX: Disable support for TSX and WAITPKG")
> Reported-by: Sashiko <sashiko-bot@kernel.org>
> Closes: https://lore.kernel.org/kvm/20260604024927.E22311F00893@smtp.kernel.org
> Cc: stable@vger.kernel.org
> Signed-off-by: Sean Christopherson <seanjc@google.com>

Given the discussion of patch2 has settled down,

Reviewed-by: Xiaoyao Li <xiaoyao.li@intel.com>