net: test nouarg before dereferencing zerocopy pointers

Zerocopy skbs without completion notification were added for packet
sockets with PACKET_TX_RING user buffers. Those signal completion
through the TP_STATUS_USER bit in the ring. Zerocopy annotation was
added only to avoid premature notification after clone or orphan, by
triggering a copy on these paths for these packets.

The mechanism had to define a special "no-uarg" mode because packet
sockets already use skb_uarg(skb) == skb_shinfo(skb)->destructor_arg
for a different pointer.

Before deferencing skb_uarg(skb), verify that it is a real pointer.

Fixes: 5cd8d46e ("packet: copy user buffers before orphan or clone")
Signed-off-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/skbuff.h

@@ -1434,10 +1434,12 @@ static inline void skb_zcopy_clear(struct sk_buff *skb, bool zerocopy)
 	struct ubuf_info *uarg = skb_zcopy(skb);
 
 	if (uarg) {
-		if (uarg->callback == sock_zerocopy_callback) {
+		if (skb_zcopy_is_nouarg(skb)) {
+			/* no notification callback */
+		} else if (uarg->callback == sock_zerocopy_callback) {
 			uarg->zerocopy = uarg->zerocopy && zerocopy;
 			sock_zerocopy_put(uarg);
-		} else if (!skb_zcopy_is_nouarg(skb)) {
+		} else {
 			uarg->callback(uarg, zerocopy);
 		}
 
@@ -2691,7 +2693,8 @@ static inline int skb_orphan_frags(struct sk_buff *skb, gfp_t gfp_mask)
 {
 	if (likely(!skb_zcopy(skb)))
 		return 0;
-	if (skb_uarg(skb)->callback == sock_zerocopy_callback)
+	if (!skb_zcopy_is_nouarg(skb) &&
+	    skb_uarg(skb)->callback == sock_zerocopy_callback)
 		return 0;
 	return skb_copy_ubufs(skb, gfp_mask);
 }