vhost/vsock: fix use-after-free in network stack callers

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.

This is CVE-2018-14625 "kernel: use-after-free Read in vhost_transport_send_pkt".

Cc: [e-mail]
Reported-and-tested-by: [e-mail]
Reported-by: [e-mail]
Reported-by: [e-mail]
Signed-off-by: Stefan Hajnoczi <[e-mail]>
Signed-off-by: Michael S. Tsirkin <[e-mail]>
Acked-by: Jason Wang <[e-mail]>

drivers/vhost/vsock.c

···
 #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"
···
 
 /* 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;
···
 	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 */
···
 	}
 
 	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
···
 	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;
 	}
···
 	spin_unlock_bh(&vsock->send_pkt_list_lock);
 
 	vhost_work_queue(&vsock->dev, &vsock->send_pkt_work);
+
+	rcu_read_unlock();
 	return len;
 }
 
···
 	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) {
···
 			vhost_poll_queue(&tx_vq->poll);
 	}
 
-	return 0;
+	ret = 0;
+out:
+	rcu_read_unlock();
+	return ret;
 }
 
 static struct virtio_vsock_pkt *
···
 	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:
···
 	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. */
···
 
 	/* 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;