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

[Tamas K Lengyel <tamas@xxxxxxxxxxxxx>]

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!

Tamas

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