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

Haozhong Zhang
2016-06-16T11:29:55Z

On 06/16/16 13:19, Paolo Bonzini wrote:
>
>
> 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.
>

My mistake. I'll remove it in patch 1.

Thanks,
Haozhong