[PATCH 1/2] KVM: arm64: Copy FGT traps to unprotected pKVM VCPU on VCPU load

[Fuad Tabba]

Hi Alexandru,

On Wed, 10 Dec 2025 at 13:21, Alexandru Elisei <alexandru.elisei at arm.com> wrote:
>
> Commit fb10ddf35c1c ("KVM: arm64: Compute per-vCPU FGTs at vcpu_load()")
> introduced per-VCPU FGT traps. For an unprotected pKVM VCPU, the untrusted
> host FGT configuration is copied in pkvm_vcpu_init_traps(), which is called
> from __pkvm_init_vcpu(). __pkvm_init_vcpu() is called once per VCPU (when
> the VCPU is first run) which means that the uninitialized, zero, values for
> the FGT registers end up being used for the entire lifetime of the VCPU.
> This causes both unwanted traps (for the inverse polarity trap bits) and
> the guest being allowed to access registers it shouldn't.
>
> Fix it by copying the FGT traps for unprotected pKVM VCPUs when the
> untrusted host loads the VCPU.
>
> Fixes: fb10ddf35c1c ("KVM: arm64: Compute per-vCPU FGTs at vcpu_load()")
> Signed-off-by: Alexandru Elisei <alexandru.elisei at arm.com>
> ---
>  arch/arm64/kvm/hyp/nvhe/hyp-main.c | 12 ++++++++++--
>  arch/arm64/kvm/hyp/nvhe/pkvm.c     |  1 -
>  2 files changed, 10 insertions(+), 3 deletions(-)
>
> diff --git a/arch/arm64/kvm/hyp/nvhe/hyp-main.c b/arch/arm64/kvm/hyp/nvhe/hyp-main.c
> index 29430c031095..ee0f1343100c 100644
> --- a/arch/arm64/kvm/hyp/nvhe/hyp-main.c
> +++ b/arch/arm64/kvm/hyp/nvhe/hyp-main.c
> @@ -167,6 +167,7 @@ static void handle___pkvm_vcpu_load(struct kvm_cpu_context *host_ctxt)
>         DECLARE_REG(unsigned int, vcpu_idx, host_ctxt, 2);
>         DECLARE_REG(u64, hcr_el2, host_ctxt, 3);
>         struct pkvm_hyp_vcpu *hyp_vcpu;
> +       struct kvm_vcpu *hyp_kvm_vcpu;
>
>         if (!is_protected_kvm_enabled())
>                 return;
> @@ -175,10 +176,17 @@ static void handle___pkvm_vcpu_load(struct kvm_cpu_context *host_ctxt)
>         if (!hyp_vcpu)
>                 return;
>
> +       hyp_kvm_vcpu = &hyp_vcpu->vcpu;
> +
>         if (pkvm_hyp_vcpu_is_protected(hyp_vcpu)) {
>                 /* Propagate WFx trapping flags */
> -               hyp_vcpu->vcpu.arch.hcr_el2 &= ~(HCR_TWE | HCR_TWI);
> -               hyp_vcpu->vcpu.arch.hcr_el2 |= hcr_el2 & (HCR_TWE | HCR_TWI);
> +               hyp_kvm_vcpu->arch.hcr_el2 &= ~(HCR_TWE | HCR_TWI);
> +               hyp_kvm_vcpu->arch.hcr_el2 |= hcr_el2 & (HCR_TWE | HCR_TWI);
> +       } else {
> +               struct kvm_vcpu *host_vcpu = hyp_vcpu->host_vcpu;
> +
> +               memcpy(&hyp_kvm_vcpu->arch.fgt, host_vcpu->arch.fgt,
> +                      sizeof(hyp_kvm_vcpu->arch.fgt));
>         }
>  }

With the fix Will mentioned (hyp-main.c never uses the variable name
hyp_kvm_vcpu, it's vcpu for the hyp, or host_vcpu for the host's).

Tested-by: Fuad Tabba <tabba at google.com>
Reviewed-by: Fuad Tabba <tabba at google.com>

Cheers,
/fuad
>
> diff --git a/arch/arm64/kvm/hyp/nvhe/pkvm.c b/arch/arm64/kvm/hyp/nvhe/pkvm.c
> index 43bde061b65d..05774aed09cb 100644
> --- a/arch/arm64/kvm/hyp/nvhe/pkvm.c
> +++ b/arch/arm64/kvm/hyp/nvhe/pkvm.c
> @@ -172,7 +172,6 @@ static int pkvm_vcpu_init_traps(struct pkvm_hyp_vcpu *hyp_vcpu)
>
>                 /* Trust the host for non-protected vcpu features. */
>                 vcpu->arch.hcrx_el2 = host_vcpu->arch.hcrx_el2;
> -               memcpy(vcpu->arch.fgt, host_vcpu->arch.fgt, sizeof(vcpu->arch.fgt));
>                 return 0;
>         }
>
> --
> 2.52.0
>
>