net: Fix skb csum races when peeking

[ Upstream commit 89c22d8c3b278212eef6a8cc66b570bc840a6f5a ]

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: Herbert Xu <herbert@gondor.apana.org.au>
Acked-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
(cherry picked from commit 58a5897a53d535bf95523e6f381f88116217f5ca)

Signed-off-by: Willy Tarreau <w@1wt.eu>

diff --git a/net/core/datagram.c b/net/core/datagram.c
index c855336b3ebbeb7394a5c12730d41ee94495c527..253d068e8cbac54be240f5d603b9466e3a6c3b84 100644 (file)
--- a/net/core/datagram.c
+++ b/net/core/datagram.c
@@ -675,7 +675,8 @@ __sum16 __skb_checksum_complete_head(struct sk_buff *skb, int len)
        if (likely(!sum)) {
                if (unlikely(skb->ip_summed == CHECKSUM_COMPLETE))
                        netdev_rx_csum_fault(skb->dev);
-               skb->ip_summed = CHECKSUM_UNNECESSARY;
+               if (!skb_shared(skb))
+                       skb->ip_summed = CHECKSUM_UNNECESSARY;
        }
        return sum;
 }