linux-kernel - Re: [PATCH] KVM: vmx: add mismatched size in vmcs

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Message-ID: <YG8pwERmjxYQoquP@google.com>
Date:   Thu, 8 Apr 2021 16:05:20 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     lihaiwei.kernel@...il.com
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        pbonzini@...hat.com, vkuznets@...hat.com, wanpengli@...cent.com,
        jmattson@...gle.com, joro@...tes.org,
        Haiwei Li <lihaiwei@...cent.com>
Subject: Re: [PATCH] KVM: vmx: add mismatched size in vmcs_check32

On Thu, Apr 08, 2021, lihaiwei.kernel@...il.com wrote:
> From: Haiwei Li <lihaiwei@...cent.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@...gle.com> 

> Signed-off-by: Haiwei Li <lihaiwei@...cent.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
>