Re: [PATCH 6/6] vhost: don't do synchronize_rcu() in vhost_uninit_vq_maps()

From: Jason Wang <jasowang@redhat.com>
To: "Michael S. Tsirkin" <mst@redhat.com>
Cc: kvm@vger.kernel.org, virtualization@lists.linux-foundation.org,
	netdev@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH 6/6] vhost: don't do synchronize_rcu() in vhost_uninit_vq_maps()
Date: Tue, 23 Jul 2019 21:16:20 +0800	[thread overview]
Message-ID: <f2074bc7-6d67-14fa-4f58-91e9c5a9cc12@redhat.com> (raw)
In-Reply-To: <20190723041144-mutt-send-email-mst@kernel.org>

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.
>