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Subject[PATCH RFC] 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.

Warning: completely untested.
Please comment.
Note: fix will be needed for -stable etc.

Signed-off-by: Michael S. Tsirkin <>
arch/x86/kvm/x86.c | 8 ++++----
1 files changed, 4 insertions(+), 4 deletions(-)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 3a09c62..4ca1d7f 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -2912,10 +2912,6 @@ 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)
@@ -2937,6 +2933,10 @@ int kvm_vm_ioctl_get_dirty_log(struct kvm *kvm,
dirty_bitmap = old_slots->memslots[log->slot].dirty_bitmap;

+ 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)) {

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