[PATCH v5 2/9] KVM: arm64: Fix Trace Buffer trap polarity for protected VMs

[Fuad Tabba]

Hi Marc,

On Wed, 26 Nov 2025 at 10:23, Marc Zyngier <maz at kernel.org> wrote:
>
> On Tue, 18 Nov 2025 10:37:59 +0000,
> Fuad Tabba <tabba at google.com> wrote:
> >
> > The E2TB bits in MDCR_EL2 control trapping of Trace Buffer system
> > register accesses. These accesses are trapped to EL2 when the bits are
> > clear.
> >
> > The trap initialization logic for protected VMs in pvm_init_traps_mdcr()
> > had the polarity inverted. When a guest did not support the Trace Buffer
> > feature, the code was setting E2TB. This incorrectly disabled the trap,
> > potentially allowing a protected guest to access registers for a feature
> > it was not given.
> >
> > Fix this by inverting the operation.
> >
> > Fixes: f50758260bff ("KVM: arm64: Group setting traps for protected VMs by control register")
> > Signed-off-by: Fuad Tabba <tabba at google.com>
> > ---
> >  arch/arm64/kvm/hyp/nvhe/pkvm.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/arch/arm64/kvm/hyp/nvhe/pkvm.c b/arch/arm64/kvm/hyp/nvhe/pkvm.c
> > index 8d06a246dfd1..f6f8996c4f97 100644
> > --- a/arch/arm64/kvm/hyp/nvhe/pkvm.c
> > +++ b/arch/arm64/kvm/hyp/nvhe/pkvm.c
> > @@ -118,7 +118,7 @@ static void pvm_init_traps_mdcr(struct kvm_vcpu *vcpu)
> >               val |= MDCR_EL2_TTRF;
> >
> >       if (!kvm_has_feat(kvm, ID_AA64DFR0_EL1, TraceBuffer, IMP))
> > -             val |= MDCR_EL2_E2TB_MASK;
> > +             val &= ~MDCR_EL2_E2TB_MASK;
>
> This does not only change the trapping logic (bit 24). It also change
> the ownership of the buffer (bit 25). I wonder whether you should do
> something for that, maybe by clearing TRBLIMITR_EL1.E, because
> otherwise, you keep tracing, but using an EL2 VA. What could possibly
> go wrong?
>
> Overall, I'm very uneasy about TRBE in the context of pKVM.

So should we clear/restore TRBLIMITR_EL1.E on guest entry/exit in
protected mode?

Cheers,
/fuad

> Thanks,
>
>         M.
>
> --
> Without deviation from the norm, progress is not possible.