RE: [PATCH v3 7/8] x86/hvm: Disallow access to unknown MSRs

["Tian, Kevin" <kevin.tian@xxxxxxxxx>]

> From: Roger Pau Monne <roger.pau@xxxxxxxxxx>
> Sent: Tuesday, September 1, 2020 6:55 PM
> 
> From: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
> 
> Change the catch-all behavior for MSR not explicitly handled. Instead
> of allow full read-access to the MSR space and silently dropping
> writes return an exception when the MSR is not explicitly handled.
> 
> Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
> [remove rdmsr_safe from default case in svm_msr_read_intercept]
> Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>

Reviewed-by: Kevin Tian <kevin.tian@xxxxxxxxx>

> ---
> Changes since v1:
>  - Fold chunk to remove explicit write handling of VMX MSRs just to
>    #GP.
>  - Remove catch-all rdmsr_safe in svm_msr_read_intercept.
> ---
>  xen/arch/x86/hvm/svm/svm.c | 10 ++++------
>  xen/arch/x86/hvm/vmx/vmx.c | 16 ++++------------
>  2 files changed, 8 insertions(+), 18 deletions(-)
> 
> diff --git a/xen/arch/x86/hvm/svm/svm.c b/xen/arch/x86/hvm/svm/svm.c
> index 0e43154c7e..66b22efdab 100644
> --- a/xen/arch/x86/hvm/svm/svm.c
> +++ b/xen/arch/x86/hvm/svm/svm.c
> @@ -1964,8 +1964,7 @@ static int svm_msr_read_intercept(unsigned int
> msr, uint64_t *msr_content)
>          break;
> 
>      default:
> -        if ( rdmsr_safe(msr, *msr_content) == 0 )
> -            break;
> +        gdprintk(XENLOG_WARNING, "RDMSR 0x%08x unimplemented\n",
> msr);
>          goto gpf;
>      }
> 
> @@ -2150,10 +2149,9 @@ static int svm_msr_write_intercept(unsigned int
> msr, uint64_t msr_content)
>          break;
> 
>      default:
> -        /* Match up with the RDMSR side; ultimately this should go away. */
> -        if ( rdmsr_safe(msr, msr_content) == 0 )
> -            break;
> -
> +        gdprintk(XENLOG_WARNING,
> +                 "WRMSR 0x%08x val 0x%016"PRIx64" unimplemented\n",
> +                 msr, msr_content);
>          goto gpf;
>      }
> 
> diff --git a/xen/arch/x86/hvm/vmx/vmx.c b/xen/arch/x86/hvm/vmx/vmx.c
> index f6657af923..9cc9d81c41 100644
> --- a/xen/arch/x86/hvm/vmx/vmx.c
> +++ b/xen/arch/x86/hvm/vmx/vmx.c
> @@ -3015,9 +3015,7 @@ static int vmx_msr_read_intercept(unsigned int
> msr, uint64_t *msr_content)
>              break;
>          }
> 
> -        if ( rdmsr_safe(msr, *msr_content) == 0 )
> -            break;
> -
> +        gdprintk(XENLOG_WARNING, "RDMSR 0x%08x unimplemented\n",
> msr);
>          goto gp_fault;
>      }
> 
> @@ -3290,11 +3288,6 @@ static int vmx_msr_write_intercept(unsigned int
> msr, uint64_t msr_content)
>          __vmwrite(GUEST_IA32_DEBUGCTL, msr_content);
>          break;
> 
> -    case MSR_IA32_FEATURE_CONTROL:
> -    case MSR_IA32_VMX_BASIC ... MSR_IA32_VMX_VMFUNC:
> -        /* None of these MSRs are writeable. */
> -        goto gp_fault;
> -
>      case MSR_IA32_MISC_ENABLE:
>          /* Silently drop writes that don't change the reported value. */
>          if ( vmx_msr_read_intercept(msr, &tmp) != X86EMUL_OKAY ||
> @@ -3320,10 +3313,9 @@ static int vmx_msr_write_intercept(unsigned int
> msr, uint64_t msr_content)
>               is_last_branch_msr(msr) )
>              break;
> 
> -        /* Match up with the RDMSR side; ultimately this should go away. */
> -        if ( rdmsr_safe(msr, msr_content) == 0 )
> -            break;
> -
> +        gdprintk(XENLOG_WARNING,
> +                 "WRMSR 0x%08x val 0x%016"PRIx64" unimplemented\n",
> +                 msr, msr_content);
>          goto gp_fault;
>      }
> 
> --
> 2.28.0