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

Started byBen Hutchings <ben@decadent.org.uk>
First post2015-11-24 23:50 +0100
Last post2015-11-25 19:10 +0100
Articles 4 — 2 participants

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  [PATCH 3.2 41/52] KVM: svm: unconditionally intercept #DB Ben Hutchings <ben@decadent.org.uk> - 2015-11-24 23:50 +0100
    Re: [PATCH 3.2 41/52] KVM: svm: unconditionally intercept #DB Paolo Bonzini <pbonzini@redhat.com> - 2015-11-25 12:40 +0100
      Re: [PATCH 3.2 41/52] KVM: svm: unconditionally intercept #DB Ben Hutchings <ben@decadent.org.uk> - 2015-11-25 19:00 +0100
        Re: [PATCH 3.2 41/52] KVM: svm: unconditionally intercept #DB Paolo Bonzini <pbonzini@redhat.com> - 2015-11-25 19:10 +0100

#1276877 — [PATCH 3.2 41/52] KVM: svm: unconditionally intercept #DB

FromBen Hutchings <ben@decadent.org.uk>
Date2015-11-24 23:50 +0100
Subject[PATCH 3.2 41/52] KVM: svm: unconditionally intercept #DB
Message-ID<qyuzq-5RA-65@gated-at.bofh.it>
3.2.74-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Paolo Bonzini <pbonzini@redhat.com>

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@suse.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
[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]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
---
 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);
@@ -1550,26 +1551,6 @@ static void svm_set_segment(struct kvm_v
 	mark_dirty(svm->vmcb, VMCB_SEG);
 }
 
-static void update_db_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;
-}
-
 static void svm_guest_debug(struct kvm_vcpu *vcpu, struct kvm_guest_debug *dbg)
 {
 	struct vcpu_svm *svm = to_svm(vcpu);
@@ -1580,8 +1561,6 @@ static void svm_guest_debug(struct kvm_v
 		svm->vmcb->save.dr7 = vcpu->arch.dr7;
 
 	mark_dirty(svm->vmcb, VMCB_DR);
-
-	update_db_intercept(vcpu);
 }
 
 static void new_asid(struct vcpu_svm *svm, struct svm_cpu_data *sd)
@@ -1655,7 +1634,6 @@ 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 +3535,6 @@ 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)

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#1277263

FromPaolo Bonzini <pbonzini@redhat.com>
Date2015-11-25 12:40 +0100
Message-ID<qyGAy-5Fi-19@gated-at.bofh.it>
In reply to#1276877

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@redhat.com>
> 
> 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@suse.com>
> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
> [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.

Paolo
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#1277656

FromBen Hutchings <ben@decadent.org.uk>
Date2015-11-25 19:00 +0100
Message-ID<qyMwj-12v-9@gated-at.bofh.it>
In reply to#1277263

[Multipart message — attachments visible in raw view] — view raw

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@redhat.com>
> > 
> > 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@suse.com>
> > Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
> > [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@redhat.com>
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@suse.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
[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@decadent.org.uk>
---
 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)

-- 
Ben Hutchings
This sentence contradicts itself - no actually it doesn't.

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#1277673

FromPaolo Bonzini <pbonzini@redhat.com>
Date2015-11-25 19:10 +0100
Message-ID<qyMFZ-1lB-35@gated-at.bofh.it>
In reply to#1277656

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@redhat.com>
>>>
>>> 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@suse.com>
>>> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
>>> [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@redhat.com>
> 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@suse.com>
> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
> [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@decadent.org.uk>
> ---
>  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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