Re: [PATCH 1/2] KVM: VMX: Bury Intel PT virtualization (guest/host mode) behind CONFIG_BROKEN

[Sean Christopherson <seanjc@google.com>]

On Mon, Nov 04, 2024, Adrian Hunter wrote:
> On 1/11/24 20:50, Sean Christopherson wrote:
> > Hide KVM's pt_mode module param behind CONFIG_BROKEN, i.e. disable support
> > for virtualizing Intel PT via guest/host mode unless BROKEN=y.  There are
> > myriad bugs in the implementation, some of which are fatal to the guest,
> > and others which put the stability and health of the host at risk.
> > 
> > For guest fatalities, the most glaring issue is that KVM fails to ensure
> > tracing is disabled, and *stays* disabled prior to VM-Enter, which is
> > necessary as hardware disallows loading (the guest's) RTIT_CTL if tracing
> > is enabled (enforced via a VMX consistency check).  Per the SDM:
> > 
> >   If the logical processor is operating with Intel PT enabled (if
> >   IA32_RTIT_CTL.TraceEn = 1) at the time of VM entry, the "load
> >   IA32_RTIT_CTL" VM-entry control must be 0.
> > 
> > On the host side, KVM doesn't validate the guest CPUID configuration
> > provided by userspace, and even worse, uses the guest configuration to
> > decide what MSRs to save/load at VM-Enter and VM-Exit.  E.g. configuring
> > guest CPUID to enumerate more address ranges than are supported in hardware
> > will result in KVM trying to passthrough, save, and load non-existent MSRs,
> > which generates a variety of WARNs, ToPA ERRORs in the host, a potential
> > deadlock, etc.
> > 
> > Fixes: f99e3daf94ff ("KVM: x86: Add Intel PT virtualization work mode")
> > Cc: stable@vger.kernel.org
> > Cc: Adrian Hunter <adrian.hunter@intel.com>
> > Signed-off-by: Sean Christopherson <seanjc@google.com>
> > ---
> >  arch/x86/kvm/vmx/vmx.c | 4 +++-
> >  1 file changed, 3 insertions(+), 1 deletion(-)
> > 
> > diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> > index 6ed801ffe33f..087504fb1589 100644
> > --- a/arch/x86/kvm/vmx/vmx.c
> > +++ b/arch/x86/kvm/vmx/vmx.c
> > @@ -217,9 +217,11 @@ module_param(ple_window_shrink, uint, 0444);
> >  static unsigned int ple_window_max        = KVM_VMX_DEFAULT_PLE_WINDOW_MAX;
> >  module_param(ple_window_max, uint, 0444);
> >  
> > -/* Default is SYSTEM mode, 1 for host-guest mode */
> > +/* Default is SYSTEM mode, 1 for host-guest mode (which is BROKEN) */
> >  int __read_mostly pt_mode = PT_MODE_SYSTEM;
> > +#ifdef CONFIG_BROKEN
> >  module_param(pt_mode, int, S_IRUGO);
> > +#endif
> 
> Side effects are:
> 1. If pt_mode is passed via modprobe, there will be a warning in kernel messages:
> 	kvm_intel: unknown parameter 'pt_mode' ignored

This is more of a feature in this case, as it's a non-fatal way of alerting the
user that trying to enable PT virtualization won't work.

> 2. The sysfs module parameter file pt_mode will be gone:
> 	# cat /sys/module/kvm_intel/parameters/pt_mode
> 	cat: /sys/module/kvm_intel/parameters/pt_mode: No such file or directory

Hrm, this could be slightly more problematic, e.g. if userspace were asserting on
the state of the parameter.  But AFAIK, module params aren't considered ABI.

Paolo, any thoughts on how best to handle this?