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

[Binbin Wu <binbin.wu@linux.intel.com>]

On 10/23/2026 3:46 PM, Xiaoyao Li wrote:
> 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.

Yes, it will be rebased.

> 
> Free feel to add
> 
> Reviewed-by: Xiaoyao Li <xiaoyao.li@intel.com>

Thanks!

> 
> if there is no big design change in the end.
> 
> [1] https://lore.kernel.org/all/20261022210134.612848-1-seanjc@google.com/