net: Fix skb csum races when peeking

When we calculate the checksum on the recv path, we store the
result in the skb as an optimisation in case we need the checksum
again down the line.

This is in fact bogus for the MSG_PEEK case as this is done without
any locking.  So multiple threads can peek and then store the result
to the same skb, potentially resulting in bogus skb states.

This patch fixes this by only storing the result if the skb is not
shared.  This preserves the optimisations for the few cases where
it can be done safely due to locking or other reasons, e.g., SIOCINQ.
Signed-off-by: NHerbert Xu <herbert@gondor.apana.org.au>
Acked-by: NEric Dumazet <edumazet@google.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/core/datagram.c

@@ -657,6 +657,7 @@ __sum16 __skb_checksum_complete_head(struct sk_buff *skb, int len)
 		    !skb->csum_complete_sw)
 			netdev_rx_csum_fault(skb->dev);
 	}
+	if (!skb_shared(skb))
 		skb->csum_valid = !sum;
 	return sum;
 }
@@ -677,11 +678,13 @@ __sum16 __skb_checksum_complete(struct sk_buff *skb)
 			netdev_rx_csum_fault(skb->dev);
 	}
 
+	if (!skb_shared(skb)) {
 		/* Save full packet checksum */
 		skb->csum = csum;
 		skb->ip_summed = CHECKSUM_COMPLETE;
 		skb->csum_complete_sw = 1;
 		skb->csum_valid = !sum;
+	}
 
 	return sum;
 }