| Message ID | 20241220082231.15884-1-yan.y.zhao@intel.com (mailing list archive) |
|---|---|
| State | New |
| Headers | show |
| Series | KVM: Do not reset dirty GFNs in a memslot not enabling dirty tracking | expand |
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.
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)) return; KVM_MMU_LOCK(kvm);
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(-)