[IPV4/IPV6] Ensure all frag_list members have NULL sk (2fdba6b0) · Commits · 戴 / test

Having frag_list members which holds wmem of an sk leads to nightmares with partially cloned frag skb's. The reason is that once you unleash a skb with a frag_list that has individual sk ownerships into the stack you can never undo those ownerships safely as they may have been cloned by things like netfilter. Since we have to undo them in order to make skb_linearize happy this approach leads to a dead-end. So let's go the other way and make this an invariant: For any skb on a frag_list, skb->sk must be NULL. That is, the socket ownership always belongs to the head skb. It turns out that the implementation is actually pretty simple. The above invariant is actually violated in the following patch for a short duration inside ip_fragment. This is OK because the offending frag_list member is either destroyed at the end of the slow path without being sent anywhere, or it is detached from the frag_list before being sent. Signed-off-by:

Herbert Xu <herbert@gondor.apana.org.au> Signed-off-by:

David S. Miller <davem@davemloft.net>

net/ipv4/ip_output.c

+8 −0

Original line number	Diff line number	Diff line
		@@ -490,6 +490,14 @@ int ip_fragment(struct sk_buff skb, int (output)(struct sk_buff*))
		/* Partially cloned skb? */
		if (skb_shared(frag))
		goto slow_path;

		BUG_ON(frag->sk);
		if (skb->sk) {
		sock_hold(skb->sk);
		frag->sk = skb->sk;
		frag->destructor = sock_wfree;
		skb->truesize -= frag->truesize;
		}
		}

		/* Everything is OK. Generate! */

net/ipv6/ip6_output.c

+8 −6

Original line number	Diff line number	Diff line
		@@ -552,13 +552,17 @@ static int ip6_fragment(struct sk_buff skb, int (output)(struct sk_buff *))
		skb_headroom(frag) < hlen)
		goto slow_path;

		/* Correct socket ownership. */
		if (frag->sk == NULL)
		goto slow_path;

		/* Partially cloned skb? */
		if (skb_shared(frag))
		goto slow_path;

		BUG_ON(frag->sk);
		if (skb->sk) {
		sock_hold(skb->sk);
		frag->sk = skb->sk;
		frag->destructor = sock_wfree;
		skb->truesize -= frag->truesize;
		}
		}

		err = 0;
		@@ -1116,12 +1120,10 @@ int ip6_push_pending_frames(struct sock *sk)
		tail_skb = &(tmp_skb->next);
		skb->len += tmp_skb->len;
		skb->data_len += tmp_skb->len;
		#if 0 /* Logically correct, but useless work, ip_fragment() will have to undo */
		skb->truesize += tmp_skb->truesize;
		__sock_put(tmp_skb->sk);
		tmp_skb->destructor = NULL;
		tmp_skb->sk = NULL;
		#endif
		}

		ipv6_addr_copy(final_dst, &fl->fl6_dst);

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