Currently SVM setup is done sequentially in
init_vmcb() -> sev_init_vmcb() -> sev_es_init_vmcb()
and tries keeping SVM/SEV/SEV-ES bits separated. One of the exceptions
is DR intercepts which is for SEV-ES before sev_es_init_vmcb() runs.

Move the SEV-ES intercept setup to sev_es_init_vmcb(). From now on
set_dr_intercepts()/clr_dr_intercepts() handle SVM/SEV only.

No functional change intended.

Suggested-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Alexey Kardashevskiy <aik@amd.com>
Reviewed-by: Santosh Shukla <santosh.shukla@amd.com>
Reviewed-by: Tom Lendacky <thomas.lendacky@amd.com>
---
Changes:
v5:
* updated the comments
* removed sev_es_guest() checks from set_dr_intercepts()/clr_dr_intercepts()
* removed remaining intercepts from clr_dr_intercepts()
---
 arch/x86/kvm/svm/sev.c | 11 ++++++
 arch/x86/kvm/svm/svm.c | 37 ++++++++------------
 2 files changed, 25 insertions(+), 23 deletions(-)

diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
index b4365622222b..f0885250252d 100644
--- a/arch/x86/kvm/svm/sev.c
+++ b/arch/x86/kvm/svm/sev.c
@@ -2946,6 +2946,7 @@ int sev_es_string_io(struct vcpu_svm *svm, int size, unsigned int port, int in)
 
 static void sev_es_init_vmcb(struct vcpu_svm *svm)
 {
+	struct vmcb *vmcb = svm->vmcb01.ptr;
 	struct kvm_vcpu *vcpu = &svm->vcpu;
 
 	svm->vmcb->control.nested_ctl |= SVM_NESTED_CTL_SEV_ES_ENABLE;
@@ -2974,6 +2975,16 @@ static void sev_es_init_vmcb(struct vcpu_svm *svm)
 	svm_set_intercept(svm, TRAP_CR4_WRITE);
 	svm_set_intercept(svm, TRAP_CR8_WRITE);
 
+	/*
+	 * DR7 access must remain intercepted for an SEV-ES guest to disallow
+	 * the guest kernel enable debugging as otherwise a VM writing to DR7
+	 * from the #DB handler may trigger infinite loop of #DB's.
+	 */
+	vmcb->control.intercepts[INTERCEPT_DR] = 0;
+	vmcb_set_intercept(&vmcb->control, INTERCEPT_DR7_READ);
+	vmcb_set_intercept(&vmcb->control, INTERCEPT_DR7_WRITE);
+	recalc_intercepts(svm);
+
 	/* Can't intercept XSETBV, HV can't modify XCR0 directly */
 	svm_clr_intercept(svm, INTERCEPT_XSETBV);
 
diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
index 179952a31d3b..0271360e8fde 100644
--- a/arch/x86/kvm/svm/svm.c
+++ b/arch/x86/kvm/svm/svm.c
@@ -691,23 +691,20 @@ static void set_dr_intercepts(struct vcpu_svm *svm)
 {
 	struct vmcb *vmcb = svm->vmcb01.ptr;
 
-	if (!sev_es_guest(svm->vcpu.kvm)) {
-		vmcb_set_intercept(&vmcb->control, INTERCEPT_DR0_READ);
-		vmcb_set_intercept(&vmcb->control, INTERCEPT_DR1_READ);
-		vmcb_set_intercept(&vmcb->control, INTERCEPT_DR2_READ);
-		vmcb_set_intercept(&vmcb->control, INTERCEPT_DR3_READ);
-		vmcb_set_intercept(&vmcb->control, INTERCEPT_DR4_READ);
-		vmcb_set_intercept(&vmcb->control, INTERCEPT_DR5_READ);
-		vmcb_set_intercept(&vmcb->control, INTERCEPT_DR6_READ);
-		vmcb_set_intercept(&vmcb->control, INTERCEPT_DR0_WRITE);
-		vmcb_set_intercept(&vmcb->control, INTERCEPT_DR1_WRITE);
-		vmcb_set_intercept(&vmcb->control, INTERCEPT_DR2_WRITE);
-		vmcb_set_intercept(&vmcb->control, INTERCEPT_DR3_WRITE);
-		vmcb_set_intercept(&vmcb->control, INTERCEPT_DR4_WRITE);
-		vmcb_set_intercept(&vmcb->control, INTERCEPT_DR5_WRITE);
-		vmcb_set_intercept(&vmcb->control, INTERCEPT_DR6_WRITE);
-	}
-
+	vmcb_set_intercept(&vmcb->control, INTERCEPT_DR0_READ);
+	vmcb_set_intercept(&vmcb->control, INTERCEPT_DR1_READ);
+	vmcb_set_intercept(&vmcb->control, INTERCEPT_DR2_READ);
+	vmcb_set_intercept(&vmcb->control, INTERCEPT_DR3_READ);
+	vmcb_set_intercept(&vmcb->control, INTERCEPT_DR4_READ);
+	vmcb_set_intercept(&vmcb->control, INTERCEPT_DR5_READ);
+	vmcb_set_intercept(&vmcb->control, INTERCEPT_DR6_READ);
+	vmcb_set_intercept(&vmcb->control, INTERCEPT_DR0_WRITE);
+	vmcb_set_intercept(&vmcb->control, INTERCEPT_DR1_WRITE);
+	vmcb_set_intercept(&vmcb->control, INTERCEPT_DR2_WRITE);
+	vmcb_set_intercept(&vmcb->control, INTERCEPT_DR3_WRITE);
+	vmcb_set_intercept(&vmcb->control, INTERCEPT_DR4_WRITE);
+	vmcb_set_intercept(&vmcb->control, INTERCEPT_DR5_WRITE);
+	vmcb_set_intercept(&vmcb->control, INTERCEPT_DR6_WRITE);
 	vmcb_set_intercept(&vmcb->control, INTERCEPT_DR7_READ);
 	vmcb_set_intercept(&vmcb->control, INTERCEPT_DR7_WRITE);
 
@@ -720,12 +717,6 @@ static void clr_dr_intercepts(struct vcpu_svm *svm)
 
 	vmcb->control.intercepts[INTERCEPT_DR] = 0;
 
-	/* DR7 access must remain intercepted for an SEV-ES guest */
-	if (sev_es_guest(svm->vcpu.kvm)) {
-		vmcb_set_intercept(&vmcb->control, INTERCEPT_DR7_READ);
-		vmcb_set_intercept(&vmcb->control, INTERCEPT_DR7_WRITE);
-	}
-
 	recalc_intercepts(svm);
 }
 
-- 
2.39.1

On Tue, Apr 11, 2023, Alexey Kardashevskiy wrote:
> Currently SVM setup is done sequentially in
> init_vmcb() -> sev_init_vmcb() -> sev_es_init_vmcb()
> and tries keeping SVM/SEV/SEV-ES bits separated. One of the exceptions
> is DR intercepts which is for SEV-ES before sev_es_init_vmcb() runs.
> 
> Move the SEV-ES intercept setup to sev_es_init_vmcb(). From now on
> set_dr_intercepts()/clr_dr_intercepts() handle SVM/SEV only.
> 
> No functional change intended.
> 
> Suggested-by: Sean Christopherson <seanjc@google.com>
> Signed-off-by: Alexey Kardashevskiy <aik@amd.com>
> Reviewed-by: Santosh Shukla <santosh.shukla@amd.com>
> Reviewed-by: Tom Lendacky <thomas.lendacky@amd.com>
> ---
> Changes:
> v5:
> * updated the comments
> * removed sev_es_guest() checks from set_dr_intercepts()/clr_dr_intercepts()
> * removed remaining intercepts from clr_dr_intercepts()
> ---
>  arch/x86/kvm/svm/sev.c | 11 ++++++
>  arch/x86/kvm/svm/svm.c | 37 ++++++++------------
>  2 files changed, 25 insertions(+), 23 deletions(-)
> 
> diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
> index b4365622222b..f0885250252d 100644
> --- a/arch/x86/kvm/svm/sev.c
> +++ b/arch/x86/kvm/svm/sev.c
> @@ -2946,6 +2946,7 @@ int sev_es_string_io(struct vcpu_svm *svm, int size, unsigned int port, int in)
>  
>  static void sev_es_init_vmcb(struct vcpu_svm *svm)
>  {
> +	struct vmcb *vmcb = svm->vmcb01.ptr;
>  	struct kvm_vcpu *vcpu = &svm->vcpu;
>  
>  	svm->vmcb->control.nested_ctl |= SVM_NESTED_CTL_SEV_ES_ENABLE;
> @@ -2974,6 +2975,16 @@ static void sev_es_init_vmcb(struct vcpu_svm *svm)
>  	svm_set_intercept(svm, TRAP_CR4_WRITE);
>  	svm_set_intercept(svm, TRAP_CR8_WRITE);
>  
> +	/*
> +	 * DR7 access must remain intercepted for an SEV-ES guest to disallow
> +	 * the guest kernel enable debugging as otherwise a VM writing to DR7
> +	 * from the #DB handler may trigger infinite loop of #DB's.

This is wrong.  The attack isn't writing DR7 in the #DB handler, it's setting up
a #DB on memory that's needed to vector a #DB, e.g. the stack, so that the _CPU_
itself gets stuck in an infinite #DB loop[*].  The guest software handler putting
itself into an infinite loop is a non-issue because it can be interrupted.

[*] https://bugzilla.redhat.com/show_bug.cgi?id=1278496

> +	 */
> +	vmcb->control.intercepts[INTERCEPT_DR] = 0;
> +	vmcb_set_intercept(&vmcb->control, INTERCEPT_DR7_READ);
> +	vmcb_set_intercept(&vmcb->control, INTERCEPT_DR7_WRITE);
> +	recalc_intercepts(svm);