Re: [PATCH v3] vhost/vsock: fix use-after-free in network stack callers

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On Tue, Nov 06, 2018 at 09:40:07AM +0000, Stefan Hajnoczi wrote:
> On Mon, Nov 05, 2018 at 10:35:47AM +0000, Stefan Hajnoczi wrote:
> > If the network stack calls .send_pkt()/.cancel_pkt() during .release(),
> > a struct vhost_vsock use-after-free is possible.  This occurs because
> > .release() does not wait for other CPUs to stop using struct
> > vhost_vsock.
> > 
> > Switch to an RCU-enabled hashtable (indexed by guest CID) so that
> > .release() can wait for other CPUs by calling synchronize_rcu().  This
> > also eliminates vhost_vsock_lock acquisition in the data path so it
> > could have a positive effect on performance.
> > 
> > Reported-and-tested-by: syzbot+bd391451452fb0b93039@xxxxxxxxxxxxxxxxxxxxxxxxx
> > Reported-by: syzbot+e3e074963495f92a89ed@xxxxxxxxxxxxxxxxxxxxxxxxx
> > Reported-by: syzbot+d5a0a170c5069658b141@xxxxxxxxxxxxxxxxxxxxxxxxx
> > Signed-off-by: Stefan Hajnoczi <stefanha@xxxxxxxxxx>
> > ---
> 
> Michael,
> This addresses CVE-2018-14625 kernel: use-after-free Read in
> vhost_transport_send_pkt.
> 
> Please consider for -stable.
> 
> Stefan

Makes sense. Will do.

> > I have now manually tested the RCU hashtable fix and am happy with this
> > patch.
> > 
> >  drivers/vhost/vsock.c | 57 +++++++++++++++++++++++++------------------
> >  1 file changed, 33 insertions(+), 24 deletions(-)
> > 
> > diff --git a/drivers/vhost/vsock.c b/drivers/vhost/vsock.c
> > index 34bc3ab40c6d..51879ed18652 100644
> > --- a/drivers/vhost/vsock.c
> > +++ b/drivers/vhost/vsock.c
> > @@ -15,6 +15,7 @@
> >  #include <net/sock.h>
> >  #include <linux/virtio_vsock.h>
> >  #include <linux/vhost.h>
> > +#include <linux/hashtable.h>
> >  
> >  #include <net/af_vsock.h>
> >  #include "vhost.h"
> > @@ -27,14 +28,14 @@ enum {
> >  
> >  /* Used to track all the vhost_vsock instances on the system. */
> >  static DEFINE_SPINLOCK(vhost_vsock_lock);
> > -static LIST_HEAD(vhost_vsock_list);
> > +static DEFINE_READ_MOSTLY_HASHTABLE(vhost_vsock_hash, 8);
> >  
> >  struct vhost_vsock {
> >  	struct vhost_dev dev;
> >  	struct vhost_virtqueue vqs[2];
> >  
> > -	/* Link to global vhost_vsock_list, protected by vhost_vsock_lock */
> > -	struct list_head list;
> > +	/* Link to global vhost_vsock_hash, writes use vhost_vsock_lock */
> > +	struct hlist_node hash;
> >  
> >  	struct vhost_work send_pkt_work;
> >  	spinlock_t send_pkt_list_lock;
> > @@ -50,11 +51,14 @@ static u32 vhost_transport_get_local_cid(void)
> >  	return VHOST_VSOCK_DEFAULT_HOST_CID;
> >  }
> >  
> > -static struct vhost_vsock *__vhost_vsock_get(u32 guest_cid)
> > +/* Callers that dereference the return value must hold vhost_vsock_lock or the
> > + * RCU read lock.
> > + */
> > +static struct vhost_vsock *vhost_vsock_get(u32 guest_cid)
> >  {
> >  	struct vhost_vsock *vsock;
> >  
> > -	list_for_each_entry(vsock, &vhost_vsock_list, list) {
> > +	hash_for_each_possible_rcu(vhost_vsock_hash, vsock, hash, guest_cid) {
> >  		u32 other_cid = vsock->guest_cid;
> >  
> >  		/* Skip instances that have no CID yet */
> > @@ -69,17 +73,6 @@ static struct vhost_vsock *__vhost_vsock_get(u32 guest_cid)
> >  	return NULL;
> >  }
> >  
> > -static struct vhost_vsock *vhost_vsock_get(u32 guest_cid)
> > -{
> > -	struct vhost_vsock *vsock;
> > -
> > -	spin_lock_bh(&vhost_vsock_lock);
> > -	vsock = __vhost_vsock_get(guest_cid);
> > -	spin_unlock_bh(&vhost_vsock_lock);
> > -
> > -	return vsock;
> > -}
> > -
> >  static void
> >  vhost_transport_do_send_pkt(struct vhost_vsock *vsock,
> >  			    struct vhost_virtqueue *vq)
> > @@ -210,9 +203,12 @@ vhost_transport_send_pkt(struct virtio_vsock_pkt *pkt)
> >  	struct vhost_vsock *vsock;
> >  	int len = pkt->len;
> >  
> > +	rcu_read_lock();
> > +
> >  	/* Find the vhost_vsock according to guest context id  */
> >  	vsock = vhost_vsock_get(le64_to_cpu(pkt->hdr.dst_cid));
> >  	if (!vsock) {
> > +		rcu_read_unlock();
> >  		virtio_transport_free_pkt(pkt);
> >  		return -ENODEV;
> >  	}
> > @@ -225,6 +221,8 @@ vhost_transport_send_pkt(struct virtio_vsock_pkt *pkt)
> >  	spin_unlock_bh(&vsock->send_pkt_list_lock);
> >  
> >  	vhost_work_queue(&vsock->dev, &vsock->send_pkt_work);
> > +
> > +	rcu_read_unlock();
> >  	return len;
> >  }
> >  
> > @@ -234,12 +232,15 @@ vhost_transport_cancel_pkt(struct vsock_sock *vsk)
> >  	struct vhost_vsock *vsock;
> >  	struct virtio_vsock_pkt *pkt, *n;
> >  	int cnt = 0;
> > +	int ret = -ENODEV;
> >  	LIST_HEAD(freeme);
> >  
> > +	rcu_read_lock();
> > +
> >  	/* Find the vhost_vsock according to guest context id  */
> >  	vsock = vhost_vsock_get(vsk->remote_addr.svm_cid);
> >  	if (!vsock)
> > -		return -ENODEV;
> > +		goto out;
> >  
> >  	spin_lock_bh(&vsock->send_pkt_list_lock);
> >  	list_for_each_entry_safe(pkt, n, &vsock->send_pkt_list, list) {
> > @@ -265,7 +266,10 @@ vhost_transport_cancel_pkt(struct vsock_sock *vsk)
> >  			vhost_poll_queue(&tx_vq->poll);
> >  	}
> >  
> > -	return 0;
> > +	ret = 0;
> > +out:
> > +	rcu_read_unlock();
> > +	return ret;
> >  }
> >  
> >  static struct virtio_vsock_pkt *
> > @@ -533,10 +537,6 @@ static int vhost_vsock_dev_open(struct inode *inode, struct file *file)
> >  	spin_lock_init(&vsock->send_pkt_list_lock);
> >  	INIT_LIST_HEAD(&vsock->send_pkt_list);
> >  	vhost_work_init(&vsock->send_pkt_work, vhost_transport_send_pkt_work);
> > -
> > -	spin_lock_bh(&vhost_vsock_lock);
> > -	list_add_tail(&vsock->list, &vhost_vsock_list);
> > -	spin_unlock_bh(&vhost_vsock_lock);
> >  	return 0;
> >  
> >  out:
> > @@ -577,9 +577,13 @@ static int vhost_vsock_dev_release(struct inode *inode, struct file *file)
> >  	struct vhost_vsock *vsock = file->private_data;
> >  
> >  	spin_lock_bh(&vhost_vsock_lock);
> > -	list_del(&vsock->list);
> > +	if (vsock->guest_cid)
> > +		hash_del_rcu(&vsock->hash);
> >  	spin_unlock_bh(&vhost_vsock_lock);
> >  
> > +	/* Wait for other CPUs to finish using vsock */
> > +	synchronize_rcu();
> > +
> >  	/* Iterating over all connections for all CIDs to find orphans is
> >  	 * inefficient.  Room for improvement here. */
> >  	vsock_for_each_connected_socket(vhost_vsock_reset_orphans);
> > @@ -620,12 +624,17 @@ static int vhost_vsock_set_cid(struct vhost_vsock *vsock, u64 guest_cid)
> >  
> >  	/* Refuse if CID is already in use */
> >  	spin_lock_bh(&vhost_vsock_lock);
> > -	other = __vhost_vsock_get(guest_cid);
> > +	other = vhost_vsock_get(guest_cid);
> >  	if (other && other != vsock) {
> >  		spin_unlock_bh(&vhost_vsock_lock);
> >  		return -EADDRINUSE;
> >  	}
> > +
> > +	if (vsock->guest_cid)
> > +		hash_del_rcu(&vsock->hash);
> > +
> >  	vsock->guest_cid = guest_cid;
> > +	hash_add_rcu(vhost_vsock_hash, &vsock->hash, guest_cid);
> >  	spin_unlock_bh(&vhost_vsock_lock);
> >  
> >  	return 0;
> > -- 
> > 2.19.1
> > 





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