Commit edde99ce authored Oct 25, 2010 by Michael S. Tsirkin Committed by Marcelo Tosatti Nov 05, 2010

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>

parent ff8b16d7

arch/x86/kvm/x86.c

+4 −4

Original line number	Diff line number	Diff line
		@@ -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);

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