Re: [PATCH 2/4] KVM: x86: Harden _regs accesses to guard against buggy input

[Sean Christopherson <seanjc@google.com>]

On Thu, May 26, 2022, Kees Cook wrote:
> On Wed, May 25, 2022 at 10:26:02PM +0000, Sean Christopherson wrote:
> > Link: https://lore.kernel.org/all/YofQlBrlx18J7h9Y@google.com
> > Cc: Robert Dinse <nanook@eskimo.com>
> > Cc: Kees Cook <keescook@chromium.org>
> > Signed-off-by: Sean Christopherson <seanjc@google.com>
> > ---
> >  arch/x86/kvm/emulate.c | 6 ++++++
> >  1 file changed, 6 insertions(+)
> > 
> > diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c
> > index 7226a127ccb4..c58366ae4da2 100644
> > --- a/arch/x86/kvm/emulate.c
> > +++ b/arch/x86/kvm/emulate.c
> > @@ -247,6 +247,9 @@ enum x86_transfer_type {
> >  
> >  static ulong reg_read(struct x86_emulate_ctxt *ctxt, unsigned nr)
> >  {
> > +	if (WARN_ON_ONCE(nr >= 16))
> > +		nr &= 16 - 1;
> 
> Instead of doing a modulo here, what about forcing it into an "unused"
> slot?
> 
> i.e. define _regs as an array of [16 + 1], and:
> 
> 	if (WARN_ON_ONCE(nr >= 16)
> 		nr = 16;
> 
> Then there is both no out-of-bounds access, but also no weird "actual"
> register indexed?

Eh, IMO it doesn't provide any meaningful value, and requires documenting why
the emulator allocates an extra register.

The guest is still going to experience data loss/corruption if KVM drops a write
or reads zeros instead whatever register it was supposed to access.  I.e. the
guest is equally hosed either way.

One idea along the lines of Vitaly's idea of KVM_BUG_ON() would be to add an
emulator hook to bug the VM, e.g.

#define KVM_EMULATOR_BUG_ON(cond, ctxt)				\
({								\
	int __ret = (cond);					\
								\
	if (WARN_ON_ONCE(__ret))				\
		ctxt->ops->vm_bugged(ctxt);			\
	unlikely(__ret);					\
})

to workaround not having access to the 'struct kvm_vcpu' in the emulator.  The
bad access will still go through, but the VM will be killed before the vCPU can
re-enter the guest and do more damage.