Re: [PATCH v5 5/5] KVM: TDX: Validate userspace CPUID input for KVM_TDX_INIT_VM

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

On 10/16/2026 5:14 PM, Binbin Wu wrote:
> Validate the CPUID configuration provided by userspace through
> KVM_TDX_INIT_VM against KVM's TDX allowlist, and drop the hardcoded
> denylist-based check.
> 
> The TDX module lets the VMM directly configure certain CPUID bits for a TD
> at initialization time, but KVM must strictly govern which of those bits
> userspace can set, otherwise a feature that clobbers host state could be
> enabled behind KVM's back.  The existing denylist rejects only TSX and
> WAITPKG, and thus would accept any bit that a future TDX module makes
> configurable, even if KVM knows nothing about the associated feature.
> 
> Add tdx_has_unsupported_cpuid_cfg_bit() and reject KVM_TDX_INIT_VM if
> userspace sets any bit outside of the mask returned by
> tdx_get_cpuid_cfg_mask(), i.e. the same mask used to filter
> KVM_TDX_CAPABILITIES.  Don't bother intersecting the userspace input with
> the bits that the TDX module reports as directly configurable, as the TDX
> module itself rejects any bit outside that set.
> 
> As with the existing check, use the subleaf reported by the TDX module to
> look up the allowed mask.  For leaves without subleaves, the TDX module's
> "no subleaf" index is normalized to 0, which matches how such leaves are
> tracked in reverse_cpuid[].
> 
> Note, the change is user-visible, as KVM_TDX_INIT_VM now rejects some
> input that was previously accepted.  The ABI itself is unchanged: KVM
> already rejected unsupported CPUID bits, albeit via a denylist, and
> userspace has always been required to consult KVM_TDX_CAPABILITIES and set
> only the bits reported as configurable.
> 
> The change isn't expected to break userspace or guests:
>  - The newly rejected bits are no longer reported as configurable by
>    KVM_TDX_CAPABILITIES, so userspace that honors KVM_TDX_CAPABILITIES
>    won't set them.
>  - CORE_CAPABILITIES, which older TDX module specs defined as fixed-1, is
>    still allowed.
>  - The dropped features are either obsolete or unsupported by KVM, and in
>    the latter case, a guest relying on them would already have run into
>    problems.
> 
> Signed-off-by: Binbin Wu <binbin.wu@linux.intel.com>
> Reviewed-by: Tony Lindgren <tony.lindgren@linux.intel.com>

The patch generally looks good to me. However, since it depends on Patch 1,
which, I assume, is going to be replaced by Sean's series[1]. If that's case,
the patch will need a rebase and adjust on top.

Free feel to add

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

if there is no big design change in the end.

[1] https://lore.kernel.org/all/20261022210134.612848-1-seanjc@google.com/