Re: [PATCH] KVM: x86: TDX: Use validated CPUID entry count for TD init

[Binbin Wu]

On 7/8/2026 4:42 PM, Thorsten Blum wrote:
> On Wed, Jul 08, 2026 at 10:29:37AM +0800, Binbin Wu wrote:
>> Use the validated CPUID entry count when parsing CPUID data for
>> KVM_TDX_INIT_VM.
>>
>> tdx_td_init() first reads user_data->cpuid.nent to size the flexible
>> kvm_tdx_init_vm copy.  The copied structure also contains cpuid.nent, and
>> that field can differ from the value used to size the allocation if
>> userspace modifies the input concurrently.  setup_tdparams_cpuids() later
>> passes init_vm->cpuid.nent to kvm_find_cpuid_entry2(), which uses it as
>> the array bound for the copied entries.
>>
>> Overwrite the copied nent with the validated count so CPUID parsing is
>> bounded by the number of entries actually copied.
>>
>> Fixes: 0bd0a4a1428b ("KVM: TDX: Replace kmalloc + copy_from_user with memdup_user in tdx_td_init()")
>> Reported-by: Sashiko:gemini-3.1-pro-preview
>> Cc: <stable@xxxxxxxxxxxxxxx>
>> Signed-off-by: Binbin Wu <binbin.wu@xxxxxxxxxxxxxxx>
>> ---
>>  arch/x86/kvm/vmx/tdx.c | 6 ++++++
>>  1 file changed, 6 insertions(+)
>>
>> diff --git a/arch/x86/kvm/vmx/tdx.c b/arch/x86/kvm/vmx/tdx.c
>> index ffe9d0db58c5..b658b03e7750 100644
>> --- a/arch/x86/kvm/vmx/tdx.c
>> +++ b/arch/x86/kvm/vmx/tdx.c
>> @@ -2802,6 +2802,12 @@ static int tdx_td_init(struct kvm *kvm, struct kvm_tdx_cmd *cmd)
>>  	if (IS_ERR(init_vm))
>>  		return PTR_ERR(init_vm);
>>  
>> +	/*
>> +	 * Use the validated entry count, as user_data->cpuid.nent may have
>> +	 * changed.
>> +	 */
>> +	init_vm->cpuid.nent = nr_user_entries;
>> +
> 
> Maybe it would be better to check for a mismatch and return -EINVAL?
> 
> 	if (init_vm->cpuid.nent != nr_user_entries) {
> 		ret = -EINVAL;
> 		goto out;
> 	}
> 
> That would make the mismatch explicit instead of silently accepting an
> inconsistent userspace snapshot.

I chose to use the snapshot value to follow KVM_SET_CPUID2's style.
KVM_SET_CPUID2 kind of uses the snapshot value of entry count.

But returning a error code is OK for me.
Let's wait and see what others prefer.


> 
>>  	if (memchr_inv(init_vm->reserved, 0, sizeof(init_vm->reserved))) {
>>  		ret = -EINVAL;
>>  		goto out;
>>
>> base-commit: 50406d35f5635e1cc523e61409d57e851b5f5df8
>> -- 
>> 2.46.0
>>
>