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, Xiaoyao Li wrote:
> On 11/2/2024 2:50 AM, 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
> 
> I like the patch, but I didn't find any other usercase of CONFIG_BROKEN in
> current Linux.

Ya, BROKEN is typically used directly in Kconfigs, e.g. "depends on BROKEN".  But
I can't think of any reason using it in this way would be problematic.