On Mon, Sep 19, 2022 at 01:46:38PM +0800, Chenyi Qiang wrote:
>
>
> On 9/17/2022 5:57 AM, Peter Xu wrote:
> > 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
>
>
> Yes, make sense. A warning log is necessary if the context is valid.
>
> > confused why KVM_X86_NOTIFY_VMEXIT_USER was set (IIUC we can just enable it
> > without setting VMEXIT_USER then).
> >
>
> VMEXIT_USR was set because KVM community prefers userspace can get notified
> and help to do some analysis or mitigation if notify window was exceeded.
>
> > 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.
> >
>
> The false positive case is not easy to happen unless some potential issues
> in silicon. But in case of it, to avoid polluting qemu log, how about:
>
> diff --git a/target/i386/kvm/kvm.c b/target/i386/kvm/kvm.c
> index ae7fb2c495..8f97133cbf 100644
> --- a/target/i386/kvm/kvm.c
> +++ b/target/i386/kvm/kvm.c
> @@ -5213,6 +5213,7 @@ int kvm_arch_handle_exit(CPUState *cs, struct kvm_run
> *run)
> break;
> case KVM_EXIT_NOTIFY:
> ret = 0;
> + warn_report_once("KVM: notify window was exceeded in guest");
Is there more informative way to dump this? If it's 99% that the guest was
doing something weird and needs attention, maybe worthwhile to point that
out directly to the admin?
> if (run->notify.flags & KVM_NOTIFY_CONTEXT_INVALID) {
> warn_report("KVM: invalid context due to notify vmexit");
> if (has_triple_fault_event) {
Adding a warning looks good to me, with that (or in any better form of
wording):
Acked-by: Peter Xu <peterx@xxxxxxxxxx>
Thanks,
--
Peter Xu
