linux-kernel - Re: [PATCH] KVM: TDX: Replace kmalloc + copy_from_user with memdup_user in tdx_td

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Message-ID: <aO16HySFc6wNVpix@google.com>
Date: Mon, 13 Oct 2025 15:15:59 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Thorsten Blum <thorsten.blum@...ux.dev>
Cc: Paolo Bonzini <pbonzini@...hat.com>, Thomas Gleixner <tglx@...utronix.de>, 
	Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>, 
	Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org, 
	"H. Peter Anvin" <hpa@...or.com>, "Kirill A. Shutemov" <kas@...nel.org>, 
	Rick Edgecombe <rick.p.edgecombe@...el.com>, kvm@...r.kernel.org, 
	linux-kernel@...r.kernel.org, linux-coco@...ts.linux.dev
Subject: Re: [PATCH] KVM: TDX: Replace kmalloc + copy_from_user with
 memdup_user in tdx_td_init

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@...ux.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).