[PATCH mptcp-net] mptcp: reclaim forward-allocated memory on RX path errors

After commit 9db5b3cec4ec ("mptcp: borrow forward memory from subflow"),
errors in the receive path prior to queueing skbs into the receive
queue do not trigger forward-allocated memory reclaiming.

Prevent forward memory from growing unboundedly in pathological drop
scenarios by explicitly reclaiming memory when skbs are dropped.

Fixes: 9db5b3cec4ec ("mptcp: borrow forward memory from subflow")
Signed-off-by: Paolo Abeni <[email protected]>
---
 net/mptcp/protocol.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/net/mptcp/protocol.c b/net/mptcp/protocol.c
index 954e20bb27de..90dc894cb976 100644
--- a/net/mptcp/protocol.c
+++ b/net/mptcp/protocol.c
@@ -149,6 +149,13 @@ struct sock *__mptcp_nmpc_sk(struct mptcp_sock *msk)
 
 static void mptcp_drop(struct sock *sk, struct sk_buff *skb)
 {
+	/*
+	 * The skb forward memory was already transferred to sk by
+	 * mptcp_borrow_fwdmem(), even before setting the destructor.
+	 */
+	if (!skb->destructor)
+		sk_mem_reclaim(sk);
+
 	sk_drops_skbadd(sk, skb);
 	__kfree_skb(skb);
 }
-- 
2.55.0