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SubjectRe: [PATCH 6/6] vhost: don't do synchronize_rcu() in vhost_uninit_vq_maps()
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On 2019/7/23 下午5:16, Michael S. Tsirkin wrote:
> On Tue, Jul 23, 2019 at 03:57:18AM -0400, Jason Wang wrote:
>> There's no need for RCU synchronization in vhost_uninit_vq_maps()
>> since we've already serialized with readers (memory accessors). This
>> also avoid the possible userspace DOS through ioctl() because of the
>> possible high latency caused by synchronize_rcu().
>>
>> Reported-by: Michael S. Tsirkin <mst@redhat.com>
>> Fixes: 7f466032dc9e ("vhost: access vq metadata through kernel virtual address")
>> Signed-off-by: Jason Wang <jasowang@redhat.com>
> I agree synchronize_rcu in both mmu notifiers and ioctl
> is a problem we must fix.
>
>> ---
>> drivers/vhost/vhost.c | 4 +++-
>> 1 file changed, 3 insertions(+), 1 deletion(-)
>>
>> diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
>> index 5b8821d00fe4..a17df1f4069a 100644
>> --- a/drivers/vhost/vhost.c
>> +++ b/drivers/vhost/vhost.c
>> @@ -334,7 +334,9 @@ static void vhost_uninit_vq_maps(struct vhost_virtqueue *vq)
>> }
>> spin_unlock(&vq->mmu_lock);
>>
>> - synchronize_rcu();
>> + /* No need for synchronize_rcu() or kfree_rcu() since we are
>> + * serialized with memory accessors (e.g vq mutex held).
>> + */
>>
>> for (i = 0; i < VHOST_NUM_ADDRS; i++)
>> if (map[i])
>> --
>> 2.18.1
> .. however we can not RCU with no synchronization in sight.
> Sometimes there are hacks like using a lock/unlock
> pair instead of sync, but here no one bothers.
>
> specifically notifiers call reset vq maps which calls
> uninit vq maps which is not under any lock.


Notifier did this:

        if (map) {
                if (uaddr->write) {
                        for (i = 0; i < map->npages; i++)
set_page_dirty(map->pages[i]);
}
                rcu_assign_pointer(vq->maps[index], NULL);
}
spin_unlock(&vq->mmu_lock);

        if (map) {
synchronize_rcu();
vhost_map_unprefetch(map);
        }

So it indeed have a synchronize_rcu() there.

Thanks


>
> You will get use after free when map is then accessed.
>
> If you always have a lock then just take that lock
> and no need for RCU.
>

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