net: qrtr: Avoid potential use after free in MHI send

It is possible that the MHI ul_callback will be invoked immediately
following the queueing of the skb for transmission, leading to the
callback decrementing the refcount of the associated sk and freeing the
skb.

As such the dereference of skb and the increment of the sk refcount must
happen before the skb is queued, to avoid the skb to be used after free
and potentially the sk to drop its last refcount..

Fixes: 6e728f32 ("net: qrtr: Add MHI transport layer")
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/qrtr/mhi.c

@@ -50,6 +50,9 @@ static int qcom_mhi_qrtr_send(struct qrtr_endpoint *ep, struct sk_buff *skb)
 	struct qrtr_mhi_dev *qdev = container_of(ep, struct qrtr_mhi_dev, ep);
 	int rc;
 
+	if (skb->sk)
+		sock_hold(skb->sk);
+
 	rc = skb_linearize(skb);
 	if (rc)
 		goto free_skb;
@@ -59,12 +62,11 @@ static int qcom_mhi_qrtr_send(struct qrtr_endpoint *ep, struct sk_buff *skb)
 	if (rc)
 		goto free_skb;
 
-	if (skb->sk)
-		sock_hold(skb->sk);
-
 	return rc;
 
 free_skb:
+	if (skb->sk)
+		sock_put(skb->sk);
 	kfree_skb(skb);
 
 	return rc;