Re: [PATCH v2] KVM: TDX: Reject concurrent change to CPUID entry count

[Thorsten Blum <thorsten.blum@linux.dev>]

On Fri, Jul 10, 2026 at 11:53:23AM +0800, Binbin Wu wrote:
> Reject KVM_TDX_INIT_VM if userspace changes cpuid.nent between the
> initial read and the subsequent copy of the initialization data.
> 
> 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.
> 
> Require the copied count to match the value used to size the allocation
> so that CPUID parsing cannot access beyond the 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@vger.kernel.org>
> Signed-off-by: Binbin Wu <binbin.wu@linux.intel.com>

LGTM, and thanks for fixing this!

Reviewed-by: Thorsten Blum <thorsten.blum@linux.dev>

> ---
> v2:
> - Reject the request if mismatch instead overwriting the value. (Thorsten, Rick)
> - Lump the check into the existing sanity check on the cpuid field. (Sean)
> - "KVM: x86: TDX:" -> "KVM: TDX:" in the shortlog.
> 
> v1:
> - https://lore.kernel.org/kvm/20260708022937.2465796-1-binbin.wu@linux.intel.com/
> ---
>  arch/x86/kvm/vmx/tdx.c | 6 +++++-
>  1 file changed, 5 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kvm/vmx/tdx.c b/arch/x86/kvm/vmx/tdx.c
> index 6ff1469e91cc..d1af0a752e97 100644
> --- a/arch/x86/kvm/vmx/tdx.c
> +++ b/arch/x86/kvm/vmx/tdx.c
> @@ -2797,7 +2797,11 @@ static int tdx_td_init(struct kvm *kvm, struct kvm_tdx_cmd *cmd)
>  		goto out;
>  	}
>  
> -	if (init_vm->cpuid.padding) {
> +	/*
> +	 * Reject the request if userspace changes cpuid.nent between the
> +	 * initial read and the subsequent copy.
> +	 */
> +	if (init_vm->cpuid.padding || init_vm->cpuid.nent != nr_user_entries) {
>  		ret = -EINVAL;
>  		goto out;
>  	}
> 
> base-commit: f1e5ada5ab62dbe32350bc161771c9afc6a896de
> -- 
> 2.46.0
>