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

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

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.

~Andrew

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