Re: [PATCH v2 2/3] KVM: VMX: validate individual bits of guest MSR_IA32_FEATURE_CONTROL

Paolo Bonzini
2016-06-16T11:19:16Z

On 16/06/2016 13:16, Haozhong Zhang wrote:
>> However, I think FEATURE_CONTROL_LOCKED should always be writable. If
>> you change that, it's simpler to just do |= and &= in the caller.
>
> These two functions (add/del) are to prevent callers from forgetting
> setting/clearing FEATURE_CONTROL_LOCKED in
> msr_ia32_feature_control_valid_bits: it should be set if any feature
> bit is set, and be cleared if all feature bits are cleared. The second
> rule could relaxed as we can always present MSR_IA32_FEATURE_CONTROL
> to guest.

Yes, this means that FEATURE_CONTROL_LOCKED effectively is always valid.
So you end up with just &= to clear and |= to set.

> I'm okey to let callers take care for the locked bit.
>
>>> + to_vmx(vcpu)->msr_ia32_feature_control_valid_bits |=
>>> + bits | FEATURE_CONTROL_LOCKED;
>>> +}
>>> +
>>> +static void feature_control_valid_bits_del(struct kvm_vcpu *vcpu, uint64_t bits)
>>> +{
>>> + uint64_t *valid_bits =
>>> + &to_vmx(vcpu)->msr_ia32_feature_control_valid_bits;
>>> + ASSERT(!(bits & ~FEATURE_CONTROL_MAX_VALID_BITS));
>>> + *valid_bits &= ~bits;
>>> + if (!(*valid_bits & ~FEATURE_CONTROL_LOCKED))
>>> + *valid_bits = 0;
>>> +}
>>> +
>>> #define VMCS12_OFFSET(x) offsetof(struct vmcs12, x)
>>> #define FIELD(number, name) [number] = VMCS12_OFFSET(name)
>>> #define FIELD64(number, name) [number] = VMCS12_OFFSET(name), \
>>> @@ -2864,6 +2897,14 @@ static int vmx_get_vmx_msr(struct kvm_vcpu *vcpu, u32 msr_index, u64 *pdata)
>>> return 0;
>>> }
>>>
>>> +static inline bool vmx_feature_control_msr_valid(struct kvm_vcpu *vcpu,
>>> + uint64_t val)
>>> +{
>>> + uint64_t valid_bits = to_vmx(vcpu)->msr_ia32_feature_control_valid_bits;
>>> +
>>> + return valid_bits && !(val & ~valid_bits);
>>> +}
>>> /*
>>> * Reads an msr value (of 'msr_index') into 'pdata'.
>>> * Returns 0 on success, non-0 otherwise.
>>> @@ -2906,7 +2947,7 @@ static int vmx_get_msr(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
>>> msr_info->data = vmcs_read64(GUEST_BNDCFGS);
>>> break;
>>> case MSR_IA32_FEATURE_CONTROL:
>>> - if (!nested_vmx_allowed(vcpu))
>>> + if (!vmx_feature_control_msr_valid(vcpu, 0))
>>
>> You can remove this if completely in patch 1. It's okay to make the MSR
>> always available.
>>
>
> But then it also allows all bits to be set by guests, even though some
> features are not available.

Note that this is "get". Of course the "if" must stay in vmx_set_msr.

Paolo