Re: [PATCH v4 2/2] x86/monitor: Add new monitor event to catch all vmexits

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Wed, Apr 13, 2022 at 09:41:52AM -0400, Tamas K Lengyel wrote:
> Add monitor event that hooks the vmexit handler allowing for both sync and
> async monitoring of events. With async monitoring an event is placed on the
> monitor ring for each exit and the rest of the vmexit handler resumes 
> normally.
> If there are additional monitor events configured those will also place their
> respective events on the monitor ring.
> 
> With the sync version an event is placed on the monitor ring but the handler
> does not get resumed, thus the sync version is only useful when the VM is not
> expected to resume normally after the vmexit. Our use-case is primarily with
> the sync version with VM forks where the fork gets reset after sync vmexit
> event, thus the rest of the vmexit handler can be safely skipped. This is
> very useful when we want to avoid Xen crashing the VM under any circumstance,
> for example during fuzzing. Collecting all vmexit information regardless of
> the root cause makes it easier to reason about the state of the VM on the
> monitor side, hence we opt to receive all events, even for external interrupt
> and NMI exits and let the monitor agent decide how to proceed.
> 
> Signed-off-by: Tamas K Lengyel <tamas.lengyel@xxxxxxxxx>
> ---
> v4: Minor tweaks and more verbose patch description.
> 
> Note: making the sync version resume-friendly is specifically out-of-scope as
> it would require significant rearrangement of the vmexit handler. As this
> feature is not required for our use-case we opt for the version that minimizes
> impact on the existing code.
> ---
>  tools/include/xenctrl.h                |  2 ++
>  tools/libs/ctrl/xc_monitor.c           | 15 +++++++++++++++
>  xen/arch/x86/hvm/monitor.c             | 18 ++++++++++++++++++
>  xen/arch/x86/hvm/vmx/vmx.c             | 12 ++++++++++++
>  xen/arch/x86/include/asm/domain.h      |  2 ++
>  xen/arch/x86/include/asm/hvm/monitor.h |  2 ++
>  xen/arch/x86/include/asm/monitor.h     |  3 ++-
>  xen/arch/x86/monitor.c                 | 14 ++++++++++++++
>  xen/include/public/domctl.h            |  6 ++++++
>  xen/include/public/vm_event.h          |  8 ++++++++
>  10 files changed, 81 insertions(+), 1 deletion(-)
> 
> diff --git a/tools/include/xenctrl.h b/tools/include/xenctrl.h
> index 1b089a2c02..159eaac050 100644
> --- a/tools/include/xenctrl.h
> +++ b/tools/include/xenctrl.h
> @@ -2096,6 +2096,8 @@ int xc_monitor_privileged_call(xc_interface *xch, 
> uint32_t domain_id,
>                                 bool enable);
>  int xc_monitor_emul_unimplemented(xc_interface *xch, uint32_t domain_id,
>                                    bool enable);
> +int xc_monitor_vmexit(xc_interface *xch, uint32_t domain_id, bool enable,
> +                      bool sync);
>  /**
>   * This function enables / disables emulation for each REP for a
>   * REP-compatible instruction.
> diff --git a/tools/libs/ctrl/xc_monitor.c b/tools/libs/ctrl/xc_monitor.c
> index 4ac823e775..c5fa62ff30 100644
> --- a/tools/libs/ctrl/xc_monitor.c
> +++ b/tools/libs/ctrl/xc_monitor.c
> @@ -246,6 +246,21 @@ int xc_monitor_emul_unimplemented(xc_interface *xch, 
> uint32_t domain_id,
>      return do_domctl(xch, &domctl);
>  }
>  
> +int xc_monitor_vmexit(xc_interface *xch, uint32_t domain_id, bool enable,
> +                      bool sync)
> +{
> +    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_VMEXIT;
> +    domctl.u.monitor_op.u.vmexit.sync = sync;
> +
> +    return do_domctl(xch, &domctl);
> +}
> +
>  /*
>   * Local variables:
>   * mode: C
> diff --git a/xen/arch/x86/hvm/monitor.c b/xen/arch/x86/hvm/monitor.c
> index b44a1e1dfe..64a38e8fa7 100644
> --- a/xen/arch/x86/hvm/monitor.c
> +++ b/xen/arch/x86/hvm/monitor.c
> @@ -328,6 +328,24 @@ bool hvm_monitor_check_p2m(unsigned long gla, gfn_t gfn, 
> uint32_t pfec,
>      return monitor_traps(curr, true, &req) >= 0;
>  }
>  
> +int hvm_monitor_vmexit(unsigned long exit_reason,
> +                       unsigned long exit_qualification)

Should this maybe live in vmx code or have 'vmx' in the name
somewhere, so that if an svm counterpart is added this doesn't need to
be renamed?

> +{
> +    struct vcpu *curr = current;
> +    struct arch_domain *ad = &curr->domain->arch;
> +    vm_event_request_t req = {};
> +
> +    ASSERT(ad->monitor.vmexit_enabled);
> +
> +    req.reason = VM_EVENT_REASON_VMEXIT;
> +    req.u.vmexit.reason = exit_reason;
> +    req.u.vmexit.qualification = exit_qualification;

You could set those fields at definition.

> +
> +    set_npt_base(curr, &req);
> +
> +    return monitor_traps(curr, ad->monitor.vmexit_sync, &req);
> +}
> +
>  /*
>   * Local variables:
>   * mode: C
> diff --git a/xen/arch/x86/hvm/vmx/vmx.c b/xen/arch/x86/hvm/vmx/vmx.c
> index c075370f64..2794db46f9 100644
> --- a/xen/arch/x86/hvm/vmx/vmx.c
> +++ b/xen/arch/x86/hvm/vmx/vmx.c
> @@ -4008,6 +4008,18 @@ void vmx_vmexit_handler(struct cpu_user_regs *regs)
>          }
>      }
>  
> +    if ( unlikely(currd->arch.monitor.vmexit_enabled) )
> +    {
> +        int rc;
> +
> +        __vmread(EXIT_QUALIFICATION, &exit_qualification);
> +        rc = hvm_monitor_vmexit(exit_reason, exit_qualification);
> +        if ( rc < 0 )
> +            goto exit_and_crash;
> +        if ( rc )
> +            return;
> +    }

Just for my understanding, is there any reason to not do this before
updating the altp2m?  AFAICT the update of the active EPTP won't
affect the call to hvm_monitor_vmexit.

> +
>      /* XXX: This looks ugly, but we need a mechanism to ensure
>       * any pending vmresume has really happened
>       */
> diff --git a/xen/arch/x86/include/asm/domain.h 
> b/xen/arch/x86/include/asm/domain.h
> index e62e109598..855db352c0 100644
> --- a/xen/arch/x86/include/asm/domain.h
> +++ b/xen/arch/x86/include/asm/domain.h
> @@ -430,6 +430,8 @@ struct arch_domain
>           */
>          unsigned int inguest_pagefault_disabled                            : 
> 1;
>          unsigned int control_register_values                               : 
> 1;
> +        unsigned int vmexit_enabled                                        : 
> 1;
> +        unsigned int vmexit_sync                                           : 
> 1;
>          struct monitor_msr_bitmap *msr_bitmap;
>          uint64_t write_ctrlreg_mask[4];
>      } monitor;
> diff --git a/xen/arch/x86/include/asm/hvm/monitor.h 
> b/xen/arch/x86/include/asm/hvm/monitor.h
> index a75cd8545c..639f6dfa37 100644
> --- a/xen/arch/x86/include/asm/hvm/monitor.h
> +++ b/xen/arch/x86/include/asm/hvm/monitor.h
> @@ -51,6 +51,8 @@ bool hvm_monitor_emul_unimplemented(void);
>  
>  bool hvm_monitor_check_p2m(unsigned long gla, gfn_t gfn, uint32_t pfec,
>                             uint16_t kind);
> +int hvm_monitor_vmexit(unsigned long exit_reason,
> +                       unsigned long exit_qualification);
>  
>  #endif /* __ASM_X86_HVM_MONITOR_H__ */
>  
> diff --git a/xen/arch/x86/include/asm/monitor.h 
> b/xen/arch/x86/include/asm/monitor.h
> index 01c6d63bb9..d8d54c5f23 100644
> --- a/xen/arch/x86/include/asm/monitor.h
> +++ b/xen/arch/x86/include/asm/monitor.h
> @@ -89,7 +89,8 @@ static inline uint32_t arch_monitor_get_capabilities(struct 
> domain *d)
>                      (1U << XEN_DOMCTL_MONITOR_EVENT_DEBUG_EXCEPTION) |
>                      (1U << XEN_DOMCTL_MONITOR_EVENT_WRITE_CTRLREG) |
>                      (1U << XEN_DOMCTL_MONITOR_EVENT_EMUL_UNIMPLEMENTED) |
> -                    (1U << XEN_DOMCTL_MONITOR_EVENT_INGUEST_PAGEFAULT));
> +                    (1U << XEN_DOMCTL_MONITOR_EVENT_INGUEST_PAGEFAULT) |
> +                    (1U << XEN_DOMCTL_MONITOR_EVENT_VMEXIT));
>  
>      if ( hvm_is_singlestep_supported() )
>          capabilities |= (1U << XEN_DOMCTL_MONITOR_EVENT_SINGLESTEP);
> diff --git a/xen/arch/x86/monitor.c b/xen/arch/x86/monitor.c
> index 3079726a8b..30ca71432c 100644
> --- a/xen/arch/x86/monitor.c
> +++ b/xen/arch/x86/monitor.c
> @@ -332,6 +332,20 @@ int arch_monitor_domctl_event(struct domain *d,
>          break;
>      }
>  
> +    case XEN_DOMCTL_MONITOR_EVENT_VMEXIT:
> +    {
> +        bool old_status = ad->monitor.vmexit_enabled;
> +
> +        if ( unlikely(old_status == requested_status) )
> +            return -EEXIST;

What about if the requested status is the same as the current one, but
vmexit sync is not?

IOW, I'm not sure this check is helpful, and you could likely avoid
the old_status local variable.

> +
> +        domain_pause(d);
> +        ad->monitor.vmexit_enabled = requested_status;
> +        ad->monitor.vmexit_sync = mop->u.vmexit.sync;
> +        domain_unpause(d);
> +        break;
> +    }
> +
>      default:
>          /*
>           * Should not be reached unless arch_monitor_get_capabilities() is
> diff --git a/xen/include/public/domctl.h b/xen/include/public/domctl.h
> index b85e6170b0..4803ed7afc 100644
> --- a/xen/include/public/domctl.h
> +++ b/xen/include/public/domctl.h
> @@ -1057,6 +1057,7 @@ struct xen_domctl_psr_cmt_op {
>  #define XEN_DOMCTL_MONITOR_EVENT_EMUL_UNIMPLEMENTED    10
>  /* Enabled by default */
>  #define XEN_DOMCTL_MONITOR_EVENT_INGUEST_PAGEFAULT     11
> +#define XEN_DOMCTL_MONITOR_EVENT_VMEXIT                12
>  
>  struct xen_domctl_monitor_op {
>      uint32_t op; /* XEN_DOMCTL_MONITOR_OP_* */
> @@ -1107,6 +1108,11 @@ struct xen_domctl_monitor_op {
>              /* Pause vCPU until response */
>              uint8_t sync;
>          } debug_exception;
> +
> +        struct {
> +            /* Send event and don't process vmexit */
> +            uint8_t sync;
> +        } vmexit;
>      } u;
>  };
>  
> diff --git a/xen/include/public/vm_event.h b/xen/include/public/vm_event.h
> index 81c2ee28cc..07f106f811 100644
> --- a/xen/include/public/vm_event.h
> +++ b/xen/include/public/vm_event.h
> @@ -175,6 +175,8 @@
>  #define VM_EVENT_REASON_DESCRIPTOR_ACCESS       13
>  /* Current instruction is not implemented by the emulator */
>  #define VM_EVENT_REASON_EMUL_UNIMPLEMENTED      14
> +/* VMEXIT */
> +#define VM_EVENT_REASON_VMEXIT                  15
>  
>  /* Supported values for the vm_event_write_ctrlreg index. */
>  #define VM_EVENT_X86_CR0    0
> @@ -394,6 +396,11 @@ struct vm_event_emul_insn_data {
>      uint8_t data[16]; /* Has to be completely filled */
>  };
>  
> +struct vm_event_vmexit {
> +    uint64_t reason;
> +    uint64_t qualification;
> +};

You are exposing an Intel specific interface publicly here.  Might be
worth adding a note, and/or adding 'intel' or 'vmx' in the structure
name: vm_event_vmx_exit, so that a vm_event_svm_exit could also be
added in the future.

Thanks, Roger.