[PATCH v1 3/4] KVM: arm64: Sync protected guest VBAR_EL1 on injecting an undef exception

[Fuad Tabba]

Hi Will,

On Tue, 5 Aug 2025 at 15:35, Will Deacon <will at kernel.org> wrote:
>
> On Tue, Aug 05, 2025 at 02:56:16PM +0100, Fuad Tabba wrote:
> > In pKVM, a race condition can occur if a guest updates its VBAR_EL1
> > register and, before a vCPU exit synchronizes this change, the
> > hypervisor needs to inject an undefined exception into a protected
> > guest.
> >
> > In this scenario, the vCPU still holds the stale VBAR_EL1 value from
> > before the guest's update. When pKVM injects the exception, it ends up
> > using the stale value.
> >
> > Explicitly read the live value of VBAR_EL1 from the guest and update the
> > vCPU value immediately before pending the exception. This ensures the
> > vCPU's value is the same as the guest's and that the exception will be
> > handled at the correct address upon resuming the guest.
> >
> > Signed-off-by: Fuad Tabba <tabba at google.com>
> > ---
> >  arch/arm64/kvm/hyp/nvhe/sys_regs.c | 1 +
> >  1 file changed, 1 insertion(+)
> >
> > diff --git a/arch/arm64/kvm/hyp/nvhe/sys_regs.c b/arch/arm64/kvm/hyp/nvhe/sys_regs.c
> > index bbd60013cf9e..b34b10be1ad7 100644
> > --- a/arch/arm64/kvm/hyp/nvhe/sys_regs.c
> > +++ b/arch/arm64/kvm/hyp/nvhe/sys_regs.c
> > @@ -253,6 +253,7 @@ static void inject_undef64(struct kvm_vcpu *vcpu)
> >
> >       *vcpu_pc(vcpu) = read_sysreg_el2(SYS_ELR);
> >       *vcpu_cpsr(vcpu) = read_sysreg_el2(SYS_SPSR);
> > +     vcpu_write_sys_reg(vcpu, read_sysreg_el1(SYS_VBAR), VBAR_EL1);
>
> You say "in pKVM" in the commit message, but why isn't this applicable
> to !VHE in general?

Because this function is only used in pKVM. I guess there's nothing
stopping anyone from using it in other non-VHE code in the future
though

Cheers,
/fuad

> Will