[PATCH net] net: test nouarg before dereferencing zerocopy pointers

[Willem de Bruijn]

From: Willem de Bruijn <will...@google.com>

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: 5cd8d46ea1562 ("packet: copy user buffers before orphan or clone")
Signed-off-by: Willem de Bruijn <will...@google.com>
---
 include/linux/skbuff.h | 9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
index 6d58fa8a65fde..2ee5e63195c02 100644
--- a/include/linux/skbuff.h
+++ b/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);
 }
-- 
2.21.0.1020.gf2820cf01a-goog