Re: [PATCH 1/2] KVM: Do not reset dirty GFNs in a memslot not enabling dirty tracking

[Sean Christopherson <seanjc@google.com>]

On Fri, Dec 20, 2024, Yan Zhao wrote:
> Do not allow resetting dirty GFNs belonging to a memslot that does not
> enable dirty tracking.
> 
> vCPUs' dirty rings are shared between userspace and KVM. After KVM sets
> dirtied entries in the dirty rings, userspace is responsible for
> harvesting/resetting the dirtied entries and calling the ioctl
> KVM_RESET_DIRTY_RINGS to inform KVM to advance the reset_index in the
> dirty rings and invoke kvm_arch_mmu_enable_log_dirty_pt_masked() to clear
> the SPTEs' dirty bits or perform write protection of GFNs.
> 
> Although KVM does not set dirty entries for GFNs in a memslot that does not
> enable dirty tracking, it is still possible for userspace to specify that
> it has harvested a GFN belonging to such a memslot. When this happens, KVM
> will be asked to clear dirty bits or perform write protection for GFNs in a
> memslot that does not enable dirty tracking, which is not desired.
> 
> For TDX, this unexpected resetting of dirty GFNs could cause inconsistency
> between the mirror SPTE and the external SPTE in hardware (e.g., the mirror
> SPTE has no write bit while it is writable in the external SPTE in
> hardware). When kvm_dirty_log_manual_protect_and_init_set() is true and
> when huge pages are enabled in TDX, this could even lead to
> kvm_mmu_slot_gfn_write_protect() being called and the external SPTE being
> removed.
> 
> Signed-off-by: Yan Zhao <yan.y.zhao@intel.com>
> ---
>  virt/kvm/dirty_ring.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/virt/kvm/dirty_ring.c b/virt/kvm/dirty_ring.c
> index d14ffc7513ee..1ce5352ea596 100644
> --- a/virt/kvm/dirty_ring.c
> +++ b/virt/kvm/dirty_ring.c
> @@ -66,7 +66,8 @@ static void kvm_reset_dirty_gfn(struct kvm *kvm, u32 slot, u64 offset, u64 mask)
>  
>  	memslot = id_to_memslot(__kvm_memslots(kvm, as_id), id);
>  
> -	if (!memslot || (offset + __fls(mask)) >= memslot->npages)
> +	if (!memslot || (offset + __fls(mask)) >= memslot->npages ||
> +	    !kvm_slot_dirty_track_enabled(memslot))

Can you add a comment explaining that it's possible to try to update a memslot
that isn't being dirty-logged if userspace is misbehaving?  And specifically that
userspace can write arbitrary data into the ring.