Re: [PATCH v3 02/13] X86/nVMX: handle_vmptrld: Copy the VMCS12 directly from guest memory

Jim Mattson
2018-10-22T21:32:52Z

On Sat, Oct 20, 2018 at 3:22 PM, KarimAllah Ahmed <karahmed@amazon.de> wrote:
> Copy the VMCS12 directly from guest memory instead of the map->copy->unmap
> sequence. This also avoids using kvm_vcpu_gpa_to_page() and kmap() which
> assumes that there is a "struct page" for guest memory.
>
> Signed-off-by: KarimAllah Ahmed <karahmed@amazon.de>
> ---
> v1 -> v2:
> - Massage commit message a bit.
> ---
> arch/x86/kvm/vmx.c | 23 +++++++++--------------
> 1 file changed, 9 insertions(+), 14 deletions(-)
>
> diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
> index 358759a..bc45347 100644
> --- a/arch/x86/kvm/vmx.c
> +++ b/arch/x86/kvm/vmx.c
> @@ -8879,33 +8879,28 @@ static int handle_vmptrld(struct kvm_vcpu *vcpu)
> }
>
> if (vmx->nested.current_vmptr != vmptr) {
> - struct vmcs12 *new_vmcs12;
> - struct page *page;
> - page = kvm_vcpu_gpa_to_page(vcpu, vmptr);
> - if (is_error_page(page)) {
> + struct vmcs12 *new_vmcs12 = (struct vmcs12 *)__get_free_page(GFP_KERNEL);
> +
> + if (!new_vmcs12 ||
> + kvm_read_guest(vcpu->kvm, vmptr, new_vmcs12,
> + sizeof(*new_vmcs12))) {
> + free_page((unsigned long)new_vmcs12);
> nested_vmx_failInvalid(vcpu);

I believe that this failure, like the one below, should be:

nested_vmx_failValid(vcpu, VMXERR_VMPTRLD_INCORRECT_VMCS_REVISION_ID);

First, if there is a valid current VMCS, then nested_vmx_failInvalid
is incorrect. Second, if there is no backing memory, then by PCI bus
error semantics, we should read all 1's, which isn't the correct VMCS
revision ID.

> return kvm_skip_emulated_instruction(vcpu);
> }
> - new_vmcs12 = kmap(page);
> +
> if (new_vmcs12->hdr.revision_id != VMCS12_REVISION ||
> (new_vmcs12->hdr.shadow_vmcs &&
> !nested_cpu_has_vmx_shadow_vmcs(vcpu))) {
> - kunmap(page);
> - kvm_release_page_clean(page);
> + free_page((unsigned long)new_vmcs12);
> nested_vmx_failValid(vcpu,
> VMXERR_VMPTRLD_INCORRECT_VMCS_REVISION_ID);
> return kvm_skip_emulated_instruction(vcpu);
> }
>
> nested_release_vmcs12(vmx);
> - /*
> - * Load VMCS12 from guest memory since it is not already
> - * cached.
> - */
> memcpy(vmx->nested.cached_vmcs12, new_vmcs12, VMCS12_SIZE);
> - kunmap(page);
> - kvm_release_page_clean(page);
> -
> + free_page((unsigned long)new_vmcs12);
> set_current_vmptr(vmx, vmptr);
> }
>
> --
> 2.7.4
>