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

[Marc Zyngier <maz@kernel.org>]

On Tue, 18 Nov 2025 10:37:59 +0000,
Fuad Tabba <tabba@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@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.

Thanks,

	M.

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