Re: [PATCH] KVM: x86/xen: Fix bug in kvm_xen_vcpu_set_attr()

[Coleman Dietsch <dietschc@csp.edu>]

On Thu, Jul 28, 2022 at 08:41:14PM +0000, Sean Christopherson wrote:
> Be more specific in the shortlog.  "Fix a bug in XYZ" doesn't provide any info
> about the bug itself, and can even become frustratingly stale if XYZ is renamed.
> I believe we should end up with two patches (see below), e.g.
> 
>   KVM: x86/xen: Initialize Xen timer only once (when it's NOT running)
> 
> and
>   
>   KVM: x86/xen: Stop Xen timer before changing the IRQ vector
> 

Got it, I will work on splitting the v2 into a patch set as you suggested
(with better names of course).

> Note, I'm assuming timer_virq is a vector of some form, I haven't actually looked
> that far into the code.
> 
> On Thu, Jul 28, 2022, Coleman Dietsch wrote:
> > This crash appears to be happening when vcpu->arch.xen.timer is already set
> 
> Instead of saying "This crash", provide the actual splat (sanitized to make it
> more readable).  That way readers, reviewers, and archaeologists don't need to
> open up a hyperlink to get details on what broken.
> 
> > and kvm_xen_init_timer(vcpu) is called.
> 
> Wrap changelogs at ~75 chars.
> 
> > During testing with the syzbot reproducer code it seemed apparent that the
> > else if statement in the KVM_XEN_VCPU_ATTR_TYPE_TIMER switch case was not
> > being reached, which is where the kvm_xen_stop_timer(vcpu) call is located.
> 
> Neither the shortlog nor the changelog actually says anything about what is actually
> being changed.
> 

I will make sure to address all these issues in the v2 patch set.

> > Link: https://syzkaller.appspot.com/bug?id=8234a9dfd3aafbf092cc5a7cd9842e3ebc45fc42
> > Reported-and-tested-by: syzbot+e54f930ed78eb0f85281@syzkaller.appspotmail.com
> > Signed-off-by: Coleman Dietsch <dietschc@csp.edu>
> > ---
> >  arch/x86/kvm/xen.c | 9 ++++++---
> >  1 file changed, 6 insertions(+), 3 deletions(-)
> > 
> > diff --git a/arch/x86/kvm/xen.c b/arch/x86/kvm/xen.c
> > index 610beba35907..4b4b985813c5 100644
> > --- a/arch/x86/kvm/xen.c
> > +++ b/arch/x86/kvm/xen.c
> > @@ -707,6 +707,12 @@ int kvm_xen_vcpu_set_attr(struct kvm_vcpu *vcpu, struct kvm_xen_vcpu_attr *data)
> >  		break;
> >  
> >  	case KVM_XEN_VCPU_ATTR_TYPE_TIMER:
> > +		/* Stop current timer if it is enabled */
> > +		if (kvm_xen_timer_enabled(vcpu)) {
> > +			kvm_xen_stop_timer(vcpu);
> > +			vcpu->arch.xen.timer_virq = 0;
> > +		}
> > +
> >  		if (data->u.timer.port) {
> >  			if (data->u.timer.priority != KVM_IRQ_ROUTING_XEN_EVTCHN_PRIO_2LEVEL) {
> >  				r = -EINVAL;
> 
> I'm not entirely sure this is correct.  Probably doesn't matter, but there's a
> subtle ABI change here in that invoking the ioctl with a "bad" priority will
> cancel any existing timer.
> 

I will try to get some clarification before I send in the next patch.

> And there appear to be two separate bugs: initializing the hrtimer while it's
> running, and not canceling a running timer before changing timer_virq.
> 

This does seem to be the case so I will be splitting v2 into a patch
set.

> Calling kvm_xen_init_timer() on "every" KVM_XEN_VCPU_ATTR_TYPE_TIMER is odd and
> unnecessary, it only needs to be called once during vCPU setup.  If Xen doesn't
> have such a hook, then a !ULL check can be done on vcpu->arch.xen.timer.function
> to initialize the timer on-demand.
> 

Yes I also thought that was a bit odd that kvm_xen_init_timer() is called on "every" KVM_XEN_VCPU_ATTR_TYPE_TIMER 

> With that out of the way, the code can be streamlined a bit, e.g. something like
> this?
> 
> 	case KVM_XEN_VCPU_ATTR_TYPE_TIMER:
> 		if (data->u.timer.port &&
> 		    data->u.timer.priority != KVM_IRQ_ROUTING_XEN_EVTCHN_PRIO_2LEVEL) {
> 			r = -EINVAL;
> 			break;
> 		}
> 
> 		if (!vcpu->arch.xen.timer.function)
> 			kvm_xen_init_timer(vcpu);
> 
> 		/* Stop the timer (if it's running) before changing the vector. */
> 		kvm_xen_stop_timer(vcpu);
> 		vcpu->arch.xen.timer_virq = data->u.timer.port;
> 
> 		if (data->u.timer.port && data->u.timer.expires_ns)
> 			kvm_xen_start_timer(vcpu, data->u.timer.expires_ns,
> 					    data->u.timer.expires_ns -
> 					    get_kvmclock_ns(vcpu->kvm));
> 		r = 0;
> 		break;
> 

I agree this code could use some cleanup, I'll see what I can do.

> > @@ -720,9 +726,6 @@ int kvm_xen_vcpu_set_attr(struct kvm_vcpu *vcpu, struct kvm_xen_vcpu_attr *data)
> >  				kvm_xen_start_timer(vcpu, data->u.timer.expires_ns,
> >  						    data->u.timer.expires_ns -
> >  						    get_kvmclock_ns(vcpu->kvm));
> > -		} else if (kvm_xen_timer_enabled(vcpu)) {
> > -			kvm_xen_stop_timer(vcpu);
> > -			vcpu->arch.xen.timer_virq = 0;
> >  		}
> >  
> >  		r = 0;
> > -- 
> > 2.34.1
> > 

Thank you for the feedback Sean, it has been most helpful!
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