Re: [PATCH v2 for-4.14 1/3] xen/monitor: Control register values

[Tamas K Lengyel <tamas@tklengyel.com>]

On Tue, Jun 2, 2020 at 5:08 AM Roger Pau Monné <roger.pau@citrix.com> wrote:
>
> On Wed, May 20, 2020 at 08:31:52PM -0600, Tamas K Lengyel wrote:
> > Extend the monitor_op domctl to include option that enables
> > controlling what values certain registers are permitted to hold
> > by a monitor subscriber.
>
> I think the change could benefit for some more detail commit message
> here. Why is this useful?

You would have to ask the Bitdefender folks who made the feature. I
don't use it. Here we are just making it optional as it is buggy so it
is disabled by default.

>
> There already seems to be some support for gating MSR writes, which
> seems to be expanded by this commit?

We don't expand on any existing features, we make an existing feature optional.

>
> Is it solving some kind of bug reported?

It does, please take a look at the cover letter.

>
> > Signed-off-by: Tamas K Lengyel <tamas@tklengyel.com>
> > ---
> >  xen/arch/x86/hvm/hvm.c       | 25 ++++++++++++++++---------
> >  xen/arch/x86/monitor.c       | 10 +++++++++-
> >  xen/include/asm-x86/domain.h |  1 +
> >  xen/include/public/domctl.h  |  1 +
> >  4 files changed, 27 insertions(+), 10 deletions(-)
> >
> > diff --git a/xen/arch/x86/hvm/hvm.c b/xen/arch/x86/hvm/hvm.c
> > index 09ee299bc7..e6780c685b 100644
> > --- a/xen/arch/x86/hvm/hvm.c
> > +++ b/xen/arch/x86/hvm/hvm.c
> > @@ -2263,7 +2263,8 @@ int hvm_set_cr0(unsigned long value, bool may_defer)
> >      {
> >          ASSERT(v->arch.vm_event);
> >
> > -        if ( hvm_monitor_crX(CR0, value, old_value) )
> > +        if ( hvm_monitor_crX(CR0, value, old_value) &&
> > +             v->domain->arch.monitor.control_register_values )
> >          {
> >              /* The actual write will occur in hvm_do_resume(), if permitted. */
> >              v->arch.vm_event->write_data.do_write.cr0 = 1;
> > @@ -2362,7 +2363,8 @@ int hvm_set_cr3(unsigned long value, bool may_defer)
> >      {
> >          ASSERT(v->arch.vm_event);
> >
> > -        if ( hvm_monitor_crX(CR3, value, old) )
> > +        if ( hvm_monitor_crX(CR3, value, old) &&
> > +             v->domain->arch.monitor.control_register_values )
> >          {
> >              /* The actual write will occur in hvm_do_resume(), if permitted. */
> >              v->arch.vm_event->write_data.do_write.cr3 = 1;
> > @@ -2443,7 +2445,8 @@ int hvm_set_cr4(unsigned long value, bool may_defer)
> >      {
> >          ASSERT(v->arch.vm_event);
> >
> > -        if ( hvm_monitor_crX(CR4, value, old_cr) )
> > +        if ( hvm_monitor_crX(CR4, value, old_cr) &&
> > +             v->domain->arch.monitor.control_register_values )
>
> I think you could return control_register_values in hvm_monitor_crX
> instead of having to add the check to each caller?

We could, but this way the code is more consistent.

>
> >          {
> >              /* The actual write will occur in hvm_do_resume(), if permitted. */
> >              v->arch.vm_event->write_data.do_write.cr4 = 1;
> > @@ -3587,13 +3590,17 @@ int hvm_msr_write_intercept(unsigned int msr, uint64_t msr_content,
> >
> >          ASSERT(v->arch.vm_event);
> >
> > -        /* The actual write will occur in hvm_do_resume() (if permitted). */
> > -        v->arch.vm_event->write_data.do_write.msr = 1;
> > -        v->arch.vm_event->write_data.msr = msr;
> > -        v->arch.vm_event->write_data.value = msr_content;
> > -
> >          hvm_monitor_msr(msr, msr_content, msr_old_content);
> > -        return X86EMUL_OKAY;
> > +
> > +        if ( v->domain->arch.monitor.control_register_values )
>
> Is there any value in limiting control_register_values to MSR that
> represent control registers, like EFER and XSS?

I don't know, you would have to ask Bitdefender about it who made this feature.

>
> > +        {
> > +            /* The actual write will occur in hvm_do_resume(), if permitted. */
> > +            v->arch.vm_event->write_data.do_write.msr = 1;
> > +            v->arch.vm_event->write_data.msr = msr;
> > +            v->arch.vm_event->write_data.value = msr_content;
> > +
> > +            return X86EMUL_OKAY;
> > +        }
>
> You seem to change the previous flow of the function here, that would
> just call hvm_monitor_msr and return previously.
>
> Don't you need to move the return from outside the added if condition
> in order to keep previous behavior? Or else the write is committed
> straight away.

That's exactly what we want to achieve. Postponing the write is buggy.
We want to make that feature optional. Before Bitdefender contributed
that feature writes were always commited straight away, so with this
patch we are actually reverting default behavior to what it was like
to start with.

>
> >      }
> >
> >      if ( (ret = guest_wrmsr(v, msr, msr_content)) != X86EMUL_UNHANDLEABLE )
> > diff --git a/xen/arch/x86/monitor.c b/xen/arch/x86/monitor.c
> > index bbcb7536c7..1517a97f50 100644
> > --- a/xen/arch/x86/monitor.c
> > +++ b/xen/arch/x86/monitor.c
> > @@ -144,7 +144,15 @@ int arch_monitor_domctl_event(struct domain *d,
> >                                struct xen_domctl_monitor_op *mop)
> >  {
> >      struct arch_domain *ad = &d->arch;
> > -    bool requested_status = (XEN_DOMCTL_MONITOR_OP_ENABLE == mop->op);
> > +    bool requested_status;
> > +
> > +    if ( XEN_DOMCTL_MONITOR_OP_CONTROL_REGISTERS == mop->op )
> > +    {
> > +        ad->monitor.control_register_values = true;
>
> I think strictly speaking you need to use 1 here, since this variable
> is not a boolean.

Sure.

Thanks,
Tamas