Re: [PATCH v2] KVM: arm64: Mask SErrors in a protected vCPU's host copy at first run

[Oliver Upton <oupton@kernel.org>]

Hi Fuad,

On Tue, Oct 06, 2026 at 10:28:26AM +0100, Fuad Tabba wrote:
> The host's copy of a protected vCPU has SErrors masked from reset, so a
> host-injected SError is pended through HCR_EL2.VSE and the guest takes
> it when it unmasks SErrors. However, the VMM can unmask SErrors in that
> copy before the first run, through PSTATE.A or SCTLR2_EL1.NMEA. KVM
> then emulates the SError's entry on the host copy, and the guest never
> takes it as an SError. Exits that copy PSTATE out set PSTATE.A again,
> but nothing clears NMEA. With NMEA set, an SError injected after a
> KVM_RUN that completes an MMIO access but returns before entering the
> guest also trips WARN_ON(INCREMENT_PC).
> 
> Mask SErrors in the host copy when the first run creates the hyp vCPU,
> after which KVM_SET_ONE_REG is rejected. An SError injected before then
> is still emulated, but only writes registers the VMM can set itself.
> 
> Fixes: 872383bd12e11 ("KVM: arm64: Add per-EC entry/exit state marshalling for protected guests")
> Reported-by: Sashiko <sashiko-bot@kernel.org>
> Closes: https://lore.kernel.org/all/20261001142109.794CA1F000FF@smtp.kernel.org/
> Signed-off-by: Fuad Tabba <fuad.tabba@linux.dev>
> ---
> v2:
>  - Set PSTATE.A and clear SCTLR2_EL1.NMEA in the host copy when the hyp
>    vCPU is created, instead of testing vcpu_is_protected() in
>    kvm_inject_serror_esr() (Marc).
> 
> Applies on kvmarm/next. A follow-up to "KVM: arm64: Confine protected VM
> vCPU state to EL2" [1], from Sashiko's review of its v4 patch 12.
> 
> v1: https://lore.kernel.org/r/20261005050352.836980-1-fuad.tabba@linux.dev/
> [1] https://lore.kernel.org/all/20261001135711.1640520-1-fuad.tabba@linux.dev/
> 
>  arch/arm64/kvm/pkvm.c | 6 ++++++
>  1 file changed, 6 insertions(+)
> 
> diff --git a/arch/arm64/kvm/pkvm.c b/arch/arm64/kvm/pkvm.c
> index d4822d8bb16b8..49973c689789f 100644
> --- a/arch/arm64/kvm/pkvm.c
> +++ b/arch/arm64/kvm/pkvm.c
> @@ -191,12 +191,18 @@ static int __pkvm_create_hyp_vcpu(struct kvm_vcpu *vcpu)
>  	/*
>  	 * Mirror EL2's seeding of power_state from mp_state. The hyp vCPU is
>  	 * published, so take mp_state_lock against kvm_psci_vcpu_on().
> +	 *
> +	 * EL2 never reads the VMM's PSTATE or SCTLR2_EL1: undo any unmasking
> +	 * so that a host SError is pended through HCR_EL2.VSE.
>  	 */
>  	if (vcpu_is_protected(vcpu)) {
>  		spin_lock(&vcpu->arch.mp_state_lock);
>  		if (kvm_arm_vcpu_stopped(vcpu))
>  			WRITE_ONCE(vcpu->arch.mp_state.mp_state, KVM_MP_STATE_UNINITIALIZED);
>  		spin_unlock(&vcpu->arch.mp_state_lock);
> +
> +		*vcpu_cpsr(vcpu) |= PSR_A_BIT;
> +		__vcpu_rmw_sys_reg(vcpu, SCTLR2_EL1, &=, ~SCTLR2_EL1_NMEA);
>  	}

Urgh, I hadn't realized that PSTATE.A is still modifiable by userspace
prior to KVM_RUN. In that case, I would prefer the vcpu_has_nv()
approach that I had recommended as a backup.

FWIW, since pVMs do not implement FEAT_SCTLR2 it should not be able to
write to the register at all.

Thanks,
Oliver