Re: OOP in ip_cmsg_recv (net-next)

[Eric Dumazet <eric.dumazet@gmail.com>]

Le lundi 03 mai 2010 à 19:04 +0200, Eric Dumazet a écrit :
> Le lundi 03 mai 2010 à 09:47 -0700, Stephen Hemminger a écrit :
> > I am getting occasional NULL pointer references with net-next kernel.
> > No test, just usual stuff (like DNS).
> > 
> > This is a new regression in net-next only.
> > 
> > 
> Hmm, skb->sk is NULL
> 
> void ip_cmsg_recv(struct msghdr *msg, struct sk_buff *skb)
> {
> 	struct inet_sock *inet = inet_sk(skb->sk);
> 	unsigned flags = inet->cmsg_flags; // CRASH
> 
> 
> So a skb_free_datagram_locked() is at fault here...
> 
> commit 4b0b72f7dd617b13abd1b04c947e15873e011a24 probably
> 
> OK, the skb_orphan() should not be done at this point, if we are not the
> only user (and last user)
> 
> Oh well, sorry for the regression ;)
> 

I'll test following patch and report results to netdev :

diff --git a/net/core/datagram.c b/net/core/datagram.c
index 95b851f..e009753 100644
--- a/net/core/datagram.c
+++ b/net/core/datagram.c
@@ -229,13 +229,18 @@ EXPORT_SYMBOL(skb_free_datagram);
 
 void skb_free_datagram_locked(struct sock *sk, struct sk_buff *skb)
 {
+	if (likely(atomic_read(&skb->users) == 1))
+		smp_rmb();
+	else if (likely(!atomic_dec_and_test(&skb->users)))
+		return;
+
 	lock_sock_bh(sk);
 	skb_orphan(skb);
 	sk_mem_reclaim_partial(sk);
 	unlock_sock_bh(sk);
 
-	/* skb is now orphaned, might be freed outside of locked section */
-	consume_skb(skb);
+	/* skb is now orphaned, can be freed outside of locked section */
+	__kfree_skb(skb);
 }
 EXPORT_SYMBOL(skb_free_datagram_locked);