From: Michael S. Tsirkin <[email protected]> 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 <[email protected]> Signed-off-by: Avi Kivity <[email protected]> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c index 48ce015..7876ec8 100644 --- a/arch/x86/kvm/x86.c +++ b/arch/x86/kvm/x86.c @@ -3208,10 +3208,6 @@ 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) @@ -3234,6 +3230,10 @@ 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); -- To unsubscribe from this list: send the line "unsubscribe kvm-commits" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html
