Fixing two (somewhat rare) endpoint-related race issues, both of which

were reported by Fernando Guzman Lugo.
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Merge tag 'rpmsg-3.5-fixes' of git://git.kernel.org/pub/scm/linux/kernel/git/ohad/rpmsg

Pull rpmsg fixes from Ohad Ben-Cohen:
 "Fixing two (somewhat rare) endpoint-related race issues, both of which
  were reported by Fernando Guzman Lugo."

* tag 'rpmsg-3.5-fixes' of git://git.kernel.org/pub/scm/linux/kernel/git/ohad/rpmsg:
  rpmsg: make sure inflight messages don't invoke just-removed callbacks
  rpmsg: avoid premature deallocation of endpoints

drivers/rpmsg/virtio_rpmsg_bus.c

@@ -188,6 +188,26 @@ static int rpmsg_uevent(struct device *dev, struct kobj_uevent_env *env)
 					rpdev->id.name);
 }
 
+/**
+ * __ept_release() - deallocate an rpmsg endpoint
+ * @kref: the ept's reference count
+ *
+ * This function deallocates an ept, and is invoked when its @kref refcount
+ * drops to zero.
+ *
+ * Never invoke this function directly!
+ */
+static void __ept_release(struct kref *kref)
+{
+	struct rpmsg_endpoint *ept = container_of(kref, struct rpmsg_endpoint,
+						  refcount);
+	/*
+	 * At this point no one holds a reference to ept anymore,
+	 * so we can directly free it
+	 */
+	kfree(ept);
+}
+
 /* for more info, see below documentation of rpmsg_create_ept() */
 static struct rpmsg_endpoint *__rpmsg_create_ept(struct virtproc_info *vrp,
 		struct rpmsg_channel *rpdev, rpmsg_rx_cb_t cb,
@@ -206,6 +226,9 @@ static struct rpmsg_endpoint *__rpmsg_create_ept(struct virtproc_info *vrp,
 		return NULL;
 	}
 
+	kref_init(&ept->refcount);
+	mutex_init(&ept->cb_lock);
+
 	ept->rpdev = rpdev;
 	ept->cb = cb;
 	ept->priv = priv;
@@ -238,7 +261,7 @@ rem_idr:
 	idr_remove(&vrp->endpoints, request);
 free_ept:
 	mutex_unlock(&vrp->endpoints_lock);
-	kfree(ept);
+	kref_put(&ept->refcount, __ept_release);
 	return NULL;
 }
 
@@ -302,11 +325,17 @@ EXPORT_SYMBOL(rpmsg_create_ept);
 static void
 __rpmsg_destroy_ept(struct virtproc_info *vrp, struct rpmsg_endpoint *ept)
 {
+	/* make sure new inbound messages can't find this ept anymore */
 	mutex_lock(&vrp->endpoints_lock);
 	idr_remove(&vrp->endpoints, ept->addr);
 	mutex_unlock(&vrp->endpoints_lock);
 
-	kfree(ept);
+	/* make sure in-flight inbound messages won't invoke cb anymore */
+	mutex_lock(&ept->cb_lock);
+	ept->cb = NULL;
+	mutex_unlock(&ept->cb_lock);
+
+	kref_put(&ept->refcount, __ept_release);
 }
 
 /**
@@ -790,12 +819,28 @@ static void rpmsg_recv_done(struct virtqueue *rvq)
 
 	/* use the dst addr to fetch the callback of the appropriate user */
 	mutex_lock(&vrp->endpoints_lock);
+
 	ept = idr_find(&vrp->endpoints, msg->dst);
+
+	/* let's make sure no one deallocates ept while we use it */
+	if (ept)
+		kref_get(&ept->refcount);
+
 	mutex_unlock(&vrp->endpoints_lock);
 
-	if (ept && ept->cb)
-		ept->cb(ept->rpdev, msg->data, msg->len, ept->priv, msg->src);
-	else
+	if (ept) {
+		/* make sure ept->cb doesn't go away while we use it */
+		mutex_lock(&ept->cb_lock);
+
+		if (ept->cb)
+			ept->cb(ept->rpdev, msg->data, msg->len, ept->priv,
+				msg->src);
+
+		mutex_unlock(&ept->cb_lock);
+
+		/* farewell, ept, we don't need you anymore */
+		kref_put(&ept->refcount, __ept_release);
+	} else
 		dev_warn(dev, "msg received with no recepient\n");
 
 	/* publish the real size of the buffer */

include/linux/rpmsg.h

@@ -38,6 +38,8 @@
 #include <linux/types.h>
 #include <linux/device.h>
 #include <linux/mod_devicetable.h>
+#include <linux/kref.h>
+#include <linux/mutex.h>
 
 /* The feature bitmap for virtio rpmsg */
 #define VIRTIO_RPMSG_F_NS	0 /* RP supports name service notifications */
@@ -120,7 +122,9 @@ typedef void (*rpmsg_rx_cb_t)(struct rpmsg_channel *, void *, int, void *, u32);
 /**
  * struct rpmsg_endpoint - binds a local rpmsg address to its user
  * @rpdev: rpmsg channel device
+ * @refcount: when this drops to zero, the ept is deallocated
  * @cb: rx callback handler
+ * @cb_lock: must be taken before accessing/changing @cb
  * @addr: local rpmsg address
  * @priv: private data for the driver's use
  *
@@ -140,7 +144,9 @@ typedef void (*rpmsg_rx_cb_t)(struct rpmsg_channel *, void *, int, void *, u32);
  */
 struct rpmsg_endpoint {
 	struct rpmsg_channel *rpdev;
+	struct kref refcount;
 	rpmsg_rx_cb_t cb;
+	struct mutex cb_lock;
 	u32 addr;
 	void *priv;
 };