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    SubjectRe: [PATCH v2] KVM: VMX: Enable Notify VM exit
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    On 5/25/2021 1:12 PM, Tao Xu wrote:
    > There are some cases that malicious virtual machines can cause CPU stuck
    > (event windows don't open up), e.g., infinite loop in microcode when
    > nested #AC (CVE-2015-5307). No event window obviously means no events,
    > e.g. NMIs, SMIs, and IRQs will all be blocked, may cause the related
    > hardware CPU can't be used by host or other VM.
    >
    > To resolve those cases, it can enable a notify VM exit if no event
    > window occur in VMX non-root mode for a specified amount of time
    > (notify window). Since CPU is first observed the risk of not causing
    > forward progress, after notify window time in a units of crystal clock,
    > Notify VM exit will happen. Notify VM exit can happen incident to delivery
    > of a vectored event.
    >
    > Expose a module param for configuring notify window, which is in unit of
    > crystal clock cycle.
    > - A negative value (e.g. -1) is to disable this feature.
    > - Make the default as 0. It is safe because an internal threshold is added
    > to notify window to ensure all the normal instructions being coverd.

    Paolo, Andy, and Sean,

    Do you have any comment on this patch? Especially about making default
    notify_window as 0.

    Thanks,
    -Xiaoyao

    > - User can set it to a large value when they want to give more cycles to
    > wait for some reasons, e.g., silicon wrongly kill some normal instruction
    > due to internal threshold is too small.
    >
    > Notify VM exit is defined in latest Intel Architecture Instruction Set
    > Extensions Programming Reference, chapter 9.2.
    >
    > Co-developed-by: Xiaoyao Li <xiaoyao.li@intel.com>
    > Signed-off-by: Xiaoyao Li <xiaoyao.li@intel.com>
    > Signed-off-by: Tao Xu <tao3.xu@intel.com>
    > ---
    >
    > Changelog:
    > v2:
    > Default set notify window to 0, less than 0 to disable.
    > Add more description in commit message.
    > ---
    > arch/x86/include/asm/vmx.h | 7 +++++
    > arch/x86/include/asm/vmxfeatures.h | 1 +
    > arch/x86/include/uapi/asm/vmx.h | 4 ++-
    > arch/x86/kvm/vmx/capabilities.h | 6 +++++
    > arch/x86/kvm/vmx/vmx.c | 42 ++++++++++++++++++++++++++++--
    > include/uapi/linux/kvm.h | 2 ++
    > 6 files changed, 59 insertions(+), 3 deletions(-)
    >
    > diff --git a/arch/x86/include/asm/vmx.h b/arch/x86/include/asm/vmx.h
    > index 0ffaa3156a4e..9104c85a973f 100644
    > --- a/arch/x86/include/asm/vmx.h
    > +++ b/arch/x86/include/asm/vmx.h
    > @@ -74,6 +74,7 @@
    > #define SECONDARY_EXEC_TSC_SCALING VMCS_CONTROL_BIT(TSC_SCALING)
    > #define SECONDARY_EXEC_ENABLE_USR_WAIT_PAUSE VMCS_CONTROL_BIT(USR_WAIT_PAUSE)
    > #define SECONDARY_EXEC_BUS_LOCK_DETECTION VMCS_CONTROL_BIT(BUS_LOCK_DETECTION)
    > +#define SECONDARY_EXEC_NOTIFY_VM_EXITING VMCS_CONTROL_BIT(NOTIFY_VM_EXITING)
    >
    > #define PIN_BASED_EXT_INTR_MASK VMCS_CONTROL_BIT(INTR_EXITING)
    > #define PIN_BASED_NMI_EXITING VMCS_CONTROL_BIT(NMI_EXITING)
    > @@ -269,6 +270,7 @@ enum vmcs_field {
    > SECONDARY_VM_EXEC_CONTROL = 0x0000401e,
    > PLE_GAP = 0x00004020,
    > PLE_WINDOW = 0x00004022,
    > + NOTIFY_WINDOW = 0x00004024,
    > VM_INSTRUCTION_ERROR = 0x00004400,
    > VM_EXIT_REASON = 0x00004402,
    > VM_EXIT_INTR_INFO = 0x00004404,
    > @@ -555,6 +557,11 @@ enum vm_entry_failure_code {
    > #define EPT_VIOLATION_EXECUTABLE (1 << EPT_VIOLATION_EXECUTABLE_BIT)
    > #define EPT_VIOLATION_GVA_TRANSLATED (1 << EPT_VIOLATION_GVA_TRANSLATED_BIT)
    >
    > +/*
    > + * Exit Qualifications for NOTIFY VM EXIT
    > + */
    > +#define NOTIFY_VM_CONTEXT_INVALID BIT(0)
    > +
    > /*
    > * VM-instruction error numbers
    > */
    > diff --git a/arch/x86/include/asm/vmxfeatures.h b/arch/x86/include/asm/vmxfeatures.h
    > index d9a74681a77d..15f0f2ab4f95 100644
    > --- a/arch/x86/include/asm/vmxfeatures.h
    > +++ b/arch/x86/include/asm/vmxfeatures.h
    > @@ -84,5 +84,6 @@
    > #define VMX_FEATURE_USR_WAIT_PAUSE ( 2*32+ 26) /* Enable TPAUSE, UMONITOR, UMWAIT in guest */
    > #define VMX_FEATURE_ENCLV_EXITING ( 2*32+ 28) /* "" VM-Exit on ENCLV (leaf dependent) */
    > #define VMX_FEATURE_BUS_LOCK_DETECTION ( 2*32+ 30) /* "" VM-Exit when bus lock caused */
    > +#define VMX_FEATURE_NOTIFY_VM_EXITING ( 2*32+ 31) /* VM-Exit when no event windows after notify window */
    >
    > #endif /* _ASM_X86_VMXFEATURES_H */
    > diff --git a/arch/x86/include/uapi/asm/vmx.h b/arch/x86/include/uapi/asm/vmx.h
    > index 946d761adbd3..ef4c80f6553e 100644
    > --- a/arch/x86/include/uapi/asm/vmx.h
    > +++ b/arch/x86/include/uapi/asm/vmx.h
    > @@ -91,6 +91,7 @@
    > #define EXIT_REASON_UMWAIT 67
    > #define EXIT_REASON_TPAUSE 68
    > #define EXIT_REASON_BUS_LOCK 74
    > +#define EXIT_REASON_NOTIFY 75
    >
    > #define VMX_EXIT_REASONS \
    > { EXIT_REASON_EXCEPTION_NMI, "EXCEPTION_NMI" }, \
    > @@ -153,7 +154,8 @@
    > { EXIT_REASON_XRSTORS, "XRSTORS" }, \
    > { EXIT_REASON_UMWAIT, "UMWAIT" }, \
    > { EXIT_REASON_TPAUSE, "TPAUSE" }, \
    > - { EXIT_REASON_BUS_LOCK, "BUS_LOCK" }
    > + { EXIT_REASON_BUS_LOCK, "BUS_LOCK" }, \
    > + { EXIT_REASON_NOTIFY, "NOTIFY"}
    >
    > #define VMX_EXIT_REASON_FLAGS \
    > { VMX_EXIT_REASONS_FAILED_VMENTRY, "FAILED_VMENTRY" }
    > diff --git a/arch/x86/kvm/vmx/capabilities.h b/arch/x86/kvm/vmx/capabilities.h
    > index 8dee8a5fbc17..8527f34a84ac 100644
    > --- a/arch/x86/kvm/vmx/capabilities.h
    > +++ b/arch/x86/kvm/vmx/capabilities.h
    > @@ -407,4 +407,10 @@ static inline u64 vmx_supported_debugctl(void)
    > return debugctl;
    > }
    >
    > +static inline bool cpu_has_notify_vm_exiting(void)
    > +{
    > + return vmcs_config.cpu_based_2nd_exec_ctrl &
    > + SECONDARY_EXEC_NOTIFY_VM_EXITING;
    > +}
    > +
    > #endif /* __KVM_X86_VMX_CAPS_H */
    > diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
    > index 4bceb5ca3a89..c0ad01c88dac 100644
    > --- a/arch/x86/kvm/vmx/vmx.c
    > +++ b/arch/x86/kvm/vmx/vmx.c
    > @@ -205,6 +205,10 @@ module_param(ple_window_max, uint, 0444);
    > int __read_mostly pt_mode = PT_MODE_SYSTEM;
    > module_param(pt_mode, int, S_IRUGO);
    >
    > +/* Default is 0, less than 0 (for example, -1) disables notify window. */
    > +static int __read_mostly notify_window;
    > +module_param(notify_window, int, 0644);
    > +
    > static DEFINE_STATIC_KEY_FALSE(vmx_l1d_should_flush);
    > static DEFINE_STATIC_KEY_FALSE(vmx_l1d_flush_cond);
    > static DEFINE_MUTEX(vmx_l1d_flush_mutex);
    > @@ -2539,7 +2543,8 @@ static __init int setup_vmcs_config(struct vmcs_config *vmcs_conf,
    > SECONDARY_EXEC_PT_USE_GPA |
    > SECONDARY_EXEC_PT_CONCEAL_VMX |
    > SECONDARY_EXEC_ENABLE_VMFUNC |
    > - SECONDARY_EXEC_BUS_LOCK_DETECTION;
    > + SECONDARY_EXEC_BUS_LOCK_DETECTION |
    > + SECONDARY_EXEC_NOTIFY_VM_EXITING;
    > if (cpu_has_sgx())
    > opt2 |= SECONDARY_EXEC_ENCLS_EXITING;
    > if (adjust_vmx_controls(min2, opt2,
    > @@ -4376,6 +4381,9 @@ static void vmx_compute_secondary_exec_control(struct vcpu_vmx *vmx)
    > if (!vcpu->kvm->arch.bus_lock_detection_enabled)
    > exec_control &= ~SECONDARY_EXEC_BUS_LOCK_DETECTION;
    >
    > + if (cpu_has_notify_vm_exiting() && notify_window < 0)
    > + exec_control &= ~SECONDARY_EXEC_NOTIFY_VM_EXITING;
    > +
    > vmx->secondary_exec_control = exec_control;
    > }
    >
    > @@ -4423,6 +4431,9 @@ static void init_vmcs(struct vcpu_vmx *vmx)
    > vmx->ple_window_dirty = true;
    > }
    >
    > + if (cpu_has_notify_vm_exiting() && notify_window >= 0)
    > + vmcs_write32(NOTIFY_WINDOW, notify_window);
    > +
    > vmcs_write32(PAGE_FAULT_ERROR_CODE_MASK, 0);
    > vmcs_write32(PAGE_FAULT_ERROR_CODE_MATCH, 0);
    > vmcs_write32(CR3_TARGET_COUNT, 0); /* 22.2.1 */
    > @@ -5642,6 +5653,31 @@ static int handle_bus_lock_vmexit(struct kvm_vcpu *vcpu)
    > return 0;
    > }
    >
    > +static int handle_notify(struct kvm_vcpu *vcpu)
    > +{
    > + unsigned long exit_qual = vmx_get_exit_qual(vcpu);
    > +
    > + if (!(exit_qual & NOTIFY_VM_CONTEXT_INVALID)) {
    > + /*
    > + * Notify VM exit happened while executing iret from NMI,
    > + * "blocked by NMI" bit has to be set before next VM entry.
    > + */
    > + if (enable_vnmi &&
    > + (exit_qual & INTR_INFO_UNBLOCK_NMI))
    > + vmcs_set_bits(GUEST_INTERRUPTIBILITY_INFO,
    > + GUEST_INTR_STATE_NMI);
    > +
    > + return 1;
    > + }
    > +
    > + vcpu->run->exit_reason = KVM_EXIT_INTERNAL_ERROR;
    > + vcpu->run->internal.suberror = KVM_INTERNAL_ERROR_NO_EVENT_WINDOW;
    > + vcpu->run->internal.ndata = 1;
    > + vcpu->run->internal.data[0] = exit_qual;
    > +
    > + return 0;
    > +}
    > +
    > /*
    > * The exit handlers return 1 if the exit was handled fully and guest execution
    > * may resume. Otherwise they set the kvm_run parameter to indicate what needs
    > @@ -5699,6 +5735,7 @@ static int (*kvm_vmx_exit_handlers[])(struct kvm_vcpu *vcpu) = {
    > [EXIT_REASON_PREEMPTION_TIMER] = handle_preemption_timer,
    > [EXIT_REASON_ENCLS] = handle_encls,
    > [EXIT_REASON_BUS_LOCK] = handle_bus_lock_vmexit,
    > + [EXIT_REASON_NOTIFY] = handle_notify,
    > };
    >
    > static const int kvm_vmx_max_exit_handlers =
    > @@ -6042,7 +6079,8 @@ static int __vmx_handle_exit(struct kvm_vcpu *vcpu, fastpath_t exit_fastpath)
    > exit_reason.basic != EXIT_REASON_EPT_VIOLATION &&
    > exit_reason.basic != EXIT_REASON_PML_FULL &&
    > exit_reason.basic != EXIT_REASON_APIC_ACCESS &&
    > - exit_reason.basic != EXIT_REASON_TASK_SWITCH)) {
    > + exit_reason.basic != EXIT_REASON_TASK_SWITCH &&
    > + exit_reason.basic != EXIT_REASON_NOTIFY)) {
    > int ndata = 3;
    >
    > vcpu->run->exit_reason = KVM_EXIT_INTERNAL_ERROR;
    > diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h
    > index 3fd9a7e9d90c..bb3b49b1fb0d 100644
    > --- a/include/uapi/linux/kvm.h
    > +++ b/include/uapi/linux/kvm.h
    > @@ -278,6 +278,8 @@ struct kvm_xen_exit {
    > #define KVM_INTERNAL_ERROR_DELIVERY_EV 3
    > /* Encounter unexpected vm-exit reason */
    > #define KVM_INTERNAL_ERROR_UNEXPECTED_EXIT_REASON 4
    > +/* Encounter notify vm-exit */
    > +#define KVM_INTERNAL_ERROR_NO_EVENT_WINDOW 5
    >
    > /* for KVM_RUN, returned by mmap(vcpu_fd, offset=0) */
    > struct kvm_run {
    >

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