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@xxxxxxxxx> > --- > 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.
