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

[Fuad Tabba]

On Wed, 26 Nov 2025 at 11:47, Marc Zyngier <maz at kernel.org> wrote:
>
> On Wed, 26 Nov 2025 10:37:57 +0000,
> Fuad Tabba <tabba at google.com> wrote:
> >
> > 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?
>
> I think you need something of the sort, yes. Overall, SPE and TRBE
> should be aligned on what they are allowed to do, as they are two
> sides of the same ugly coin.

I'll fix this (or add a patch to do this, depending on how it looks)
when I respin.

Cheers,
/fuad

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