[PATCH 1/2] KVM: x86: Handle triple fault in L2 without killing L1

Sean Christopherson
2021-03-03T07:23:43Z

Synthesize a nested VM-Exit if L2 triggers an emulated triple fault
instead of exiting to userspace, which likely will kill L1. Any flow
that does KVM_REQ_TRIPLE_FAULT is suspect, but the most common scenario
for L2 killing L1 is if L0 (KVM) intercepts a contributory exception that
is _not_intercepted by L1. E.g. if KVM is intercepting #GPs for the
VMware backdoor, a #GP that occurs in L2 while vectoring an injected #DF
will cause KVM to emulate triple fault.

Cc: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Cc: Jim Mattson <jmattson@google.com>
Cc: stable@vger.kernel.org
Signed-off-by: Sean Christopherson <seanjc@google.com>
---
arch/x86/include/asm/kvm_host.h | 1 +
arch/x86/kvm/lapic.c | 2 +-
arch/x86/kvm/svm/nested.c | 14 ++++++++++++++
arch/x86/kvm/vmx/nested.c | 9 +++++++++
arch/x86/kvm/x86.c | 29 +++++++++++++++++++++++------
arch/x86/kvm/x86.h | 2 ++
6 files changed, 50 insertions(+), 7 deletions(-)

diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
index 6db60ea8ee5b..3668f38bad60 100644
--- a/arch/x86/include/asm/kvm_host.h
+++ b/arch/x86/include/asm/kvm_host.h
@@ -1333,6 +1333,7 @@ struct kvm_x86_ops {
struct kvm_x86_nested_ops {
int (*check_events)(struct kvm_vcpu *vcpu);
bool (*hv_timer_pending)(struct kvm_vcpu *vcpu);
+ void (*triple_fault)(struct kvm_vcpu *vcpu);
int (*get_state)(struct kvm_vcpu *vcpu,
struct kvm_nested_state __user *user_kvm_nested_state,
unsigned user_data_size);
diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
index 45d40bfacb7c..9faed6e6960a 100644
--- a/arch/x86/kvm/lapic.c
+++ b/arch/x86/kvm/lapic.c
@@ -2859,7 +2859,7 @@ void kvm_apic_accept_events(struct kvm_vcpu *vcpu)
return;

if (is_guest_mode(vcpu)) {
- r = kvm_x86_ops.nested_ops->check_events(vcpu);
+ r = kvm_check_nested_events(vcpu);
if (r < 0)
return;
/*
diff --git a/arch/x86/kvm/svm/nested.c b/arch/x86/kvm/svm/nested.c
index 90a1704b5752..93a61ed76e5b 100644
--- a/arch/x86/kvm/svm/nested.c
+++ b/arch/x86/kvm/svm/nested.c
@@ -671,6 +671,9 @@ int nested_svm_vmexit(struct vcpu_svm *svm)
struct kvm_host_map map;
int rc;

+ /* Triple faults in L2 should never escape. */
+ WARN_ON_ONCE(kvm_check_request(KVM_REQ_TRIPLE_FAULT, vcpu));
+
rc = kvm_vcpu_map(vcpu, gpa_to_gfn(svm->nested.vmcb12_gpa), &map);
if (rc) {
if (rc == -EINVAL)
@@ -792,6 +795,16 @@ int nested_svm_vmexit(struct vcpu_svm *svm)
return 0;
}

+static void nested_svm_triple_fault(struct kvm_vcpu *vcpu)
+{
+ struct vcpu_svm *svm = to_svm(vcpu);
+
+ svm->vmcb->control.exit_code = SVM_EXIT_SHUTDOWN;
+ svm->vmcb->control.exit_info_1 = 0;
+ svm->vmcb->control.exit_info_2 = 0;
+ nested_svm_vmexit(svm);
+}
+
int svm_allocate_nested(struct vcpu_svm *svm)
{
struct page *vmcb02_page;
@@ -1327,6 +1340,7 @@ static int svm_set_nested_state(struct kvm_vcpu *vcpu,

struct kvm_x86_nested_ops svm_nested_ops = {
.check_events = svm_check_nested_events,
+ .triple_fault = nested_svm_triple_fault,
.get_nested_state_pages = svm_get_nested_state_pages,
.get_state = svm_get_nested_state,
.set_state = svm_set_nested_state,
diff --git a/arch/x86/kvm/vmx/nested.c b/arch/x86/kvm/vmx/nested.c
index fdd80dd8e781..3493a48411a8 100644
--- a/arch/x86/kvm/vmx/nested.c
+++ b/arch/x86/kvm/vmx/nested.c
@@ -4416,6 +4416,9 @@ void nested_vmx_vmexit(struct kvm_vcpu *vcpu, u32 vm_exit_reason,
/* trying to cancel vmlaunch/vmresume is a bug */
WARN_ON_ONCE(vmx->nested.nested_run_pending);

+ /* Similarly, triple faults in L2 should never escape. */
+ WARN_ON_ONCE(kvm_check_request(KVM_REQ_TRIPLE_FAULT, vcpu));
+
kvm_clear_request(KVM_REQ_GET_NESTED_STATE_PAGES, vcpu);

/* Service the TLB flush request for L2 before switching to L1. */
@@ -4552,6 +4555,11 @@ void nested_vmx_vmexit(struct kvm_vcpu *vcpu, u32 vm_exit_reason,
vmx->fail = 0;
}

+static void nested_vmx_triple_fault(struct kvm_vcpu *vcpu)
+{
+ nested_vmx_vmexit(vcpu, EXIT_REASON_TRIPLE_FAULT, 0, 0);
+}
+
/*
* Decode the memory-address operand of a vmx instruction, as recorded on an
* exit caused by such an instruction (run by a guest hypervisor).
@@ -6593,6 +6601,7 @@ __init int nested_vmx_hardware_setup(int (*exit_handlers[])(struct kvm_vcpu *))
struct kvm_x86_nested_ops vmx_nested_ops = {
.check_events = vmx_check_nested_events,
.hv_timer_pending = nested_vmx_preemption_timer_pending,
+ .triple_fault = nested_vmx_triple_fault,
.get_state = vmx_get_nested_state,
.set_state = vmx_set_nested_state,
.get_nested_state_pages = vmx_get_nested_state_pages,
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 828de7d65074..11746f488f04 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -8349,6 +8349,19 @@ static void update_cr8_intercept(struct kvm_vcpu *vcpu)
static_call(kvm_x86_update_cr8_intercept)(vcpu, tpr, max_irr);
}

+int kvm_check_nested_events(struct kvm_vcpu *vcpu)
+{
+ if (WARN_ON_ONCE(!is_guest_mode(vcpu)))
+ return -EIO;
+
+ if (kvm_check_request(KVM_REQ_TRIPLE_FAULT, vcpu)) {
+ kvm_x86_ops.nested_ops->triple_fault(vcpu);
+ return 1;
+ }
+
+ return kvm_x86_ops.nested_ops->check_events(vcpu);
+}
+
static void kvm_inject_exception(struct kvm_vcpu *vcpu)
{
if (vcpu->arch.exception.error_code && !is_protmode(vcpu))
@@ -8401,7 +8414,7 @@ static void inject_pending_event(struct kvm_vcpu *vcpu, bool *req_immediate_exit
* from L2 to L1.
*/
if (is_guest_mode(vcpu)) {
- r = kvm_x86_ops.nested_ops->check_events(vcpu);
+ r = kvm_check_nested_events(vcpu);
if (r < 0)
goto busy;
}
@@ -8964,10 +8977,14 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
goto out;
}
if (kvm_check_request(KVM_REQ_TRIPLE_FAULT, vcpu)) {
- vcpu->run->exit_reason = KVM_EXIT_SHUTDOWN;
- vcpu->mmio_needed = 0;
- r = 0;
- goto out;
+ if (is_guest_mode(vcpu)) {
+ kvm_x86_ops.nested_ops->triple_fault(vcpu);
+ } else {
+ vcpu->run->exit_reason = KVM_EXIT_SHUTDOWN;
+ vcpu->mmio_needed = 0;
+ r = 0;
+ goto out;
+ }
}
if (kvm_check_request(KVM_REQ_APF_HALT, vcpu)) {
/* Page is swapped out. Do synthetic halt */
@@ -9265,7 +9282,7 @@ static inline int vcpu_block(struct kvm *kvm, struct kvm_vcpu *vcpu)
static inline bool kvm_vcpu_running(struct kvm_vcpu *vcpu)
{
if (is_guest_mode(vcpu))
- kvm_x86_ops.nested_ops->check_events(vcpu);
+ kvm_check_nested_events(vcpu);

return (vcpu->arch.mp_state == KVM_MP_STATE_RUNNABLE &&
!vcpu->arch.apf.halted);
diff --git a/arch/x86/kvm/x86.h b/arch/x86/kvm/x86.h
index ee6e01067884..daccf20fbcd5 100644
--- a/arch/x86/kvm/x86.h
+++ b/arch/x86/kvm/x86.h
@@ -56,6 +56,8 @@ static inline unsigned int __shrink_ple_window(unsigned int val,

#define MSR_IA32_CR_PAT_DEFAULT 0x0007040600070406ULL

+int kvm_check_nested_events(struct kvm_vcpu *vcpu);
+
static inline void kvm_clear_exception_queue(struct kvm_vcpu *vcpu)
{
vcpu->arch.exception.pending = false;