[PATCH 3.10 40/56] net: Fix skb csum races when peeking

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

3.10-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Herbert Xu <herbert@gondor.apana.org.au>

[ 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: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
 net/core/datagram.c |    3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

--- a/net/core/datagram.c
+++ b/net/core/datagram.c
@@ -700,7 +700,8 @@ __sum16 __skb_checksum_complete_head(str
 	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;
 }