Re: [PATCH] KVM: TDX: Replace kmalloc + copy_from_user with memdup_user in tdx_td_init

Sean Christopherson
2025-10-13T22:16:01Z

On Tue, Sep 16, 2025, Thorsten Blum wrote:
> Use get_user() to retrieve the number of entries instead of allocating
> memory for 'init_vm' with the maximum size, copying 'cmd->data' to it,
> only to then read the actual entry count 'cpuid.nent' from the copy.
>
> Return -E2BIG early if 'nr_user_entries' exceeds KVM_MAX_CPUID_ENTRIES.

I think I'll drop this line from the changelog. At first glance I thought you
were calling out a change in behavior, and my hackles went up. :-)

> Use memdup_user() to allocate just enough memory to fit all entries and
> to copy 'cmd->data' from userspace. Use struct_size() instead of
> manually calculating the number of bytes to allocate and copy.
>
> No functional changes intended.
>
> Signed-off-by: Thorsten Blum <thorsten.blum@linux.dev>
> ---
> Compile-tested only.
> ---
> arch/x86/kvm/vmx/tdx.c | 32 ++++++++++++--------------------
> 1 file changed, 12 insertions(+), 20 deletions(-)
>
> diff --git a/arch/x86/kvm/vmx/tdx.c b/arch/x86/kvm/vmx/tdx.c
> index 66744f5768c8..87510541d2a2 100644
> --- a/arch/x86/kvm/vmx/tdx.c
> +++ b/arch/x86/kvm/vmx/tdx.c
> @@ -2742,8 +2742,10 @@ static int tdx_read_cpuid(struct kvm_vcpu *vcpu, u32 leaf, u32 sub_leaf,
> static int tdx_td_init(struct kvm *kvm, struct kvm_tdx_cmd *cmd)
> {
> struct kvm_tdx *kvm_tdx = to_kvm_tdx(kvm);
> + struct kvm_tdx_init_vm __user *user_init_vm;

Any objection to calling this user_data instead of user_init_vm? I keep reading
user_init_vm as a flag or command, e.g. "user initialized VM" or something, not
as a pointer to user data.

No need for a v2, I'll fixup to whatever we settle on (assuming no one jumps in
with a crazy idea).