KVM: Write protect memory after slot swap

I have observed the following bug trigger:

1. userspace calls GET_DIRTY_LOG
2. kvm_mmu_slot_remove_write_access is called and makes a page ro
3. page fault happens and makes the page writeable
   fault is logged in the bitmap appropriately
4. kvm_vm_ioctl_get_dirty_log swaps slot pointers

a lot of time passes

5. guest writes into the page
6. userspace calls GET_DIRTY_LOG

At point (5), bitmap is clean and page is writeable,
thus, guest modification of memory is not logged
and GET_DIRTY_LOG returns an empty bitmap.

The rule is that all pages are either dirty in the current bitmap,
or write-protected, which is violated here.

It seems that just moving kvm_mmu_slot_remove_write_access down
to after the slot pointer swap should fix this bug.

KVM-Stable-Tag.
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Avi Kivity <avi@redhat.com>

1 files changed, 4 insertions, 4 deletions

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 2288ad829b32..b0818f672064 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -3169,10 +3169,6 @@ int kvm_vm_ioctl_get_dirty_log(struct kvm *kvm,
                 struct kvm_memslots *slots, *old_slots;
                 unsigned long *dirty_bitmap;
 
-                spin_lock(&kvm->mmu_lock);
-                kvm_mmu_slot_remove_write_access(kvm, log->slot);
-                spin_unlock(&kvm->mmu_lock);
-
                 r = -ENOMEM;
                 dirty_bitmap = vmalloc(n);
                 if (!dirty_bitmap)
@@ -3194,6 +3190,10 @@ int kvm_vm_ioctl_get_dirty_log(struct kvm *kvm,
                 dirty_bitmap = old_slots->memslots[log->slot].dirty_bitmap;
                 kfree(old_slots);
 
+                spin_lock(&kvm->mmu_lock);
+                kvm_mmu_slot_remove_write_access(kvm, log->slot);
+                spin_unlock(&kvm->mmu_lock);
+
                 r = -EFAULT;
                 if (copy_to_user(log->dirty_bitmap, dirty_bitmap, n)) {
                         vfree(dirty_bitmap);