dccp/tcp: Avoid negative sk_forward_alloc by ipv6_pinfo.pktoptions.

Eric Dumazet pointed out [0] that when we call skb_set_owner_r()
for ipv6_pinfo.pktoptions, sk_rmem_schedule() has not been called,
resulting in a negative sk_forward_alloc.

We add a new helper which clones a skb and sets its owner only
when sk_rmem_schedule() succeeds.

Note that we move skb_set_owner_r() forward in (dccp|tcp)_v6_do_rcv()
because tcp_send_synack() can make sk_forward_alloc negative before
ipv6_opt_accepted() in the crossed SYN-ACK or self-connect() cases.

[0]: https://lore.kernel.org/netdev/CANn89iK9oc20Jdi_41jb9URdF210r7d1Y-+uypbMSbOfY6jqrg@mail.gmail.com/

Fixes: 323fbd0 ("net: dccp: Add handling of IPV6_PKTOPTIONS to dccp_v6_do_rcv()")
Fixes: 3df80d9 ("[DCCP]: Introduce DCCPv6")
Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Author: q2ven

include/net/sock.h

@@ -2434,6 +2434,19 @@ static inline __must_check bool skb_set_owner_sk_safe(struct sk_buff *skb, struc
 	return false;
 }
 
+static inline struct sk_buff *skb_clone_and_charge_r(struct sk_buff *skb, struct sock *sk)
+{
+	skb = skb_clone(skb, sk_gfp_mask(sk, GFP_ATOMIC));
+	if (skb) {
+		if (sk_rmem_schedule(sk, skb, skb->truesize)) {
+			skb_set_owner_r(skb, sk);
+			return skb;
+		}
+		__kfree_skb(skb);
+	}
+	return NULL;
+}
+
 static inline void skb_prepare_for_gro(struct sk_buff *skb)
 {
 	if (skb->destructor != sock_wfree) {

net/dccp/ipv6.c

@@ -551,11 +551,9 @@ static struct sock *dccp_v6_request_recv_sock(const struct sock *sk,
 	*own_req = inet_ehash_nolisten(newsk, req_to_sk(req_unhash), NULL);
 	/* Clone pktoptions received with SYN, if we own the req */
 	if (*own_req && ireq->pktopts) {
-		newnp->pktoptions = skb_clone(ireq->pktopts, GFP_ATOMIC);
+		newnp->pktoptions = skb_clone_and_charge_r(ireq->pktopts, newsk);
 		consume_skb(ireq->pktopts);
 		ireq->pktopts = NULL;
-		if (newnp->pktoptions)
-			skb_set_owner_r(newnp->pktoptions, newsk);
 	}
 
 	return newsk;
@@ -615,7 +613,7 @@ static int dccp_v6_do_rcv(struct sock *sk, struct sk_buff *skb)
 					       --ANK (980728)
 	 */
 	if (np->rxopt.all)
-		opt_skb = skb_clone(skb, GFP_ATOMIC);
+		opt_skb = skb_clone_and_charge_r(skb, sk);
 
 	if (sk->sk_state == DCCP_OPEN) { /* Fast path */
 		if (dccp_rcv_established(sk, skb, dccp_hdr(skb), skb->len))
@@ -679,7 +677,6 @@ static int dccp_v6_do_rcv(struct sock *sk, struct sk_buff *skb)
 			np->flow_label = ip6_flowlabel(ipv6_hdr(opt_skb));
 		if (ipv6_opt_accepted(sk, opt_skb,
 				      &DCCP_SKB_CB(opt_skb)->header.h6)) {
-			skb_set_owner_r(opt_skb, sk);
 			memmove(IP6CB(opt_skb),
 				&DCCP_SKB_CB(opt_skb)->header.h6,
 				sizeof(struct inet6_skb_parm));

net/ipv6/tcp_ipv6.c

@@ -1388,14 +1388,11 @@ static struct sock *tcp_v6_syn_recv_sock(const struct sock *sk, struct sk_buff *
 
 		/* Clone pktoptions received with SYN, if we own the req */
 		if (ireq->pktopts) {
-			newnp->pktoptions = skb_clone(ireq->pktopts,
-						      sk_gfp_mask(sk, GFP_ATOMIC));
+			newnp->pktoptions = skb_clone_and_charge_r(ireq->pktopts, newsk);
 			consume_skb(ireq->pktopts);
 			ireq->pktopts = NULL;
-			if (newnp->pktoptions) {
+			if (newnp->pktoptions)
 				tcp_v6_restore_cb(newnp->pktoptions);
-				skb_set_owner_r(newnp->pktoptions, newsk);
-			}
 		}
 	} else {
 		if (!req_unhash && found_dup_sk) {
@@ -1467,7 +1464,7 @@ int tcp_v6_do_rcv(struct sock *sk, struct sk_buff *skb)
 					       --ANK (980728)
 	 */
 	if (np->rxopt.all)
-		opt_skb = skb_clone(skb, sk_gfp_mask(sk, GFP_ATOMIC));
+		opt_skb = skb_clone_and_charge_r(skb, sk);
 
 	reason = SKB_DROP_REASON_NOT_SPECIFIED;
 	if (sk->sk_state == TCP_ESTABLISHED) { /* Fast path */
@@ -1553,7 +1550,6 @@ int tcp_v6_do_rcv(struct sock *sk, struct sk_buff *skb)
 		if (np->repflow)
 			np->flow_label = ip6_flowlabel(ipv6_hdr(opt_skb));
 		if (ipv6_opt_accepted(sk, opt_skb, &TCP_SKB_CB(opt_skb)->header.h6)) {
-			skb_set_owner_r(opt_skb, sk);
 			tcp_v6_restore_cb(opt_skb);
 			opt_skb = xchg(&np->pktoptions, opt_skb);
 		} else {