Re: [PATCH v2 3/3] KVM: x86: #GP when attempts to write reserved bits of Variable Range MTRRs

Wanpeng Li
2014-09-01T00:20:29Z

On Fri, Aug 29, 2014 at 06:47:54PM +0200, Paolo Bonzini wrote:
>Il 19/08/2014 11:04, Wanpeng Li ha scritto:
>> Section 11.11.2.3 of the SDM mentions "All other bits in the IA32_MTRR_PHYSBASEn
>> and IA32_MTRR_PHYSMASKn registers are reserved; the processor generates a
>> general-protection exception(#GP) if software attempts to write to them". This
>> patch do it in kvm.
>>
>> Signed-off-by: Wanpeng Li <wanpeng.li@linux.intel.com>
>
>This breaks if the guest maxphyaddr is higher than the host's (which
>sometimes happens depending on your hardware and how QEMU is
>configured).
>
>You need to use cpuid_maxphyaddr, like this
>
>diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
>index a375dfc42f6a..916e89515210 100644
>--- a/arch/x86/kvm/x86.c
>+++ b/arch/x86/kvm/x86.c
>@@ -1726,7 +1726,7 @@ static bool valid_mtrr_type(unsigned t)
> static bool mtrr_valid(struct kvm_vcpu *vcpu, u32 msr, u64 data)
> {
> int i;
>- u64 mask = 0;
>+ u64 mask;
>
> if (!msr_mtrr_valid(msr))
> return false;
>@@ -1750,8 +1750,7 @@ static bool mtrr_valid(struct kvm_vcpu *vcpu, u32 msr, u64 data)
> /* variable MTRRs */
> WARN_ON(!(msr >= 0x200 && msr < 0x200 + 2 * KVM_NR_VAR_MTRR));
>
>- for (i = 63; i > boot_cpu_data.x86_phys_bits; i--)
>- mask |= (1ULL << i);
>+ mask = (~0ULL) << cpuid_maxphyaddr(vcpu);
> if ((msr & 1) == 0) {
> /* MTRR base */
> if (!valid_mtrr_type(data & 0xff))
>
>

Got it, thanks Paolo and Jan.

Regards,
Wanpeng Li

>Jan, can you see if this patch fixes the SeaBIOS triple fault you reported?
>
>Paolo
>
>> ---
>> arch/x86/kvm/x86.c | 18 +++++++++++++++---
>> 1 file changed, 15 insertions(+), 3 deletions(-)
>>
>> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
>> index fb3ea7a..b85da5f 100644
>> --- a/arch/x86/kvm/x86.c
>> +++ b/arch/x86/kvm/x86.c
>> @@ -1726,6 +1726,7 @@ static bool valid_mtrr_type(unsigned t)
>> static bool mtrr_valid(struct kvm_vcpu *vcpu, u32 msr, u64 data)
>> {
>> int i;
>> + u64 mask = 0;
>>
>> if (!msr_mtrr_valid(msr))
>> return false;
>> @@ -1749,10 +1750,21 @@ static bool mtrr_valid(struct kvm_vcpu *vcpu, u32 msr, u64 data)
>> /* variable MTRRs */
>> WARN_ON(!(msr >= 0x200 && msr < 0x200 + 2 * KVM_NR_VAR_MTRR));
>>
>> - if ((msr & 1) == 0)
>> + for (i = 63; i > boot_cpu_data.x86_phys_bits; i--)
>> + mask |= (1ULL << i);
>> + if ((msr & 1) == 0) {
>> /* MTRR base */
>> - return valid_mtrr_type(data & 0xff);
>> - /* MTRR mask */
>> + if (!valid_mtrr_type(data & 0xff))
>> + return false;
>> + mask |= 0xf00;
>> + } else
>> + /* MTRR mask */
>> + mask |= 0x7ff;
>> + if (data & mask) {
>> + kvm_inject_gp(vcpu, 0);
>> + return false;
>> + }
>> +
>> return true;
>> }
>>
>>