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

[Yan Zhao <yan.y.zhao@intel.com>]

On Fri, Dec 20, 2024 at 09:31:35AM -0800, Sean Christopherson wrote:
> 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.
Yes, will do. Thanks!