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SubjectRe: [PATCH 2/6] vhost: validate MMU notifier registration
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On 2019/7/23 下午5:17, Michael S. Tsirkin wrote:
> On Tue, Jul 23, 2019 at 03:57:14AM -0400, Jason Wang wrote:
>> The return value of mmu_notifier_register() is not checked in
>> vhost_vring_set_num_addr(). This will cause an out of sync between mm
>> and MMU notifier thus a double free. To solve this, introduce a
>> boolean flag to track whether MMU notifier is registered and only do
>> unregistering when it was true.
>>
>> Reported-and-tested-by:
>> syzbot+e58112d71f77113ddb7b@syzkaller.appspotmail.com
>> Fixes: 7f466032dc9e ("vhost: access vq metadata through kernel virtual address")
>> Signed-off-by: Jason Wang <jasowang@redhat.com>
> Right. This fixes the bug.
> But it's not great that simple things like
> setting vq address put pressure on memory allocator.
> Also, if we get a single during processing
> notifier register will fail, disabling optimization permanently.


Yes, but do we really care about this case. E.g we even fail for -ENOMEM
for set owner.


>
> In fact, see below:
>
>
>> ---
>> drivers/vhost/vhost.c | 19 +++++++++++++++----
>> drivers/vhost/vhost.h | 1 +
>> 2 files changed, 16 insertions(+), 4 deletions(-)
>>
>> diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
>> index 34c0d970bcbc..058191d5efad 100644
>> --- a/drivers/vhost/vhost.c
>> +++ b/drivers/vhost/vhost.c
>> @@ -630,6 +630,7 @@ void vhost_dev_init(struct vhost_dev *dev,
>> dev->iov_limit = iov_limit;
>> dev->weight = weight;
>> dev->byte_weight = byte_weight;
>> + dev->has_notifier = false;
>> init_llist_head(&dev->work_list);
>> init_waitqueue_head(&dev->wait);
>> INIT_LIST_HEAD(&dev->read_list);
>> @@ -731,6 +732,7 @@ long vhost_dev_set_owner(struct vhost_dev *dev)
>> if (err)
>> goto err_mmu_notifier;
>> #endif
>> + dev->has_notifier = true;
>>
>> return 0;
>>
> I just noticed that set owner now fails if we get a signal.
> Userspace could retry in theory but it does not:
> this is userspace abi breakage since it used to only
> fail on invalid input.


Well, at least kthread_create() and vhost_dev_alloc_iovecs() will
allocate memory.

Thanks


>
>> @@ -960,7 +962,11 @@ void vhost_dev_cleanup(struct vhost_dev *dev)
>> }
>> if (dev->mm) {
>> #if VHOST_ARCH_CAN_ACCEL_UACCESS
>> - mmu_notifier_unregister(&dev->mmu_notifier, dev->mm);
>> + if (dev->has_notifier) {
>> + mmu_notifier_unregister(&dev->mmu_notifier,
>> + dev->mm);
>> + dev->has_notifier = false;
>> + }
>> #endif
>> mmput(dev->mm);
>> }
>> @@ -2065,8 +2071,10 @@ static long vhost_vring_set_num_addr(struct vhost_dev *d,
>> /* Unregister MMU notifer to allow invalidation callback
>> * can access vq->uaddrs[] without holding a lock.
>> */
>> - if (d->mm)
>> + if (d->has_notifier) {
>> mmu_notifier_unregister(&d->mmu_notifier, d->mm);
>> + d->has_notifier = false;
>> + }
>>
>> vhost_uninit_vq_maps(vq);
>> #endif
>> @@ -2086,8 +2094,11 @@ static long vhost_vring_set_num_addr(struct vhost_dev *d,
>> if (r == 0)
>> vhost_setup_vq_uaddr(vq);
>>
>> - if (d->mm)
>> - mmu_notifier_register(&d->mmu_notifier, d->mm);
>> + if (d->mm) {
>> + r = mmu_notifier_register(&d->mmu_notifier, d->mm);
>> + if (!r)
>> + d->has_notifier = true;
>> + }
>> #endif
>>
>> mutex_unlock(&vq->mutex);
>> diff --git a/drivers/vhost/vhost.h b/drivers/vhost/vhost.h
>> index 819296332913..a62f56a4cf72 100644
>> --- a/drivers/vhost/vhost.h
>> +++ b/drivers/vhost/vhost.h
>> @@ -214,6 +214,7 @@ struct vhost_dev {
>> int iov_limit;
>> int weight;
>> int byte_weight;
>> + bool has_notifier;
>> };
>>
>> bool vhost_exceeds_weight(struct vhost_virtqueue *vq, int pkts, int total_len);
>> --
>> 2.18.1

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