linux-kernel - Re: [PATCH v2 2/3] KVM: VMX: validate individual bits of guest MSR_IA32_FEATURE

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Date:	Thu, 16 Jun 2016 13:19:16 +0200
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	kvm@...r.kernel.org, rkrcmar@...hat.com,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H . Peter Anvin" <hpa@...or.com>, x86@...nel.org,
	linux-kernel@...r.kernel.org, Gleb Natapov <gleb@...nel.org>,
	Boris Petkov <bp@...e.de>, Tony Luck <tony.luck@...el.com>,
	Andi Kleen <andi.kleen@...el.com>,
	Ashok Raj <ashok.raj@...el.com>
Subject: Re: [PATCH v2 2/3] KVM: VMX: validate individual bits of guest
 MSR_IA32_FEATURE_CONTROL



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