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KVM: arm64: protected VM: Handle user writes to CNTVCT_EL0/CNTPCT_EL0

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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: f7d05ee84a ("KVM: arm64: Prevent host from managing timer offsets for protected VMs")
Suggested-by: Marc Zyngier <maz@kernel.org>
Reviewed-by: Gavin Shan <gshan@redhat.com>
Tested-by: Gavin Shan <gshan@redhat.com>
Signed-off-by: Suzuki K Poulose <suzuki.poulose@arm.com>
Reviewed-by: Fuad Tabba <fuad.tabba@linux.dev>
Link: https://patch.msgid.link/20261007073537.2454351-2-suzuki.poulose@arm.com
Signed-off-by: Marc Zyngier <maz@kernel.org>
This commit is contained in:
Suzuki K Poulose 2026-10-07 08:35:24 +01:00 committed by Marc Zyngier
parent fd73f4a665
commit 3d736a339e
1 changed files with 10 additions and 2 deletions
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12
arch/arm64/kvm/arch_timer.c
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@ -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)