Subject: Re: [PATCH] KVM: arm64: Sanitise host vCPU fields in flush_hyp_vcpu()

From: Hyunwoo Kim <[email-masked]>
Date: Thu, 4 Jun 2026 22:35:20 +0900

On Thu, Jun 04, 2026 at 02:01:17PM +0100, Fuad Tabba wrote:
> Hi Hyunwoo,
> 
> On Thu, 4 Jun 2026 at 12:18, Hyunwoo Kim <[email-masked]> wrote:
> >
> > flush_hyp_vcpu() copies the host vCPU context and vGIC state into the
> > hyp's private vCPU on every run. ctxt_to_vcpu() expects a guest context
> > to have a NULL __hyp_running_vcpu, which is only ever set on the host
> > context, so that it resolves the vCPU via container_of(). The vGIC list
> > register save and restore expect used_lrs to stay within the number of
> > implemented list registers. While this is generally the case,
> > flush_hyp_vcpu() copies both fields verbatim from the host vCPU and
> > enforces neither expectation.
> >
> > Fix by clearing __hyp_running_vcpu and clamping used_lrs after the copy.
> 
> Nice catch, both fixes are correct.

Thanks for the review.

> 
> Please split this into two patches, one per field. They are independent
> bugs that just happen to share a Fixes: tag and the function. Both are
> host -> EL2, so worth stating that in the commit messages.

I'll split this into two patches and resend it as a series.

> 
> Otherwise this looks right to me.
> 
> Cheers,
> /fuad
> 
> 
> >
> > Fixes: be66e67f1750 ("KVM: arm64: Use the pKVM hyp vCPU structure in handle___kvm_vcpu_run()")
> > Signed-off-by: Hyunwoo Kim <[email-masked]>
> > ---
> >  arch/arm64/kvm/hyp/nvhe/hyp-main.c | 11 +++++++++++
> >  1 file changed, 11 insertions(+)
> >
> > diff --git a/arch/arm64/kvm/hyp/nvhe/hyp-main.c b/arch/arm64/kvm/hyp/nvhe/hyp-main.c
> > index 06db299c37a89..ef9318ff0c25e 100644
> > --- a/arch/arm64/kvm/hyp/nvhe/hyp-main.c
> > +++ b/arch/arm64/kvm/hyp/nvhe/hyp-main.c
> > @@ -7,6 +7,7 @@
> >  #include <hyp/adjust_pc.h>
> >  #include <hyp/switch.h>
> >
> > +#include <asm/arch_gicv3.h>
> >  #include <asm/pgtable-types.h>
> >  #include <asm/kvm_asm.h>
> >  #include <asm/kvm_emulate.h>
> > @@ -128,6 +129,9 @@ static void flush_hyp_vcpu(struct pkvm_hyp_vcpu *hyp_vcpu)
> >
> >         hyp_vcpu->vcpu.arch.ctxt        = host_vcpu->arch.ctxt;
> >
> > +       /* A guest context must keep a NULL __hyp_running_vcpu. */
> > +       hyp_vcpu->vcpu.arch.ctxt.__hyp_running_vcpu = NULL;
> > +
> >         hyp_vcpu->vcpu.arch.mdcr_el2    = host_vcpu->arch.mdcr_el2;
> >         hyp_vcpu->vcpu.arch.hcr_el2 &= ~(HCR_TWI | HCR_TWE);
> >         hyp_vcpu->vcpu.arch.hcr_el2 |= READ_ONCE(host_vcpu->arch.hcr_el2) &
> > @@ -139,6 +143,13 @@ static void flush_hyp_vcpu(struct pkvm_hyp_vcpu *hyp_vcpu)
> >
> >         hyp_vcpu->vcpu.arch.vgic_cpu.vgic_v3 = host_vcpu->arch.vgic_cpu.vgic_v3;
> >
> > +       /* Bound the host-provided used_lrs by the implemented list registers. */
> > +       if (static_branch_unlikely(&kvm_vgic_global_state.gicv3_cpuif))
> > +               hyp_vcpu->vcpu.arch.vgic_cpu.vgic_v3.used_lrs =
> > +                       min_t(unsigned int,
> > +                             hyp_vcpu->vcpu.arch.vgic_cpu.vgic_v3.used_lrs,
> > +                             (read_gicreg(ICH_VTR_EL2) & 0xf) + 1);
> > +
> >         hyp_vcpu->vcpu.arch.pid = host_vcpu->arch.pid;
> >  }
> >
> > --
> > 2.43.0
> >
> >

Best regards,
Hyunwoo Kim