From 3d736a339e9cbeec3841da1a821f56fb3230fd68 Mon Sep 17 00:00:00 2001 From: Suzuki K Poulose Date: Wed, 7 Oct 2026 08:35:24 +0100 Subject: [PATCH] KVM: arm64: protected VM: Handle user writes to CNTVCT_EL0/CNTPCT_EL0 Protected VMs doesn't allow setting offsets for virtual and physical counters, as the offset is always fixed to 0. The VM ioctl is filtered out based on the cap. However we don't prevent the userspace from trying to write to the CNTVCT/CNTPCT registers. This would lead to KVM triggering a WARN() in timer_set_offset() as the vm_offset pointer is set to NULL. Fix this by always "fixing" the timer offsets to 0 and marking that the timer offset is set in the kvm->arch.flags at KVM init time for protected VMs. This prevents the access to the VM specific vm_offset at low cost. A userspace writing to the CNT*CT_EL0 would observe success, without any real effect. This is cleaner over spilling "*_is_protected()" checks and "matches" what we really do in practise. i.e., always run with "fixed counter offset of 0". Reported by Sashiko Link: https://lore.kernel.org/all/20260908164641.416911F00A3A@smtp.kernel.org Fixes: f7d05ee84a6a ("KVM: arm64: Prevent host from managing timer offsets for protected VMs") Suggested-by: Marc Zyngier Reviewed-by: Gavin Shan Tested-by: Gavin Shan Signed-off-by: Suzuki K Poulose Reviewed-by: Fuad Tabba Link: https://patch.msgid.link/20261007073537.2454351-2-suzuki.poulose@arm.com Signed-off-by: Marc Zyngier --- arch/arm64/kvm/arch_timer.c | 12 ++++++++++-- 1 file changed, 10 insertions(+), 2 deletions(-) diff --git a/arch/arm64/kvm/arch_timer.c b/arch/arm64/kvm/arch_timer.c index 6ac3321f4c57..a45845f4ae4e 100644 --- a/arch/arm64/kvm/arch_timer.c +++ b/arch/arm64/kvm/arch_timer.c @@ -1110,8 +1110,7 @@ void kvm_timer_vcpu_init(struct kvm_vcpu *vcpu) timer_context_init(vcpu, i); /* Synchronize offsets across timers of a VM if not already provided */ - if (!vcpu_is_protected(vcpu) && - !test_bit(KVM_ARCH_FLAG_VM_COUNTER_OFFSET, &vcpu->kvm->arch.flags)) { + if (!test_bit(KVM_ARCH_FLAG_VM_COUNTER_OFFSET, &vcpu->kvm->arch.flags)) { timer_set_offset(vcpu_vtimer(vcpu), kvm_phys_timer_read()); timer_set_offset(vcpu_ptimer(vcpu), 0); } @@ -1133,6 +1132,15 @@ void kvm_timer_init_vm(struct kvm *kvm) */ for (int i = 0; i < NR_KVM_TIMERS; i++) kvm->arch.timer_data.ppi[i] = get_vgic_ppi(kvm, default_ppi[i]); + + /* + * Protected VMs don't allow the userspace to set counter offsets, + * either set via counter register writes or the dedicated ioctls. + * Pretend the offset has already been set and rely on the default + * offset being 0. + */ + if (kvm_vm_is_protected(kvm)) + set_bit(KVM_ARCH_FLAG_VM_COUNTER_OFFSET, &kvm->arch.flags); } void kvm_timer_cpu_up(void)