Re: [Xen-devel] [PATCH v2] x86/altp2m: Allow setting the #VE info page for an arbitrary VCPU

[Wei Liu <wei.liu2@xxxxxxxxxx>]

On Thu, Sep 20, 2018 at 11:11:37AM +0300, Razvan Cojocaru wrote:
> On 9/6/18 1:27 AM, Tamas K Lengyel wrote:
> > On Wed, Sep 5, 2018 at 12:45 PM Andrew Cooper <andrew.cooper3@xxxxxxxxxx> 
> > wrote:
> >>
> >> On 05/09/18 19:40, Tamas K Lengyel wrote:
> >>> On Wed, Sep 5, 2018 at 10:40 AM Razvan Cojocaru
> >>> <rcojocaru@xxxxxxxxxxxxxxx> wrote:
> >>>> On 9/5/18 7:28 PM, Tamas K Lengyel wrote:
> >>>>> On Tue, Sep 4, 2018 at 2:58 PM Razvan Cojocaru
> >>>>> <rcojocaru@xxxxxxxxxxxxxxx> wrote:
> >>>>>> On 9/4/18 11:40 PM, Tamas K Lengyel wrote:
> >>>>>>> On Mon, Sep 3, 2018 at 10:59 PM Adrian Pop <apop@xxxxxxxxxxxxxxx> 
> >>>>>>> wrote:
> >>>>>>>> In a classic HVI + Xen setup, the introspection engine would monitor
> >>>>>>>> legacy guest page-tables by marking them read-only inside the EPT; 
> >>>>>>>> this
> >>>>>>>> way any modification explicitly made by the guest or implicitly made 
> >>>>>>>> by
> >>>>>>>> the CPU page walker would trigger an EPT violation, which would be
> >>>>>>>> forwarded by Xen to the SVA and thus the HVI agent.  The HVI agent 
> >>>>>>>> would
> >>>>>>>> analyse the modification, and act upon it - for example, a virtual 
> >>>>>>>> page
> >>>>>>>> may be remapped (its guest physical address changed inside the
> >>>>>>>> page-table), in which case the introspection logic would update the
> >>>>>>>> protection accordingly (remove EPT hook on the old gpa, and place a 
> >>>>>>>> new
> >>>>>>>> EPT hook on the new gpa).  In other cases, the modification may be 
> >>>>>>>> of no
> >>>>>>>> interest to the introspection engine - for example, the 
> >>>>>>>> accessed/dirty
> >>>>>>>> bits may be cleared by the operating system or the accessed/dirty 
> >>>>>>>> bits
> >>>>>>>> may be set by the CPU page walker.
> >>>>>>>>
> >>>>>>>> In our tests we discovered that the vast majority of guest page-table
> >>>>>>>> modifications fall in the second category (especially on Windows 10 
> >>>>>>>> RS4
> >>>>>>>> x64 - more than 95% of ALL the page-table modifications are 
> >>>>>>>> irrelevant to
> >>>>>>>> us) - they are of no interest to the introspection logic, but they
> >>>>>>>> trigger a very costly EPT violation nonetheless.  Therefore, we 
> >>>>>>>> decided
> >>>>>>>> to make use of the new #VE & VMFUNC features in recent Intel CPUs to
> >>>>>>>> accelerate the guest page-tables monitoring in the following way:
> >>>>>>>>
> >>>>>>>> 1. Each monitored page-table would be flagged as being convertible
> >>>>>>>>    inside the EPT, thus enabling the CPU to deliver a virtualization
> >>>>>>>>    exception to he guest instead of generating a traditional EPT
> >>>>>>>>    violation.
> >>>>>>>> 2. We inject a small filtering driver inside the protected guest VM,
> >>>>>>>>    which would intercept the virtualization exception in order to 
> >>>>>>>> handle
> >>>>>>>>    guest page-table modifications.
> >>>>>>>> 3. We create a dedicated EPT view (altp2m) for the in-guest agent, 
> >>>>>>>> which
> >>>>>>>>    would isolate the agent from the rest of the operating system; the
> >>>>>>>>    agent will switch in and out of the protected EPT view via the 
> >>>>>>>> VMFUNC
> >>>>>>>>    instruction placed inside a trampoline page, thus making the agent
> >>>>>>>>    immune to malicious code inside the guest.
> >>>>>>>>
> >>>>>>>> This way, all the page-table accesses would generate a
> >>>>>>>> virtualization-exception inside the guest instead of a costly EPT
> >>>>>>>> violation; the #VE agent would emulate and analyse the modification, 
> >>>>>>>> and
> >>>>>>>> decide whether it is relevant for the main introspection logic; if 
> >>>>>>>> it is
> >>>>>>>> relevant, it would do a VMCALL and notify the introspection engine
> >>>>>>>> about the modification; otherwise, it would resume normal instruction
> >>>>>>>> execution, thus avoiding a very costly VM exit.
> >>>>>>>>
> >>>>>>>> Signed-off-by: Adrian Pop <apop@xxxxxxxxxxxxxxx>
> >>>>>>>> ---
> >>>>>>>> Changes in v2:
> >>>>>>>> - remove the "__get_vcpu()" helper
> >>>>>>>> ---
> >>>>>>>>  tools/libxc/xc_altp2m.c |  1 -
> >>>>>>>>  xen/arch/x86/hvm/hvm.c  | 19 ++++++++++---------
> >>>>>>>>  2 files changed, 10 insertions(+), 10 deletions(-)
> >>>>>>>>
> >>>>>>>> diff --git a/tools/libxc/xc_altp2m.c b/tools/libxc/xc_altp2m.c
> >>>>>>>> index ce4a1e4d60..528e929d7a 100644
> >>>>>>>> --- a/tools/libxc/xc_altp2m.c
> >>>>>>>> +++ b/tools/libxc/xc_altp2m.c
> >>>>>>>> @@ -68,7 +68,6 @@ int xc_altp2m_set_domain_state(xc_interface 
> >>>>>>>> *handle, uint32_t dom, bool state)
> >>>>>>>>      return rc;
> >>>>>>>>  }
> >>>>>>>>
> >>>>>>>> -/* This is a bit odd to me that it acts on current.. */
> >>>>>>>>  int xc_altp2m_set_vcpu_enable_notify(xc_interface *handle, uint32_t 
> >>>>>>>> domid,
> >>>>>>>>                                       uint32_t vcpuid, xen_pfn_t gfn)
> >>>>>>>>  {
> >>>>>>>> diff --git a/xen/arch/x86/hvm/hvm.c b/xen/arch/x86/hvm/hvm.c
> >>>>>>>> index 72c51faecb..49c3bbee94 100644
> >>>>>>>> --- a/xen/arch/x86/hvm/hvm.c
> >>>>>>>> +++ b/xen/arch/x86/hvm/hvm.c
> >>>>>>>> @@ -4533,8 +4533,7 @@ static int do_altp2m_op(
> >>>>>>>>          return -EOPNOTSUPP;
> >>>>>>>>      }
> >>>>>>>>
> >>>>>>>> -    d = ( a.cmd != HVMOP_altp2m_vcpu_enable_notify ) ?
> >>>>>>>> -        rcu_lock_domain_by_any_id(a.domain) : 
> >>>>>>>> rcu_lock_current_domain();
> >>>>>>>> +    d = rcu_lock_domain_by_any_id(a.domain);
> >>>>>>> Does rcu_lock_domain_by_any_id work if its from the current domain? If
> >>>>>>> not, doesn't that change this function's accessibility to be from
> >>>>>>> exclusively usable only by the outside agent?
> >>>>>> The code says it should be safe:
> >>>>>>
> >>>>>>  633 struct domain *rcu_lock_domain_by_any_id(domid_t dom)
> >>>>>>  634 {
> >>>>>>  635     if ( dom == DOMID_SELF )
> >>>>>>  636         return rcu_lock_current_domain();
> >>>>>>  637     return rcu_lock_domain_by_id(dom);
> >>>>>>  638 }
> >>>>>>
> >>>>>> as long as dom == DOMID_SELF. I think the old behaviour assumed that
> >>>>>> HVMOP_altp2m_vcpu_enable_notify alone would only ever be used from the
> >>>>>> current domain, and this change expands its usability (Adrian should
> >>>>>> correct me if I'm wrong here).
> >>>>> Sounds good, thanks!
> >>>> May we take that as an Acked-by, or are there are other things you think
> >>>> we should address?
> >>> A Reviewed-by would be appropriate, I don't think the files touched in
> >>> this patch fall under our umbrella.
> >>
> >> That doesn't prohibit you providing a Reviewed-by: tag :)
> >>
> >> The statement itself is useful and hold weight, even if it isn't in code
> >> you are a maintainer of.
> > 
> > Indeed :)
> > 
> > Reviewed-by: Tamas K Lengyel <tamas@xxxxxxxxxxxxx>
> 
> Are there any issues preventing this patch to go in? Possibly missing acks?

I don't think so, unless an explicit ack for the deletion of that
comment is required. In any case:

Acked-by: Wei Liu <wei.liu2@xxxxxxxxxx>

I will commit this patch shortly.

Wei.

> 
> 
> Thanks,
> Razvan

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