Re: [PATCH] KVM: arm64: vgic: drop WARN from vgic_get_irq

[Ricardo Koller <ricarkol@google.com>]

On Wed, Aug 18, 2021 at 02:34:03PM -0700, Oliver Upton wrote:
> Hi Ricardo,
> 
> On Wed, Aug 18, 2021 at 2:32 PM Ricardo Koller <ricarkol@google.com> wrote:
> >
> > vgic_get_irq(intid) is used all over the vgic code in order to get a
> > reference to a struct irq. It warns whenever intid is not a valid number
> > (like when it's a reserved IRQ number). The issue is that this warning
> > can be triggered from userspace (e.g., KVM_IRQ_LINE for intid 1020).
> >
> > Drop the WARN call from vgic_get_irq.
> >
> > Signed-off-by: Ricardo Koller <ricarkol@google.com>
> > ---
> >  arch/arm64/kvm/vgic/vgic.c | 1 -
> >  1 file changed, 1 deletion(-)
> >
> > diff --git a/arch/arm64/kvm/vgic/vgic.c b/arch/arm64/kvm/vgic/vgic.c
> > index 111bff47e471..81cec508d413 100644
> > --- a/arch/arm64/kvm/vgic/vgic.c
> > +++ b/arch/arm64/kvm/vgic/vgic.c
> > @@ -106,7 +106,6 @@ struct vgic_irq *vgic_get_irq(struct kvm *kvm, struct kvm_vcpu *vcpu,
> >         if (intid >= VGIC_MIN_LPI)
> >                 return vgic_get_lpi(kvm, intid);
> >
> > -       WARN(1, "Looking up struct vgic_irq for reserved INTID");
> 
> Could we maybe downgrade the message to WARN_ONCE() (to get a stack)
> or pr_warn_ratelimited()? I agree it is problematic that userspace can
> cause this WARN to fire, but it'd be helpful for debugging too.
> 

Was thinking about that, until I found this in bug.h:

	/*
	 * WARN(), WARN_ON(), WARN_ON_ONCE, and so on can be used to report
	 * significant kernel issues that need prompt attention if they should ever
	 * appear at runtime.
	 *
	 * Do not use these macros when checking for invalid external inputs
	 * (e.g. invalid system call arguments, or invalid data coming from
	 * network/devices),

Just in case, KVM_IRQ_LINE returns -EINVAL for an invalid intid (like
1020). I think it's more appropriate for the vmm to log it. What do you
think?

Thanks,
Ricardo

> --
> Thanks,
> Oliver
> 
> >         return NULL;
> >  }
> >
> > --
> > 2.33.0.rc2.250.ged5fa647cd-goog
> >
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