Re: [PATCH 3.2 41/52] KVM: svm: unconditionally intercept #DB

[Paolo Bonzini]

On 25/11/2015 18:56, Ben Hutchings wrote:
> On Wed, 2015-11-25 at 12:31 +0100, Paolo Bonzini wrote:
>>
>> On 24/11/2015 23:33, Ben Hutchings wrote:
>>> 3.2.74-rc1 review patch.  If anyone has any objections, please let me know.
>>>
>>> ------------------
>>>
>>> From: Paolo Bonzini <pbonzini@xxxxxxxxxx>
>>>
>>> commit cbdb967af3d54993f5814f1cee0ed311a055377d upstream.
>>>
>>> This is needed to avoid the possibility that the guest triggers
>>> an infinite stream of #DB exceptions (CVE-2015-8104).
>>>
>>> VMX is not affected: because it does not save DR6 in the VMCS,
>>> it already intercepts #DB unconditionally.
>>>
>>> Reported-by: Jan Beulich <jbeulich@xxxxxxxx>
>>> Signed-off-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>
>>> [bwh: Backported to 3.2: #DB and #BP did not share a function, and there is
>>>  no operation pointer referring to it, so remove update_db_intercept()
>>>  entirely]
>>
>> This is wrong, you still need to check the BP intercept in the
>> (incorrectly named as of 3.2) update_db_intercept function.
>>
>> Something like:
>>
>> -static void update_db_intercept(struct kvm_vcpu *vcpu)
>> +static void update_bp_intercept(struct kvm_vcpu *vcpu)
>>  {
>>  > 	> struct vcpu_svm *svm = to_svm(vcpu);
>>
>> -> 	> clr_exception_intercept(svm, DB_VECTOR);
>>  > 	> clr_exception_intercept(svm, BP_VECTOR);
>> -
>> -> 	> if (svm->nmi_singlestep)
>> -> 	> 	> set_exception_intercept(svm, DB_VECTOR);
>> -
>>  > 	> if (vcpu->guest_debug & KVM_GUESTDBG_ENABLE) {
>> -> 	> 	> if (vcpu->guest_debug &
>> -> 	> 	>     (KVM_GUESTDBG_SINGLESTEP | KVM_GUESTDBG_USE_HW_BP))
>> -> 	> 	> 	> set_exception_intercept(svm, DB_VECTOR);
>>  > 	> 	> if (vcpu->guest_debug & KVM_GUESTDBG_USE_SW_BP)
>>  > 	> 	> 	> set_exception_intercept(svm, BP_VECTOR);
>>  > 	> } else
>> 	> 	> vcpu->guest_debug = 0;
>>  }
>>
>>
>> Then the calls in db_interception and enable_nmi_window can be removed,
>> but the one in svm_guest_debug is important.
> 
> Sorry about that.  I now have with this version:
> 
> From: Paolo Bonzini <pbonzini@xxxxxxxxxx>
> Date: Tue, 10 Nov 2015 09:14:39 +0100
> Subject: KVM: svm: unconditionally intercept #DB
> 
> commit cbdb967af3d54993f5814f1cee0ed311a055377d upstream.
> 
> This is needed to avoid the possibility that the guest triggers
> an infinite stream of #DB exceptions (CVE-2015-8104).
> 
> VMX is not affected: because it does not save DR6 in the VMCS,
> it already intercepts #DB unconditionally.
> 
> Reported-by: Jan Beulich <jbeulich@xxxxxxxx>
> Signed-off-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>
> [bwh: Backported to 3.2, with thanks to Paolo:
>  - update_db_bp_intercept() was called update_db_intercept()
>  - The remaining call is in svm_guest_debug() rather than through svm_x86_ops]
> Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
> ---
>  arch/x86/kvm/svm.c | 14 +++-----------
>  1 file changed, 3 insertions(+), 11 deletions(-)
> 
> --- a/arch/x86/kvm/svm.c
> +++ b/arch/x86/kvm/svm.c
> @@ -1015,6 +1015,7 @@ static void init_vmcb(struct vcpu_svm *s
>  	set_exception_intercept(svm, UD_VECTOR);
>  	set_exception_intercept(svm, MC_VECTOR);
>  	set_exception_intercept(svm, AC_VECTOR);
> +	set_exception_intercept(svm, DB_VECTOR);
>  
>  	set_intercept(svm, INTERCEPT_INTR);
>  	set_intercept(svm, INTERCEPT_NMI);
> @@ -1550,20 +1551,13 @@ static void svm_set_segment(struct kvm_v
>  	mark_dirty(svm->vmcb, VMCB_SEG);
>  }
>  
> -static void update_db_intercept(struct kvm_vcpu *vcpu)
> +static void update_bp_intercept(struct kvm_vcpu *vcpu)
>  {
>  	struct vcpu_svm *svm = to_svm(vcpu);
>  
> -	clr_exception_intercept(svm, DB_VECTOR);
>  	clr_exception_intercept(svm, BP_VECTOR);
>  
> -	if (svm->nmi_singlestep)
> -		set_exception_intercept(svm, DB_VECTOR);
> -
>  	if (vcpu->guest_debug & KVM_GUESTDBG_ENABLE) {
> -		if (vcpu->guest_debug &
> -		    (KVM_GUESTDBG_SINGLESTEP | KVM_GUESTDBG_USE_HW_BP))
> -			set_exception_intercept(svm, DB_VECTOR);
>  		if (vcpu->guest_debug & KVM_GUESTDBG_USE_SW_BP)
>  			set_exception_intercept(svm, BP_VECTOR);
>  	} else
> @@ -1581,7 +1575,7 @@ static void svm_guest_debug(struct kvm_v
>  
>  	mark_dirty(svm->vmcb, VMCB_DR);
>  
> -	update_db_intercept(vcpu);
> +	update_bp_intercept(vcpu);
>  }
>  
>  static void new_asid(struct vcpu_svm *svm, struct svm_cpu_data *sd)
> @@ -1655,7 +1649,6 @@ static int db_interception(struct vcpu_s
>  		if (!(svm->vcpu.guest_debug & KVM_GUESTDBG_SINGLESTEP))
>  			svm->vmcb->save.rflags &=
>  				~(X86_EFLAGS_TF | X86_EFLAGS_RF);
> -		update_db_intercept(&svm->vcpu);
>  	}
>  
>  	if (svm->vcpu.guest_debug &
> @@ -3557,7 +3550,6 @@ static void enable_nmi_window(struct kvm
>  	 */
>  	svm->nmi_singlestep = true;
>  	svm->vmcb->save.rflags |= (X86_EFLAGS_TF | X86_EFLAGS_RF);
> -	update_db_intercept(vcpu);
>  }
>  
>  static int svm_set_tss_addr(struct kvm *kvm, unsigned int addr)
> 

Thanks, this looks good.

Paolo
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