linux-kernel - Re: [PATCH v4 02/14] X86/nVMX: handle_vmptrld: Copy the VMCS12 directly from guest memory

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Date:   Mon, 3 Dec 2018 13:59:23 +0100
From:   David Hildenbrand <david@...hat.com>
To:     KarimAllah Ahmed <karahmed@...zon.de>, rkrcmar@...hat.com,
        pbonzini@...hat.com, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org, jmattson@...gle.com
Subject: Re: [PATCH v4 02/14] X86/nVMX: handle_vmptrld: Copy the VMCS12
 directly from guest memory

On 03.12.18 10:30, KarimAllah Ahmed 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@...zon.de>
> ---
> v3 -> v4:
> - Return VMXERR_VMPTRLD_INCORRECT_VMCS_REVISION_ID on failure (jmattson@)
> v1 -> v2:
> - Massage commit message a bit.
> ---
>  arch/x86/kvm/vmx.c | 24 ++++++++++++------------
>  1 file changed, 12 insertions(+), 12 deletions(-)
> 
> diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
> index b84f230..75817cb 100644
> --- a/arch/x86/kvm/vmx.c
> +++ b/arch/x86/kvm/vmx.c
> @@ -9301,20 +9301,22 @@ static int handle_vmptrld(struct kvm_vcpu *vcpu)
>  		return 1;
>  
>  	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))
> -			return nested_vmx_failInvalid(vcpu);
> +		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);
> +			return nested_vmx_failValid(vcpu,
> +						    VMXERR_VMPTRLD_INCORRECT_VMCS_REVISION_ID);
> +		}

If we fail to read, VMXERR_VMPTRLD_INCORRECT_VMCS_REVISION_ID seems t be
the right thing to do (I remember that reading from non-existent memory
returns all 1's).

I somewhat dislike that we are converting -ENOMEM to
VMXERR_VMPTRLD_INCORRECT_VMCS_REVISION_ID. Isn't there a way we can
report -ENOMEM to user space? (or do we really want to avoid crashing
the guest here? if so, comment + print a warning or something like that?)

>  
> -		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);
>  			return nested_vmx_failValid(vcpu,
> -				VMXERR_VMPTRLD_INCORRECT_VMCS_REVISION_ID);
> +						    VMXERR_VMPTRLD_INCORRECT_VMCS_REVISION_ID);
>  		}
>  
>  		nested_release_vmcs12(vcpu);
> @@ -9324,9 +9326,7 @@ static int handle_vmptrld(struct kvm_vcpu *vcpu)
>  		 * 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);
>  	}
>  
> 


-- 

Thanks,

David / dhildenb