Instally dummy, empty memslots immediately after unregistering KVM's mmu_notifier during VM destruction to harden against accessing memory from the wrong address space when tearing down a VM. Because kvm_destroy_vm() often runs when the associated VM's file is being released, current->mm is often no longer kvm->mm, i.e. using the memslots to access userspace memory is inherently broken/dangerous.
While KVM's APIs to read/write guest memory explicitly reject accesses if current->mm != kvm->mm, taking away the memslots adds another layer of defense and helps guard against rogue accesses that don't go through KVM's standard API, or that do GUP+kmap(). Note, simply hoisting memslot destruction above kvm_arch_destroy_vm() without installing dummy slots is not a viable alternative. Doing so would require auditing all the paths of kvm_arch_destroy_vm(), which is nearly infeasible, and missing even one case would result in a NULL pointer dereference and/or use-after-free, neither of which is a substantially better outcome than the status quo. Signed-off-by: Sean Christopherson <[email protected]> --- virt/kvm/kvm_main.c | 45 +++++++++++++++++++++++++++++++-------------- 1 file changed, 31 insertions(+), 14 deletions(-) diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c index f368240aa1cd..770a2c3bd445 100644 --- a/virt/kvm/kvm_main.c +++ b/virt/kvm/kvm_main.c @@ -955,23 +955,42 @@ static void kvm_free_memslot(struct kvm *kvm, struct kvm_memory_slot *slot) kfree(slot); } -static void kvm_free_memslots(struct kvm *kvm, struct kvm_memslots *slots) +static const struct kvm_memslots kvm_empty_memslots = { + .generation = -1ull, + .hva_tree = RB_ROOT_CACHED, + .gfn_tree = RB_ROOT, + .id_hash[0 ... (ARRAY_SIZE(kvm_empty_memslots.id_hash) - 1)] = HLIST_HEAD_INIT, + .node_idx = 0, +}; + +static void kvm_destroy_memslots(struct kvm *kvm) { struct hlist_node *idnode; struct kvm_memory_slot *memslot; - int bkt; + int bkt, i; + + /* + * Install empty memslots to guard against memslot lookups while the VM + * is being destroyed. Consuming memslots at this stage is a KVM bug, + * but "gracefully do nothing" is a much better outcome than "crash the + * host" or "corrupt random memory" when there inevitably is a bug. + */ + mutex_lock(&kvm->slots_lock); + for (i = 0; i < kvm_arch_nr_memslot_as_ids(kvm); i++) + rcu_assign_pointer(kvm->memslots[i], &kvm_empty_memslots); + + synchronize_srcu_expedited(&kvm->srcu); + mutex_unlock(&kvm->slots_lock); /* * The same memslot objects live in both active and inactive sets, - * arbitrarily free using index '1' so the second invocation of this - * function isn't operating over a structure with dangling pointers - * (even though this function isn't actually touching them). + * arbitrarily free using index '1'. */ - if (!slots->node_idx) - return; - - hash_for_each_safe(slots->id_hash, bkt, idnode, memslot, id_node[1]) - kvm_free_memslot(kvm, memslot); + for (i = 0; i < kvm_arch_nr_memslot_as_ids(kvm); i++) { + hash_for_each_safe(kvm->__memslots[i][1].id_hash, bkt, idnode, + memslot, id_node[1]) + kvm_free_memslot(kvm, memslot); + } } static umode_t kvm_stats_debugfs_mode(const struct kvm_stats_desc *desc) @@ -1302,12 +1321,10 @@ static void kvm_destroy_vm(struct kvm *kvm) kvm->mn_active_invalidate_count = 0; else WARN_ON(kvm->mmu_invalidate_in_progress); + kvm_destroy_memslots(kvm); + kvm_arch_destroy_vm(kvm); kvm_destroy_devices(kvm); - for (i = 0; i < kvm_arch_nr_memslot_as_ids(kvm); i++) { - kvm_free_memslots(kvm, &kvm->__memslots[i][0]); - kvm_free_memslots(kvm, &kvm->__memslots[i][1]); - } cleanup_srcu_struct(&kvm->irq_srcu); srcu_barrier(&kvm->srcu); cleanup_srcu_struct(&kvm->srcu); -- 2.56.0.rc1.315.gc6ed9934b7-goog
