Re: [PATCH v22 01/23] KVM: arm64: protected VM: Handle user writes to CNTVCT_EL0/CNTPCT_EL0

[Gavin Shan]

On 10/5/26 7:07 PM, Suzuki K Poulose wrote:
> 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@xxxxxxxxxxxxxxx
> Fixes: f7d05ee84a6a ("KVM: arm64: Prevent host from managing timer offsets for protected VMs")
> Suggested-by: Marc Zyngier <maz@xxxxxxxxxx>
> Tested-by: Gavin Shan <gshan@xxxxxxxxxx>
> Signed-off-by: Suzuki K Poulose <suzuki.poulose@xxxxxxx>
> ---
>   Changes since v19:
>    - Fix typos in commit description and explain why we choose the approach.
>    - Improve comment in the code
>   Changes since v18:
>    - Retain NULL vm_offset for protected VMs to avoid host tampering with the
>      offset.
>    - Moved the flag setting into kvm_timer_init_vm(), where it should have been
>      in the first place
> ---
>   arch/arm64/kvm/arch_timer.c | 12 ++++++++++--
>   1 file changed, 10 insertions(+), 2 deletions(-)
Reviewed-by: Gavin Shan <gshan@xxxxxxxxxx>