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

[Razvan Cojocaru <rcojocaru@xxxxxxxxxxxxxxx>]

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).


Thanks,
Razvan

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