Subject: Re: [PATCH] KVM: arm64: vgic-its: reject restored DTE with out-of-range num_eventid_bits

From: Marc Zyngier <[email-masked]>
Date: Wed, 20 May 2026 07:52:58 +0100

On Wed, 20 May 2026 07:04:26 +0100,
Yao Yuan <[email-masked]> wrote:
> 
> On Mon, May 18, 2026 at 09:23:08AM +0800, Marc Zyngier wrote:
> > On Mon, 18 May 2026 07:05:13 +0100,
> > Yao Yuan <[email-masked]> wrote:
> > >
> > > On Sun, May 17, 2026 at 01:49:55PM +0800, Michael Bommarito wrote:
> >
> > [...]
> >
> > > > diff --git a/arch/arm64/kvm/vgic/vgic-its.c b/arch/arm64/kvm/vgic/vgic-its.c
> > > > index 2ea9f1c7ebcd0..a5dcf9a6a2854 100644
> > > > --- a/arch/arm64/kvm/vgic/vgic-its.c
> > > > +++ b/arch/arm64/kvm/vgic/vgic-its.c
> > > > @@ -2307,6 +2307,15 @@ static int vgic_its_restore_dte(struct vgic_its *its, u32 id,
> > > >  	/* dte entry is valid */
> > > >  	offset = (entry & KVM_ITS_DTE_NEXT_MASK) >> KVM_ITS_DTE_NEXT_SHIFT;
> > > >
> > > > +	/*
> > > > +	 * The MAPD command rejects this case; mirror the cap here so a
> > > > +	 * restored DTE cannot install an out-of-range num_eventid_bits
> > > > +	 * that vgic_its_restore_itt() would then convert into a
> > > > +	 * sign-extended scan_its_table() length.
> > > > +	 */
> > > > +	if (num_eventid_bits > VITS_TYPER_IDBITS)
> > > > +		return -EINVAL;
> > >
> > > Hi,
> > >
> > > IIUC, the same issue is still there when VITS_TYPER_IDBITS
> > > change to >=28, I know it's limited to 16 in GITS_TYPER's
> > > definition. I mean the issue is still there w/o really be
> > > fixed.
> >
> > Change how? This is a hard-coded limit that reflect a practical use of
> > the ITS (and is already 32 times larger than what PCIe allows).
> >
> > Are you suggesting a possibility of making this userspace
> > configurable?
> 
> So far No, may until one day it has more than 16 definition :-)

This is 16 *bits*. That 2^16 possible MSIs *per device*. Please show
me a device that exceeds this requirement.

	M.

-- 
Jazz isn't dead. It just smells funny.