Re: [PATCH V4 0/1] KVM: TDX: Decrease TDX VM shutdown time

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

On 7/14/2025 9:56 PM, Sean Christopherson wrote:
> On Mon, Jul 14, 2025, Xiaoyao Li wrote:
>> On 7/12/2025 7:17 AM, Edgecombe, Rick P wrote:
>>> On Fri, 2025-07-11 at 16:05 -0700, Sean Christopherson wrote:
>>>>> Zero the reserved area in struct kvm_tdx_capabilities so that fields added
>>>>> in
>>>>> the reserved area won't disturb any userspace that previously had garbage
>>>>> there.
>>>>
>>>> It's not only about disturbing userspace, it's also about actually being able
>>>> to repurpose the reserved fields in the future without needing *another* flag
>>>> to tell userspace that it's ok to read the previously-reserved fields.  I care
>>>> about this much more than I care about userspace using reserved fields as
>>>> scratch space.
>>>
>>> If, before calling KVM_TDX_CAPABILITIES, userspace zeros the new field that it
>>> knows about, but isn't sure if the kernel does, it's the same no?
> 
> Heh, yeah, this crossed my mind about 5 minutes after I logged off :-)
> 
>>> Did you see that the way KVM_TDX_CAPABILITIES is implemented today is a little
>>> weird? It actually copies the whole struct kvm_tdx_capabilities from userspace
>>> and then sets some fields (not reserved) and then copies it back. So userspace
>>> can zero any fields it wants to know about before calling KVM_TDX_CAPABILITIES.
>>> Then it could know the same things as if the kernel zeroed it.
>>>
>>> I was actually wondering if we want to change the kernel to zero reserved, if it
>>> might make more sense to just copy caps->cpuid.nent field from userspace, and
>>> then populate the whole thing starting from a zero'd buffer in the kernel.
>>
>> +1 to zero the whole buffer of *caps in the kernel.
> 
> Ya, I almost suggested that, but assumed there was a reason for copying the entire
> structure.
> 
>> current code seems to have issue on the caps->kernel_tdvmcallinfo_1_r11/kernel_tdvmcallinfo_1_r12/user_tdvmcallinfo_1_r12,
>> as KVM cannot guarantee zero'ed value are returned to userspace.
> 
> This?  (untested)

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

> diff --git a/arch/x86/kvm/vmx/tdx.c b/arch/x86/kvm/vmx/tdx.c
> index f4d4fd5cc6e8..42cb328d8a7d 100644
> --- a/arch/x86/kvm/vmx/tdx.c
> +++ b/arch/x86/kvm/vmx/tdx.c
> @@ -2270,25 +2270,26 @@ static int tdx_get_capabilities(struct kvm_tdx_cmd *cmd)
>          const struct tdx_sys_info_td_conf *td_conf = &tdx_sysinfo->td_conf;
>          struct kvm_tdx_capabilities __user *user_caps;
>          struct kvm_tdx_capabilities *caps = NULL;
> +       u32 nr_user_entries;
>          int ret = 0;
>   
>          /* flags is reserved for future use */
>          if (cmd->flags)
>                  return -EINVAL;
>   
> -       caps = kmalloc(sizeof(*caps) +
> +       caps = kzalloc(sizeof(*caps) +
>                         sizeof(struct kvm_cpuid_entry2) * td_conf->num_cpuid_config,
>                         GFP_KERNEL);
>          if (!caps)
>                  return -ENOMEM;
>   
>          user_caps = u64_to_user_ptr(cmd->data);
> -       if (copy_from_user(caps, user_caps, sizeof(*caps))) {
> +       if (get_user(nr_user_entries, &user_caps->cpuid.nent)) {
>                  ret = -EFAULT;
>                  goto out;
>          }
>   
> -       if (caps->cpuid.nent < td_conf->num_cpuid_config) {
> +       if (nr_user_entries < td_conf->num_cpuid_config) {
>                  ret = -E2BIG;
>                  goto out;
>          }