[PATCH] KVM: vmx: add mismatched size in vmcs_check32

[PATCH] KVM: vmx: add mismatched size in vmcs_check32

[lihaiwei.kernel]

From: Haiwei Li <lihaiwei@tencent.com>

vmcs_check32 misses the check for 64-bit and 64-bit high.

Signed-off-by: Haiwei Li <lihaiwei@tencent.com>
---
 arch/x86/kvm/vmx/vmx_ops.h | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/arch/x86/kvm/vmx/vmx_ops.h b/arch/x86/kvm/vmx/vmx_ops.h
index 692b0c3..164b64f 100644
--- a/arch/x86/kvm/vmx/vmx_ops.h
+++ b/arch/x86/kvm/vmx/vmx_ops.h
@@ -37,6 +37,10 @@ static __always_inline void vmcs_check32(unsigned long field)
 {
 	BUILD_BUG_ON_MSG(__builtin_constant_p(field) && ((field) & 0x6000) == 0,
 			 "32-bit accessor invalid for 16-bit field");
+	BUILD_BUG_ON_MSG(__builtin_constant_p(field) && ((field) & 0x6001) == 0x2000,
+			 "32-bit accessor invalid for 64-bit field");
+	BUILD_BUG_ON_MSG(__builtin_constant_p(field) && ((field) & 0x6001) == 0x2001,
+			 "32-bit accessor invalid for 64-bit high field");
 	BUILD_BUG_ON_MSG(__builtin_constant_p(field) && ((field) & 0x6000) == 0x6000,
 			 "32-bit accessor invalid for natural width field");
 }
-- 
1.8.3.1

Re: [PATCH] KVM: vmx: add mismatched size in vmcs_check32

[Sean Christopherson]

On Thu, Apr 08, 2021, lihaiwei.kernel@gmail.com wrote:
> From: Haiwei Li <lihaiwei@tencent.com>
> 
> vmcs_check32 misses the check for 64-bit and 64-bit high.

Can you clarify in the changelog that, while it is architecturally legal to
access 64-bit and 64-bit high fields with a 32-bit read/write in 32-bit mode,
KVM should never do partial accesses to VMCS fields.  And/or note that the
32-bit accesses are done in vmcs_{read,write}64() when necessary?  Hmm, maybe:

  Add compile-time assertions in vmcs_check32() to disallow accesses to
  64-bit and 64-bit high fields via vmcs_{read,write}32().  Upper level
  KVM code should never do partial accesses to VMCS fields.  KVM handles
  the split accesses automatically in vmcs_{read,write}64() when running
  as a 32-bit kernel.

With something along those lines:

Reviewed-and-tested-by: Sean Christopherson <seanjc@google.com> 

> Signed-off-by: Haiwei Li <lihaiwei@tencent.com>
> ---
>  arch/x86/kvm/vmx/vmx_ops.h | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/arch/x86/kvm/vmx/vmx_ops.h b/arch/x86/kvm/vmx/vmx_ops.h
> index 692b0c3..164b64f 100644
> --- a/arch/x86/kvm/vmx/vmx_ops.h
> +++ b/arch/x86/kvm/vmx/vmx_ops.h
> @@ -37,6 +37,10 @@ static __always_inline void vmcs_check32(unsigned long field)
>  {
>  	BUILD_BUG_ON_MSG(__builtin_constant_p(field) && ((field) & 0x6000) == 0,
>  			 "32-bit accessor invalid for 16-bit field");
> +	BUILD_BUG_ON_MSG(__builtin_constant_p(field) && ((field) & 0x6001) == 0x2000,
> +			 "32-bit accessor invalid for 64-bit field");
> +	BUILD_BUG_ON_MSG(__builtin_constant_p(field) && ((field) & 0x6001) == 0x2001,
> +			 "32-bit accessor invalid for 64-bit high field");
>  	BUILD_BUG_ON_MSG(__builtin_constant_p(field) && ((field) & 0x6000) == 0x6000,
>  			 "32-bit accessor invalid for natural width field");
>  }
> -- 
> 1.8.3.1
> 

Re: [PATCH] KVM: vmx: add mismatched size in vmcs_check32

[Paolo Bonzini]

On 08/04/21 18:05, Sean Christopherson wrote:
>    Add compile-time assertions in vmcs_check32() to disallow accesses to
>    64-bit and 64-bit high fields via vmcs_{read,write}32().  Upper level
>    KVM code should never do partial accesses to VMCS fields.  KVM handles
>    the split accesses automatically in vmcs_{read,write}64() when running
>    as a 32-bit kernel.

KVM also uses raw vmread/vmwrite (__vmcs_readl/__vmcs_writel) when 
copying to and from the shadow VMCS, so that path will not go through 
vmcs_check32 either.

Paolo

Re: [PATCH] KVM: vmx: add mismatched size in vmcs_check32

[Haiwei Li]

On Fri, Apr 9, 2021 at 12:05 AM Sean Christopherson <seanjc@google.com> wrote:
>
> On Thu, Apr 08, 2021, lihaiwei.kernel@gmail.com wrote:
> > From: Haiwei Li <lihaiwei@tencent.com>
> >
> > vmcs_check32 misses the check for 64-bit and 64-bit high.
>
> Can you clarify in the changelog that, while it is architecturally legal to
> access 64-bit and 64-bit high fields with a 32-bit read/write in 32-bit mode,
> KVM should never do partial accesses to VMCS fields.  And/or note that the
> 32-bit accesses are done in vmcs_{read,write}64() when necessary?  Hmm, maybe:
>
>   Add compile-time assertions in vmcs_check32() to disallow accesses to
>   64-bit and 64-bit high fields via vmcs_{read,write}32().  Upper level
>   KVM code should never do partial accesses to VMCS fields.  KVM handles
>   the split accesses automatically in vmcs_{read,write}64() when running
>   as a 32-bit kernel.

Good suggestion, thanks. I will send v2.