Re: [PATCH v4] x86/monitor: Notify monitor if an emulation fails.

[Tamas K Lengyel <tamas@tklengyel.com>]

On Wed, Jul 12, 2017 at 11:21 AM, Petre Pircalabu
<ppircalabu@bitdefender.com> wrote:
> If case of a vm_event with the emulate_flags set, if the instruction
> cannot be emulated, the monitor should be notified instead of directly
> injecting a hw exception.
> This behavior can be used to re-execute an instruction not supported by
> the emulator using the real processor (e.g. altp2m) instead of just
> crashing.
>
> Signed-off-by: Petre Pircalabu <ppircalabu@bitdefender.com>
>
> ---
> Changed since v1:
>   * Removed the emulation kind check when calling hvm_inject_hw_exception
>
> Changed since v2:
>   * Removed a file added by mistake
>
> Changed since v3:
>   * Removed extra stray line
>   * Added the _enabled suffix to the emul_unhandleable monitor option
> ---
>  tools/libxc/include/xenctrl.h     |  2 ++
>  tools/libxc/xc_monitor.c          | 14 ++++++++++++++
>  xen/arch/x86/hvm/emulate.c        |  5 ++++-
>  xen/arch/x86/hvm/monitor.c        | 18 ++++++++++++++++++
>  xen/arch/x86/monitor.c            | 12 ++++++++++++
>  xen/include/asm-x86/domain.h      |  1 +
>  xen/include/asm-x86/hvm/monitor.h |  1 +
>  xen/include/asm-x86/monitor.h     |  3 ++-
>  xen/include/public/domctl.h       |  1 +
>  xen/include/public/vm_event.h     |  2 ++
>  10 files changed, 57 insertions(+), 2 deletions(-)
>
> diff --git a/tools/libxc/include/xenctrl.h b/tools/libxc/include/xenctrl.h
> index c51bb3b..8deb5ac 100644
> --- a/tools/libxc/include/xenctrl.h
> +++ b/tools/libxc/include/xenctrl.h
> @@ -2029,6 +2029,8 @@ int xc_monitor_debug_exceptions(xc_interface *xch, domid_t domain_id,
>  int xc_monitor_cpuid(xc_interface *xch, domid_t domain_id, bool enable);
>  int xc_monitor_privileged_call(xc_interface *xch, domid_t domain_id,
>                                 bool enable);
> +int xc_monitor_emul_unhandleable(xc_interface *xch, domid_t domain_id,
> +                                 bool enable);
>  /**
>   * This function enables / disables emulation for each REP for a
>   * REP-compatible instruction.
> diff --git a/tools/libxc/xc_monitor.c b/tools/libxc/xc_monitor.c
> index b44ce93..8e72c6c 100644
> --- a/tools/libxc/xc_monitor.c
> +++ b/tools/libxc/xc_monitor.c
> @@ -216,6 +216,20 @@ int xc_monitor_privileged_call(xc_interface *xch, domid_t domain_id,
>      return do_domctl(xch, &domctl);
>  }
>
> +int xc_monitor_emul_unhandleable(xc_interface *xch, domid_t domain_id,
> +                                 bool enable)
> +{
> +    DECLARE_DOMCTL;
> +
> +    domctl.cmd = XEN_DOMCTL_monitor_op;
> +    domctl.domain = domain_id;
> +    domctl.u.monitor_op.op = enable ? XEN_DOMCTL_MONITOR_OP_ENABLE
> +                                    : XEN_DOMCTL_MONITOR_OP_DISABLE;
> +    domctl.u.monitor_op.event = XEN_DOMCTL_MONITOR_EVENT_EMUL_UNHANDLEABLE;
> +
> +    return do_domctl(xch, &domctl);
> +}
> +
>  /*
>   * Local variables:
>   * mode: C
> diff --git a/xen/arch/x86/hvm/emulate.c b/xen/arch/x86/hvm/emulate.c
> index e97aa69..f52aa87 100644
> --- a/xen/arch/x86/hvm/emulate.c
> +++ b/xen/arch/x86/hvm/emulate.c
> @@ -14,12 +14,14 @@
>  #include <xen/sched.h>
>  #include <xen/paging.h>
>  #include <xen/trace.h>
> +#include <xen/vm_event.h>
>  #include <asm/event.h>
>  #include <asm/i387.h>
>  #include <asm/xstate.h>
>  #include <asm/hvm/emulate.h>
>  #include <asm/hvm/hvm.h>
>  #include <asm/hvm/ioreq.h>
> +#include <asm/hvm/monitor.h>
>  #include <asm/hvm/trace.h>
>  #include <asm/hvm/support.h>
>  #include <asm/hvm/svm/svm.h>
> @@ -2101,7 +2103,8 @@ void hvm_emulate_one_vm_event(enum emul_kind kind, unsigned int trapnr,
>          return;
>      case X86EMUL_UNHANDLEABLE:
>          hvm_dump_emulation_state(XENLOG_G_DEBUG, "Mem event", &ctx);
> -        hvm_inject_hw_exception(trapnr, errcode);
> +        if ( !hvm_monitor_emul_unhandleable() )
> +            hvm_inject_hw_exception(trapnr, errcode);
>          break;
>      case X86EMUL_EXCEPTION:
>          hvm_inject_event(&ctx.ctxt.event);
> diff --git a/xen/arch/x86/hvm/monitor.c b/xen/arch/x86/hvm/monitor.c
> index a7ccfc4..977a96b 100644
> --- a/xen/arch/x86/hvm/monitor.c
> +++ b/xen/arch/x86/hvm/monitor.c
> @@ -57,6 +57,24 @@ bool_t hvm_monitor_cr(unsigned int index, unsigned long value, unsigned long old
>      return 0;
>  }
>
> +bool hvm_monitor_emul_unhandleable(void)
> +{
> +    struct vcpu *curr = current;
> +    struct domain *d = curr->domain;
> +
> +    /*
> +     * Send a vm_event to the monitor to signal that the current
> +     * instruction couldn't be emulated.
> +     */
> +    vm_event_request_t req = {
> +        .reason = VM_EVENT_REASON_EMUL_UNHANDLEABLE,
> +        .vcpu_id  = curr->vcpu_id,
> +    };
> +
> +    return ( d->arch.monitor.emul_unhandleable_enabled &&
> +             monitor_traps(curr, true, &req) );

So what happens if monitor_traps fails and returns -1? Since this gets
treated as a boolean, we will return true here and just silently
swallow the problem and likely make the VM hang in a loop. I think it
would be appropriate to crash the vm here if we can't emulate and also
can't notify the listener.

> +}
> +
>  void hvm_monitor_msr(unsigned int msr, uint64_t value)
>  {
>      struct vcpu *curr = current;
> diff --git a/xen/arch/x86/monitor.c b/xen/arch/x86/monitor.c
> index 706454f..f791372 100644
> --- a/xen/arch/x86/monitor.c
> +++ b/xen/arch/x86/monitor.c
> @@ -283,6 +283,18 @@ int arch_monitor_domctl_event(struct domain *d,
>          break;
>      }
>
> +    case XEN_DOMCTL_MONITOR_EVENT_EMUL_UNHANDLEABLE:
> +    {
> +        bool old_status = ad->monitor.emul_unhandleable_enabled;
> +        if ( unlikely(old_status == requested_status) )
> +            return -EEXIST;
> +
> +        domain_pause(d);
> +        ad->monitor.emul_unhandleable_enabled = requested_status;
> +        domain_unpause(d);
> +        break;
> +    }
> +
>      default:
>          /*
>           * Should not be reached unless arch_monitor_get_capabilities() is
> diff --git a/xen/include/asm-x86/domain.h b/xen/include/asm-x86/domain.h
> index c10522b..ae2d04d 100644
> --- a/xen/include/asm-x86/domain.h
> +++ b/xen/include/asm-x86/domain.h
> @@ -405,6 +405,7 @@ struct arch_domain
>          unsigned int debug_exception_sync        : 1;
>          unsigned int cpuid_enabled               : 1;
>          unsigned int descriptor_access_enabled   : 1;
> +        unsigned int emul_unhandleable_enabled   : 1;
>          struct monitor_msr_bitmap *msr_bitmap;
>          uint64_t write_ctrlreg_mask[4];
>      } monitor;
> diff --git a/xen/include/asm-x86/hvm/monitor.h b/xen/include/asm-x86/hvm/monitor.h
> index d9efb35..4030be7 100644
> --- a/xen/include/asm-x86/hvm/monitor.h
> +++ b/xen/include/asm-x86/hvm/monitor.h
> @@ -47,6 +47,7 @@ int hvm_monitor_cpuid(unsigned long insn_length, unsigned int leaf,
>                        unsigned int subleaf);
>  void hvm_monitor_interrupt(unsigned int vector, unsigned int type,
>                             unsigned int err, uint64_t cr2);
> +bool hvm_monitor_emul_unhandleable(void);
>
>  #endif /* __ASM_X86_HVM_MONITOR_H__ */
>
> diff --git a/xen/include/asm-x86/monitor.h b/xen/include/asm-x86/monitor.h
> index c5c323b..38ba0ff 100644
> --- a/xen/include/asm-x86/monitor.h
> +++ b/xen/include/asm-x86/monitor.h
> @@ -77,7 +77,8 @@ static inline uint32_t arch_monitor_get_capabilities(struct domain *d)
>                     (1U << XEN_DOMCTL_MONITOR_EVENT_GUEST_REQUEST) |
>                     (1U << XEN_DOMCTL_MONITOR_EVENT_DEBUG_EXCEPTION) |
>                     (1U << XEN_DOMCTL_MONITOR_EVENT_CPUID) |
> -                   (1U << XEN_DOMCTL_MONITOR_EVENT_INTERRUPT);
> +                   (1U << XEN_DOMCTL_MONITOR_EVENT_INTERRUPT) |
> +                   (1U << XEN_DOMCTL_MONITOR_EVENT_EMUL_UNHANDLEABLE);
>
>      /* Since we know this is on VMX, we can just call the hvm func */
>      if ( hvm_is_singlestep_supported() )
> diff --git a/xen/include/public/domctl.h b/xen/include/public/domctl.h
> index ff39762..6bd8666 100644
> --- a/xen/include/public/domctl.h
> +++ b/xen/include/public/domctl.h
> @@ -1083,6 +1083,7 @@ DEFINE_XEN_GUEST_HANDLE(xen_domctl_psr_cmt_op_t);
>  #define XEN_DOMCTL_MONITOR_EVENT_PRIVILEGED_CALL       7
>  #define XEN_DOMCTL_MONITOR_EVENT_INTERRUPT             8
>  #define XEN_DOMCTL_MONITOR_EVENT_DESC_ACCESS           9
> +#define XEN_DOMCTL_MONITOR_EVENT_EMUL_UNHANDLEABLE     10
>
>  struct xen_domctl_monitor_op {
>      uint32_t op; /* XEN_DOMCTL_MONITOR_OP_* */
> diff --git a/xen/include/public/vm_event.h b/xen/include/public/vm_event.h
> index f01e471..5548afb 100644
> --- a/xen/include/public/vm_event.h
> +++ b/xen/include/public/vm_event.h
> @@ -148,6 +148,8 @@
>  #define VM_EVENT_REASON_INTERRUPT               12
>  /* A descriptor table register was accessed. */
>  #define VM_EVENT_REASON_DESCRIPTOR_ACCESS       13
> +/* Current instruction couldn't be emulated */
> +#define VM_EVENT_REASON_EMUL_UNHANDLEABLE       14
>
>  /* Supported values for the vm_event_write_ctrlreg index. */
>  #define VM_EVENT_X86_CR0    0
> --
> 2.7.4

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