Nested hypervisor should not see singlestep VM exits if singlestepping
was enabled internally by KVM. Windows is particularly sensitive to this
and known to bluescreen on unexpected VM exits.
Signed-off-by: Ladi Prosek <lprosek@xxxxxxxxxx>
---
arch/x86/kvm/svm.c | 45 ++++++++++++++++++++++++++++++++++++++++-----
1 file changed, 40 insertions(+), 5 deletions(-)
diff --git a/arch/x86/kvm/svm.c b/arch/x86/kvm/svm.c
index 27521a0..83c4e21 100644
--- a/arch/x86/kvm/svm.c
+++ b/arch/x86/kvm/svm.c
@@ -189,6 +189,7 @@ struct vcpu_svm {
struct nested_state nested;
bool nmi_singlestep;
+ u64 nmi_singlestep_guest_rflags;
unsigned int3_injected;
unsigned long int3_rip;
@@ -966,9 +967,13 @@ static void svm_disable_lbrv(struct vcpu_svm *svm)
static void disable_nmi_singlestep(struct vcpu_svm *svm)
{
svm->nmi_singlestep = false;
- if (!(svm->vcpu.guest_debug & KVM_GUESTDBG_SINGLESTEP))
- svm->vmcb->save.rflags &=
- ~(X86_EFLAGS_TF | X86_EFLAGS_RF);
+ if (!(svm->vcpu.guest_debug & KVM_GUESTDBG_SINGLESTEP)) {
+ /* Clear our flags if they were not set by the guest */
+ if (!(svm->nmi_singlestep_guest_rflags & X86_EFLAGS_TF))
+ svm->vmcb->save.rflags &= ~X86_EFLAGS_TF;
+ if (!(svm->nmi_singlestep_guest_rflags & X86_EFLAGS_RF))
+ svm->vmcb->save.rflags &= ~X86_EFLAGS_RF;
+ }
}
/* Note:
@@ -2538,6 +2543,31 @@ static int nested_svm_exit_handled_msr(struct vcpu_svm *svm)
return (value & mask) ? NESTED_EXIT_DONE : NESTED_EXIT_HOST;
}
+/* DB exceptions for our internal use must not cause vmexit */
+static int nested_svm_intercept_db(struct vcpu_svm *svm)
+{
+ unsigned long dr6;
+
+ /* if we're not singlestepping, it's not ours */
+ if (!svm->nmi_singlestep)
+ return NESTED_EXIT_DONE;
+
+ /* if it's not a singlestep exception, it's not ours */
+ if (kvm_get_dr(&svm->vcpu, 6, &dr6))
+ return NESTED_EXIT_DONE;
+ if (!(dr6 & DR6_BS))
+ return NESTED_EXIT_DONE;
+
+ /* if the guest is singlestepping, it should get the vmexit */
+ if (svm->nmi_singlestep_guest_rflags & X86_EFLAGS_TF) {
+ disable_nmi_singlestep(svm);
+ return NESTED_EXIT_DONE;
+ }
+
+ /* it's ours, the nested hypervisor must not see this one */
+ return NESTED_EXIT_HOST;
+}
+
static int nested_svm_exit_special(struct vcpu_svm *svm)
{
u32 exit_code = svm->vmcb->control.exit_code;
@@ -2593,8 +2623,12 @@ static int nested_svm_intercept(struct vcpu_svm *svm)
}
case SVM_EXIT_EXCP_BASE ... SVM_EXIT_EXCP_BASE + 0x1f: {
u32 excp_bits = 1 << (exit_code - SVM_EXIT_EXCP_BASE);
- if (svm->nested.intercept_exceptions & excp_bits)
- vmexit = NESTED_EXIT_DONE;
+ if (svm->nested.intercept_exceptions & excp_bits) {
+ if (exit_code == SVM_EXIT_EXCP_BASE + DB_VECTOR)
+ vmexit = nested_svm_intercept_db(svm);
+ else
+ vmexit = NESTED_EXIT_DONE;
+ }
/* async page fault always cause vmexit */
else if ((exit_code == SVM_EXIT_EXCP_BASE + PF_VECTOR) &&
svm->apf_reason != 0)
@@ -4635,6 +4669,7 @@ static void enable_nmi_window(struct kvm_vcpu *vcpu)
* Something prevents NMI from been injected. Single step over possible
* problem (IRET or exception injection or interrupt shadow)
*/
+ svm->nmi_singlestep_guest_rflags = svm_get_rflags(vcpu);
svm->nmi_singlestep = true;
svm->vmcb->save.rflags |= (X86_EFLAGS_TF | X86_EFLAGS_RF);
}
--
2.9.3
