Linux Virtualization - Feb 2014 - [PATCH net v2] vhost: fix a theoretical race in device cleanup

vhost_zerocopy_callback accesses VQ right after it drops a ubuf
reference.  In theory, this could race with device removal which waits
on the ubuf kref, and crash on use after free.

Do all accesses within rcu read side critical section, and synchronize
on release.

Since callbacks are always invoked from bh, synchronize_rcu_bh seems
enough and will help release complete a bit faster.

Signed-off-by: Michael S. Tsirkin <mst at redhat.com>
---

This is was previously posted as part of patch
series, but it's an independent fix really.
Theoretical race so not needed for stable I think.

changes from v1:
	fixed typo in commit log

 drivers/vhost/net.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c
index b12176f..f1be80d 100644
--- a/drivers/vhost/net.c
+++ b/drivers/vhost/net.c
@@ -308,6 +308,8 @@ static void vhost_zerocopy_callback(struct ubuf_info *ubuf,
bool success)
 	struct vhost_virtqueue *vq = ubufs->vq;
 	int cnt;
 
+	rcu_read_lock_bh();
+
 	/* set len to mark this desc buffers done DMA */
 	vq->heads[ubuf->desc].len = success ?
 		VHOST_DMA_DONE_LEN : VHOST_DMA_FAILED_LEN;
@@ -322,6 +324,8 @@ static void vhost_zerocopy_callback(struct ubuf_info *ubuf,
bool success)
 	 */
 	if (cnt <= 1 || !(cnt % 16))
 		vhost_poll_queue(&vq->poll);
+
+	rcu_read_unlock_bh();
 }
 
 /* Expects to be always run from workqueue - which acts as
@@ -804,6 +808,8 @@ static int vhost_net_release(struct inode *inode, struct
file *f)
 		fput(tx_sock->file);
 	if (rx_sock)
 		fput(rx_sock->file);
+	/* Make sure no callbacks are outstanding */
+	synchronize_rcu_bh();
 	/* We do an extra flush before freeing memory,
 	 * since jobs can re-queue themselves. */
 	vhost_net_flush(n);
-- 
MST

On 02/13/2014 05:45 PM, Michael S. Tsirkin wrote:
> vhost_zerocopy_callback accesses VQ right after it drops a ubuf
> reference.  In theory, this could race with device removal which waits
> on the ubuf kref, and crash on use after free.
>
> Do all accesses within rcu read side critical section, and synchronize
> on release.
>
> Since callbacks are always invoked from bh, synchronize_rcu_bh seems
> enough and will help release complete a bit faster.
>
> Signed-off-by: Michael S. Tsirkin <mst at redhat.com>
> ---
>
> This is was previously posted as part of patch
> series, but it's an independent fix really.
> Theoretical race so not needed for stable I think.
>
> changes from v1:
> 	fixed typo in commit log
>
>  drivers/vhost/net.c | 6 ++++++
>  1 file changed, 6 insertions(+)
>
> diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c
> index b12176f..f1be80d 100644
> --- a/drivers/vhost/net.c
> +++ b/drivers/vhost/net.c
> @@ -308,6 +308,8 @@ static void vhost_zerocopy_callback(struct ubuf_info
*ubuf, bool success)
>  	struct vhost_virtqueue *vq = ubufs->vq;
>  	int cnt;
>  
> +	rcu_read_lock_bh();
> +
>  	/* set len to mark this desc buffers done DMA */
>  	vq->heads[ubuf->desc].len = success ?
>  		VHOST_DMA_DONE_LEN : VHOST_DMA_FAILED_LEN;
> @@ -322,6 +324,8 @@ static void vhost_zerocopy_callback(struct ubuf_info
*ubuf, bool success)
>  	 */
>  	if (cnt <= 1 || !(cnt % 16))
>  		vhost_poll_queue(&vq->poll);
> +
> +	rcu_read_unlock_bh();
>  }
>  
>  /* Expects to be always run from workqueue - which acts as
> @@ -804,6 +808,8 @@ static int vhost_net_release(struct inode *inode,
struct file *f)
>  		fput(tx_sock->file);
>  	if (rx_sock)
>  		fput(rx_sock->file);
> +	/* Make sure no callbacks are outstanding */
> +	synchronize_rcu_bh();
>  	/* We do an extra flush before freeing memory,
>  	 * since jobs can re-queue themselves. */
>  	vhost_net_flush(n);
Acked-by: Jason Wang <jasowang at redhat.com>

From: "Michael S. Tsirkin" <mst at redhat.com>
Date: Thu, 13 Feb 2014 11:45:11 +0200
> vhost_zerocopy_callback accesses VQ right after it drops a ubuf
> reference.  In theory, this could race with device removal which waits
> on the ubuf kref, and crash on use after free.
> 
> Do all accesses within rcu read side critical section, and synchronize
> on release.
> 
> Since callbacks are always invoked from bh, synchronize_rcu_bh seems
> enough and will help release complete a bit faster.
> 
> Signed-off-by: Michael S. Tsirkin <mst at redhat.com>
> ---
> 
> This is was previously posted as part of patch
> series, but it's an independent fix really.
> Theoretical race so not needed for stable I think.
Ok, no -stable, applied.