Subject: Re: KASAN: use-after-free Read in vhost_chr_write_iter

From: "Michael S. Tsirkin" <[email-masked]>
Date: Mon, 21 May 2018 17:42:16 +0300

On Mon, May 21, 2018 at 10:38:10AM +0800, Jason Wang wrote:
> 
> 
> On 2018年05月18日 17:24, Jason Wang wrote:
> > 
> > 
> > On 2018年05月17日 21:45, DaeRyong Jeong wrote:
> > > We report the crash: KASAN: use-after-free Read in vhost_chr_write_iter
> > > 
> > > This crash has been found in v4.17-rc1 using RaceFuzzer (a modified
> > > version of Syzkaller), which we describe more at the end of this
> > > report. Our analysis shows that the race occurs when invoking two
> > > syscalls concurrently, write$vnet and ioctl$VHOST_RESET_OWNER.
> > > 
> > > 
> > > Analysis:
> > > We think the concurrent execution of vhost_process_iotlb_msg() and
> > > vhost_dev_cleanup() causes the crash.
> > > Both of functions can run concurrently (please see call sequence below),
> > > and possibly, there is a race on dev->iotlb.
> > > If the switch occurs right after vhost_dev_cleanup() frees
> > > dev->iotlb, vhost_process_iotlb_msg() still sees the non-null value
> > > and it
> > > keep executing without returning -EFAULT. Consequently, use-after-free
> > > occures
> > > 
> > > 
> > > Thread interleaving:
> > > CPU0 (vhost_process_iotlb_msg)                CPU1 (vhost_dev_cleanup)
> > > (In the case of both VHOST_IOTLB_UPDATE and
> > > VHOST_IOTLB_INVALIDATE)
> > > =====                            =====
> > >                             vhost_umem_clean(dev->iotlb);
> > > if (!dev->iotlb) {
> > >             ret = -EFAULT;
> > >                 break;
> > > }
> > >                             dev->iotlb = NULL;
> > > 
> > > 
> > > Call Sequence:
> > > CPU0
> > > =====
> > > vhost_net_chr_write_iter
> > >     vhost_chr_write_iter
> > >         vhost_process_iotlb_msg
> > > 
> > > CPU1
> > > =====
> > > vhost_net_ioctl
> > >     vhost_net_reset_owner
> > >         vhost_dev_reset_owner
> > >             vhost_dev_cleanup
> > 
> > Thanks a lot for the analysis.
> > 
> > This could be addressed by simply protect it with dev mutex.
> > 
> > Will post a patch.
> > 
> 
> Could you please help to test the attached patch? I've done some smoking
> test.
> 
> Thanks

> >From 88328386f3f652e684ee33dc4cf63dcaed871aea Mon Sep 17 00:00:00 2001
> From: Jason Wang <[email-masked]>
> Date: Fri, 18 May 2018 17:33:27 +0800
> Subject: [PATCH] vhost: synchronize IOTLB message with dev cleanup
> 
> DaeRyong Jeong reports a race between vhost_dev_cleanup() and
> vhost_process_iotlb_msg():
> 
> Thread interleaving:
> CPU0 (vhost_process_iotlb_msg)			CPU1 (vhost_dev_cleanup)
> (In the case of both VHOST_IOTLB_UPDATE and
> VHOST_IOTLB_INVALIDATE)
> =====						=====
> 						vhost_umem_clean(dev->iotlb);
> if (!dev->iotlb) {
> 	        ret = -EFAULT;
> 		        break;
> }
> 						dev->iotlb = NULL;
> 
> The reason is we don't synchronize between them, fixing by protecting
> vhost_process_iotlb_msg() with dev mutex.
> 
> Reported-by: DaeRyong Jeong <[email-masked]>
> Fixes: 6b1e6cc7855b0 ("vhost: new device IOTLB API")
> Reported-by: DaeRyong Jeong <[email-masked]>

Long terms we might want to move iotlb into vqs
so that messages can be processed in parallel.
Not sure how to do it yet.

> ---
>  drivers/vhost/vhost.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
> index f3bd8e9..f0be5f3 100644
> --- a/drivers/vhost/vhost.c
> +++ b/drivers/vhost/vhost.c
> @@ -981,6 +981,7 @@ static int vhost_process_iotlb_msg(struct vhost_dev *dev,
>  {
>  	int ret = 0;
>  
> +	mutex_lock(&dev->mutex);
>  	vhost_dev_lock_vqs(dev);
>  	switch (msg->type) {
>  	case VHOST_IOTLB_UPDATE:
> @@ -1016,6 +1017,8 @@ static int vhost_process_iotlb_msg(struct vhost_dev *dev,
>  	}
>  
>  	vhost_dev_unlock_vqs(dev);
> +	mutex_unlock(&dev->mutex);
> +
>  	return ret;
>  }
>  ssize_t vhost_chr_write_iter(struct vhost_dev *dev,
> -- 
> 2.7.4
>