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

[Razvan Cojocaru <rcojocaru@xxxxxxxxxxxxxxx>]

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?


Thanks,
Razvan

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