| From | Andi Kleen <> | Subject | [PATCH] [104/223] KVM: Write protect memory after slot swap | Date | Mon, 13 Dec 2010 00:46:44 +0100 (CET) |
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2.6.35-longterm review patch. If anyone has any objections, please let me know.
------------------ From: Michael S. Tsirkin <mst@redhat.com>
commit edde99ce05290e50ce0b3495d209e54e6349ab47 upstream.
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.
Signed-off-by: Michael S. Tsirkin <mst@redhat.com> Signed-off-by: Avi Kivity <avi@redhat.com> Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de> Signed-off-by: Andi Kleen <ak@linux.intel.com>
--- arch/x86/kvm/x86.c | 9 +++++---- 1 file changed, 5 insertions(+), 4 deletions(-)
Index: linux/arch/x86/kvm/x86.c =================================================================== --- linux.orig/arch/x86/kvm/x86.c +++ linux/arch/x86/kvm/x86.c @@ -2825,10 +2825,6 @@ int kvm_vm_ioctl_get_dirty_log(struct kv if (is_dirty) { struct kvm_memslots *slots, *old_slots; - spin_lock(&kvm->mmu_lock); - kvm_mmu_slot_remove_write_access(kvm, log->slot); - spin_unlock(&kvm->mmu_lock); - slots = kzalloc(sizeof(struct kvm_memslots), GFP_KERNEL); if (!slots) goto out_free; @@ -2841,6 +2837,11 @@ int kvm_vm_ioctl_get_dirty_log(struct kv synchronize_srcu_expedited(&kvm->srcu); 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 = 0;
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