On Thu, Sep 15, 2022 at 05:28:39PM +0800, Chenyi Qiang wrote:
> There are cases that malicious virtual machine can cause CPU stuck (due
> to event windows don't open up), e.g., infinite loop in microcode when
> nested #AC (CVE-2015-5307). No event window means no event (NMI, SMI and
> IRQ) can be delivered. It leads the CPU to be unavailable to host or
> other VMs. Notify VM exit is introduced to mitigate such kind of
> attacks, which will generate a VM exit if no event window occurs in VM
> non-root mode for a specified amount of time (notify window).
>
> A new KVM capability KVM_CAP_X86_NOTIFY_VMEXIT is exposed to user space
> so that the user can query the capability and set the expected notify
> window when creating VMs. The format of the argument when enabling this
> capability is as follows:
> Bit 63:32 - notify window specified in qemu command
> Bit 31:0 - some flags (e.g. KVM_X86_NOTIFY_VMEXIT_ENABLED is set to
> enable the feature.)
>
> Because there are some concerns, e.g. a notify VM exit may happen with
> VM_CONTEXT_INVALID set in exit qualification (no cases are anticipated
> that would set this bit), which means VM context is corrupted. To avoid
> the false positive and a well-behaved guest gets killed, make this
> feature disabled by default. Users can enable the feature by a new
> machine property:
> qemu -machine notify_vmexit=on,notify_window=0 ...
>
> Note that notify_window is only valid when notify_vmexit is on. The valid
> range of notify_window is non-negative. It is even safe to set it to zero
> since there's an internal hardware threshold to be added to ensure no false
> positive.
>
> A new KVM exit reason KVM_EXIT_NOTIFY is defined for notify VM exit. If
> it happens with VM_INVALID_CONTEXT, hypervisor exits to user space to
> inform the fatal case. Then user space can inject a SHUTDOWN event to
> the target vcpu. This is implemented by injecting a sythesized triple
> fault event.
>
> Signed-off-by: Chenyi Qiang <chenyi.qiang@xxxxxxxxx>
> ---
> hw/i386/x86.c | 45 +++++++++++++++++++++++++++++++++++++++++++
> include/hw/i386/x86.h | 5 +++++
> qemu-options.hx | 10 +++++++++-
> target/i386/kvm/kvm.c | 28 +++++++++++++++++++++++++++
> 4 files changed, 87 insertions(+), 1 deletion(-)
>
> diff --git a/hw/i386/x86.c b/hw/i386/x86.c
> index 050eedc0c8..1eccbd3deb 100644
> --- a/hw/i386/x86.c
> +++ b/hw/i386/x86.c
> @@ -1379,6 +1379,37 @@ static void machine_set_sgx_epc(Object *obj, Visitor *v, const char *name,
> qapi_free_SgxEPCList(list);
> }
>
> +static bool x86_machine_get_notify_vmexit(Object *obj, Error **errp)
> +{
> + X86MachineState *x86ms = X86_MACHINE(obj);
> +
> + return x86ms->notify_vmexit;
> +}
> +
> +static void x86_machine_set_notify_vmexit(Object *obj, bool value, Error **errp)
> +{
> + X86MachineState *x86ms = X86_MACHINE(obj);
> +
> + x86ms->notify_vmexit = value;
> +}
> +
> +static void x86_machine_get_notify_window(Object *obj, Visitor *v,
> + const char *name, void *opaque, Error **errp)
> +{
> + X86MachineState *x86ms = X86_MACHINE(obj);
> + uint32_t notify_window = x86ms->notify_window;
> +
> + visit_type_uint32(v, name, ¬ify_window, errp);
> +}
> +
> +static void x86_machine_set_notify_window(Object *obj, Visitor *v,
> + const char *name, void *opaque, Error **errp)
> +{
> + X86MachineState *x86ms = X86_MACHINE(obj);
> +
> + visit_type_uint32(v, name, &x86ms->notify_window, errp);
> +}
> +
> static void x86_machine_initfn(Object *obj)
> {
> X86MachineState *x86ms = X86_MACHINE(obj);
> @@ -1392,6 +1423,8 @@ static void x86_machine_initfn(Object *obj)
> x86ms->oem_table_id = g_strndup(ACPI_BUILD_APPNAME8, 8);
> x86ms->bus_lock_ratelimit = 0;
> x86ms->above_4g_mem_start = 4 * GiB;
> + x86ms->notify_vmexit = false;
> + x86ms->notify_window = 0;
> }
>
> static void x86_machine_class_init(ObjectClass *oc, void *data)
> @@ -1461,6 +1494,18 @@ static void x86_machine_class_init(ObjectClass *oc, void *data)
> NULL, NULL);
> object_class_property_set_description(oc, "sgx-epc",
> "SGX EPC device");
> +
> + object_class_property_add(oc, X86_MACHINE_NOTIFY_WINDOW, "uint32_t",
> + x86_machine_get_notify_window,
> + x86_machine_set_notify_window, NULL, NULL);
> + object_class_property_set_description(oc, X86_MACHINE_NOTIFY_WINDOW,
> + "Set the notify window required by notify VM exit");
> +
> + object_class_property_add_bool(oc, X86_MACHINE_NOTIFY_VMEXIT,
> + x86_machine_get_notify_vmexit,
> + x86_machine_set_notify_vmexit);
> + object_class_property_set_description(oc, X86_MACHINE_NOTIFY_VMEXIT,
> + "Enable notify VM exit");
> }
>
> static const TypeInfo x86_machine_info = {
> diff --git a/include/hw/i386/x86.h b/include/hw/i386/x86.h
> index 62fa5774f8..5707329fa7 100644
> --- a/include/hw/i386/x86.h
> +++ b/include/hw/i386/x86.h
> @@ -85,6 +85,9 @@ struct X86MachineState {
> * which means no limitation on the guest's bus locks.
> */
> uint64_t bus_lock_ratelimit;
> +
> + bool notify_vmexit;
> + uint32_t notify_window;
> };
>
> #define X86_MACHINE_SMM "smm"
> @@ -94,6 +97,8 @@ struct X86MachineState {
> #define X86_MACHINE_OEM_ID "x-oem-id"
> #define X86_MACHINE_OEM_TABLE_ID "x-oem-table-id"
> #define X86_MACHINE_BUS_LOCK_RATELIMIT "bus-lock-ratelimit"
> +#define X86_MACHINE_NOTIFY_VMEXIT "notify-vmexit"
> +#define X86_MACHINE_NOTIFY_WINDOW "notify-window"
>
> #define TYPE_X86_MACHINE MACHINE_TYPE_NAME("x86")
> OBJECT_DECLARE_TYPE(X86MachineState, X86MachineClass, X86_MACHINE)
> diff --git a/qemu-options.hx b/qemu-options.hx
> index 31c04f7eea..3cdeeac8f3 100644
> --- a/qemu-options.hx
> +++ b/qemu-options.hx
> @@ -37,7 +37,8 @@ DEF("machine", HAS_ARG, QEMU_OPTION_machine, \
> " memory-encryption=@var{} memory encryption object to use (default=none)\n"
> " hmat=on|off controls ACPI HMAT support (default=off)\n"
> " memory-backend='backend-id' specifies explicitly provided backend for main RAM (default=none)\n"
> - " cxl-fmw.0.targets.0=firsttarget,cxl-fmw.0.targets.1=secondtarget,cxl-fmw.0.size=size[,cxl-fmw.0.interleave-granularity=granularity]\n",
> + " cxl-fmw.0.targets.0=firsttarget,cxl-fmw.0.targets.1=secondtarget,cxl-fmw.0.size=size[,cxl-fmw.0.interleave-granularity=granularity]\n"
> + " notify_vmexit=on|off,notify_window=n controls notify VM exit support (default=off) and specifies the notify window size (default=0)\n",
> QEMU_ARCH_ALL)
> SRST
> ``-machine [type=]name[,prop=value[,...]]``
> @@ -157,6 +158,13 @@ SRST
> ::
>
> -machine cxl-fmw.0.targets.0=cxl.0,cxl-fmw.0.targets.1=cxl.1,cxl-fmw.0.size=128G,cxl-fmw.0.interleave-granularity=512k
> +
> + ``notify_vmexit=on|off,notify_window=n``
> + Enables or disables Notify VM exit support on x86 host and specify
> + the corresponding notify window to trigger the VM exit if enabled.
> + This feature can mitigate the CPU stuck issue due to event windows
> + don't open up for a specified of time (notify window).
> + The default is off.
> ERST
>
> DEF("M", HAS_ARG, QEMU_OPTION_M,
> diff --git a/target/i386/kvm/kvm.c b/target/i386/kvm/kvm.c
> index 3838827134..ae7fb2c495 100644
> --- a/target/i386/kvm/kvm.c
> +++ b/target/i386/kvm/kvm.c
> @@ -2597,6 +2597,20 @@ int kvm_arch_init(MachineState *ms, KVMState *s)
> ratelimit_set_speed(&bus_lock_ratelimit_ctrl,
> x86ms->bus_lock_ratelimit, BUS_LOCK_SLICE_TIME);
> }
> +
> + if (x86ms->notify_vmexit &&
> + kvm_check_extension(s, KVM_CAP_X86_NOTIFY_VMEXIT)) {
> + uint64_t notify_window_flags = ((uint64_t)x86ms->notify_window << 32) |
> + KVM_X86_NOTIFY_VMEXIT_ENABLED |
> + KVM_X86_NOTIFY_VMEXIT_USER;
It'll always request a user exit here as long as enabled, then...
> + ret = kvm_vm_enable_cap(s, KVM_CAP_X86_NOTIFY_VMEXIT, 0,
> + notify_window_flags);
> + if (ret < 0) {
> + error_report("kvm: Failed to enable notify vmexit cap: %s",
> + strerror(-ret));
> + return ret;
> + }
> + }
> }
>
> return 0;
> @@ -5141,6 +5155,7 @@ int kvm_arch_handle_exit(CPUState *cs, struct kvm_run *run)
> X86CPU *cpu = X86_CPU(cs);
> uint64_t code;
> int ret;
> + struct kvm_vcpu_events events = {};
>
> switch (run->exit_reason) {
> case KVM_EXIT_HLT:
> @@ -5196,6 +5211,19 @@ int kvm_arch_handle_exit(CPUState *cs, struct kvm_run *run)
> /* already handled in kvm_arch_post_run */
> ret = 0;
> break;
> + case KVM_EXIT_NOTIFY:
> + ret = 0;
> + if (run->notify.flags & KVM_NOTIFY_CONTEXT_INVALID) {
> + warn_report("KVM: invalid context due to notify vmexit");
> + if (has_triple_fault_event) {
> + events.flags |= KVM_VCPUEVENT_VALID_TRIPLE_FAULT;
> + events.triple_fault.pending = true;
> + ret = kvm_vcpu_ioctl(cs, KVM_SET_VCPU_EVENTS, &events);
> + } else {
> + ret = -1;
> + }
> + }
... should we do something even if the context is valid? Or I'm a bit
confused why KVM_X86_NOTIFY_VMEXIT_USER was set (IIUC we can just enable it
without setting VMEXIT_USER then).
Not sure some warning would be also useful here, but I really don't know
the whole context so I can't tell whether there can easily be false
positives to pollute qemu log.
> + break;
> default:
> fprintf(stderr, "KVM: unknown exit reason %d\n", run->exit_reason);
> ret = -1;
> --
> 2.17.1
>
--
Peter Xu
