On Thu, Mar 23, 2017 at 06:24:19PM +0100, David Hildenbrand wrote:
> No caller currently checks the return value of
> kvm_io_bus_unregister_dev(). This is evil, as all callers silently go on
> freeing their device. A stale reference will remain in the io_bus,
> getting at least used again, when the iobus gets teared down on
> kvm_destroy_vm() - leading to use after free errors.
>
> There is nothing the callers could do, except retrying over and over
> again.
>
> So let's simply remove the bus altogether, print an error and make
> sure no one can access this broken bus again (returning -ENOMEM on any
> attempt to access it).
>
> Fixes: e93f8a0f821e ("KVM: convert io_bus to SRCU")
> Cc: stable@xxxxxxxxxxxxxxx # 3.4+
> Reported-by: Dmitry Vyukov <dvyukov@xxxxxxxxxx>
> Signed-off-by: David Hildenbrand <david@xxxxxxxxxx>
> ---
>
> Based on kvm/queue, where we just got 2a108a4e7c1 ("KVM: x86: clear bus
> pointer when destroyed"), which added a check we need here.
>
> v1 -> v2:
> - added a check in kvm_destroy_vm()
> - added a check in virt/kvm/eventfd.c
>
> Using 'git grep -C 4 "kvm->buses"' should help to find all users. The other
> user in virt/kvm/eventfd.c should be fine.
>
> ---
> include/linux/kvm_host.h | 4 ++--
> virt/kvm/eventfd.c | 3 ++-
> virt/kvm/kvm_main.c | 42 +++++++++++++++++++++++++-----------------
> 3 files changed, 29 insertions(+), 20 deletions(-)
>
> diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
> index 2c14ad9..d025074 100644
> --- a/include/linux/kvm_host.h
> +++ b/include/linux/kvm_host.h
> @@ -162,8 +162,8 @@ int kvm_io_bus_read(struct kvm_vcpu *vcpu, enum kvm_bus bus_idx, gpa_t addr,
> int len, void *val);
> int kvm_io_bus_register_dev(struct kvm *kvm, enum kvm_bus bus_idx, gpa_t addr,
> int len, struct kvm_io_device *dev);
> -int kvm_io_bus_unregister_dev(struct kvm *kvm, enum kvm_bus bus_idx,
> - struct kvm_io_device *dev);
> +void kvm_io_bus_unregister_dev(struct kvm *kvm, enum kvm_bus bus_idx,
> + struct kvm_io_device *dev);
> struct kvm_io_device *kvm_io_bus_get_dev(struct kvm *kvm, enum kvm_bus bus_idx,
> gpa_t addr);
>
> diff --git a/virt/kvm/eventfd.c b/virt/kvm/eventfd.c
> index a29786d..4d28a9d 100644
> --- a/virt/kvm/eventfd.c
> +++ b/virt/kvm/eventfd.c
> @@ -870,7 +870,8 @@ kvm_deassign_ioeventfd_idx(struct kvm *kvm, enum kvm_bus bus_idx,
> continue;
>
> kvm_io_bus_unregister_dev(kvm, bus_idx, &p->dev);
> - kvm->buses[bus_idx]->ioeventfd_count--;
> + if (kvm->buses[bus_idx])
> + kvm->buses[bus_idx]->ioeventfd_count--;
> ioeventfd_release(p);
> ret = 0;
> break;
> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> index 7445566..ef1aa7f 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -728,7 +728,8 @@ static void kvm_destroy_vm(struct kvm *kvm)
> spin_unlock(&kvm_lock);
> kvm_free_irq_routing(kvm);
> for (i = 0; i < KVM_NR_BUSES; i++) {
> - kvm_io_bus_destroy(kvm->buses[i]);
> + if (kvm->buses[i])
> + kvm_io_bus_destroy(kvm->buses[i]);
> kvm->buses[i] = NULL;
> }
> kvm_coalesced_mmio_free(kvm);
> @@ -3476,6 +3477,8 @@ int kvm_io_bus_write(struct kvm_vcpu *vcpu, enum kvm_bus bus_idx, gpa_t addr,
> };
>
> bus = srcu_dereference(vcpu->kvm->buses[bus_idx], &vcpu->kvm->srcu);
> + if (!bus)
> + return -ENOMEM;
> r = __kvm_io_bus_write(vcpu, bus, &range, val);
> return r < 0 ? r : 0;
> }
> @@ -3493,6 +3496,8 @@ int kvm_io_bus_write_cookie(struct kvm_vcpu *vcpu, enum kvm_bus bus_idx,
> };
>
> bus = srcu_dereference(vcpu->kvm->buses[bus_idx], &vcpu->kvm->srcu);
> + if (!bus)
> + return -ENOMEM;
>
> /* First try the device referenced by cookie. */
> if ((cookie >= 0) && (cookie < bus->dev_count) &&
> @@ -3543,6 +3548,8 @@ int kvm_io_bus_read(struct kvm_vcpu *vcpu, enum kvm_bus bus_idx, gpa_t addr,
> };
>
> bus = srcu_dereference(vcpu->kvm->buses[bus_idx], &vcpu->kvm->srcu);
> + if (!bus)
> + return -ENOMEM;
> r = __kvm_io_bus_read(vcpu, bus, &range, val);
> return r < 0 ? r : 0;
> }
> @@ -3555,6 +3562,9 @@ int kvm_io_bus_register_dev(struct kvm *kvm, enum kvm_bus bus_idx, gpa_t addr,
> struct kvm_io_bus *new_bus, *bus;
>
> bus = kvm->buses[bus_idx];
> + if (!bus)
> + return -ENOMEM;
> +
> /* exclude ioeventfd which is limited by maximum fd */
> if (bus->dev_count - bus->ioeventfd_count > NR_IOBUS_DEVS - 1)
> return -ENOSPC;
> @@ -3574,45 +3584,41 @@ int kvm_io_bus_register_dev(struct kvm *kvm, enum kvm_bus bus_idx, gpa_t addr,
> }
>
> /* Caller must hold slots_lock. */
> -int kvm_io_bus_unregister_dev(struct kvm *kvm, enum kvm_bus bus_idx,
> - struct kvm_io_device *dev)
> +void kvm_io_bus_unregister_dev(struct kvm *kvm, enum kvm_bus bus_idx,
> + struct kvm_io_device *dev)
> {
> - int i, r;
> + int i;
> struct kvm_io_bus *new_bus, *bus;
>
> bus = kvm->buses[bus_idx];
> -
> - /*
> - * It's possible the bus being released before hand. If so,
> - * we're done here.
> - */
> if (!bus)
> - return 0;
> + return;
>
> - r = -ENOENT;
> for (i = 0; i < bus->dev_count; i++)
> if (bus->range[i].dev == dev) {
> - r = 0;
> break;
> }
>
> - if (r)
> - return r;
> + if (i == bus->dev_count)
> + return;
>
> new_bus = kmalloc(sizeof(*bus) + ((bus->dev_count - 1) *
> sizeof(struct kvm_io_range)), GFP_KERNEL);
> - if (!new_bus)
> - return -ENOMEM;
> + if (!new_bus) {
> + pr_err("kvm: failed to shrink bus, removing it completely\n");
> + goto broken;
The guest will fail in mysterious ways, if you do this (and
io_bus_unregister_dev can be called during runtime): in-kernel device
accesses will fail with unknown behaviour in the guest.
Can't you retry a handful of times with GFP_KERNEL before switching to GFP_ATOMIC?
(which in case fails the machine is likely to be crashing soon).
