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

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 04.09.18 at 22:58, <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).

But a guest exposed interface can't be changed like this: If a.domain
was ignored for this sub-op before, it needs to continue to be ignored.

Jan


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