Re: [PATCH] KVM: X86: Also reload the debug registers before kvm_x86->run() when the host is using them

[Sean Christopherson <seanjc@google.com>]

On Thu, Jul 08, 2021, Paolo Bonzini wrote:
> On 28/06/21 19:26, Lai Jiangshan wrote:
> > From: Lai Jiangshan <laijs@linux.alibaba.com>
> > 
> > When the host is using debug registers but the guest is not using them
> > nor is the guest in guest-debug state, the kvm code does not reset
> > the host debug registers before kvm_x86->run().  Rather, it relies on
> > the hardware vmentry instruction to automatically reset the dr7 registers
> > which ensures that the host breakpoints do not affect the guest.
> > 
> > But there are still problems:
> > 	o The addresses of the host breakpoints can leak into the guest
> > 	  and the guest may use these information to attack the host.
> 
> I don't think this is true, because DRn reads would exit (if they don't,
> switch_db_regs would be nonzero).  But otherwise it makes sense to do at
> least the DR7 write, and we might as well do all of them.
> 
> > 	o It violates the non-instrumentable nature around VM entry and
> > 	  exit.  For example, when a host breakpoint is set on
> > 	  vcpu->arch.cr2, #DB will hit aftr kvm_guest_enter_irqoff().
> > 
> > Beside the problems, the logic is not consistent either. When the guest
> > debug registers are active, the host breakpoints are reset before
> > kvm_x86->run(). But when the guest debug registers are inactive, the
> > host breakpoints are delayed to be disabled.  The host tracing tools may
> > see different results depending on there is any guest running or not.
> 
> More precisely, the host tracing tools may see different results depending
> on what the guest is doing.
> 
> Queued (with fixed commit message), thanks!
> 
> Paolo
> 
> > To fix the problems, we also reload the debug registers before
> > kvm_x86->run() when the host is using them whenever the guest is using
> > them or not.
> > 
> > Signed-off-by: Lai Jiangshan <laijs@linux.alibaba.com>
> > ---
> >   arch/x86/kvm/x86.c | 2 +-
> >   1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> > index b594275d49b5..cce316655d3c 100644
> > --- a/arch/x86/kvm/x86.c
> > +++ b/arch/x86/kvm/x86.c
> > @@ -9320,7 +9320,7 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
> >   	if (test_thread_flag(TIF_NEED_FPU_LOAD))
> >   		switch_fpu_return();
> > -	if (unlikely(vcpu->arch.switch_db_regs)) {
> > +	if (unlikely(vcpu->arch.switch_db_regs || hw_breakpoint_active())) {
> >   		set_debugreg(0, 7);

I would prefer zero only dr7, e.g.

	if (unlikely(vcpu->arch.switch_db_regs)) {
		...
	} else if (hw_breakpoint_active()) {
		set_debugreg(0, 7);
	}

Stuffing all DRs isn't a bug because hw_breakpoint_restore() will restore all DRs,
but loading stale state into DRs is weird.

> >   		set_debugreg(vcpu->arch.eff_db[0], 0);
> >   		set_debugreg(vcpu->arch.eff_db[1], 1);
> > 
>