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Description of problem:
We need write protect memory after the slot swap in kvm_vm_ioctl_get_dirty_log() otherwise we may miss dirty pages.
This issue have been fixed in upstream in:
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>
Signed-off-by: Avi Kivity <avi>
Version-Release number of selected component (if applicable):
6.0
How reproducible:
Steps to Reproduce:
1.
2.
3.
Actual results:
Expected results:
Additional info:
Description of problem: We need write protect memory after the slot swap in kvm_vm_ioctl_get_dirty_log() otherwise we may miss dirty pages. This issue have been fixed in upstream in: 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> Signed-off-by: Avi Kivity <avi> Version-Release number of selected component (if applicable): 6.0 How reproducible: Steps to Reproduce: 1. 2. 3. Actual results: Expected results: Additional info: