[PATCH v6] sctp: fix refcount bug in sctp_wfree

[Qiujun Huang <hqjagain@xxxxxxxxx>]

We should iterate over the datamsgs to move
all chunks(skbs) to newsk.

The following case cause the bug:
for the trouble SKB, it was in outq->transmitted list

sctp_outq_sack
        sctp_check_transmitted
                SKB was moved to outq->sacked list
        then throw away the sack queue
                SKB was deleted from outq->sacked
(but it was held by datamsg at sctp_datamsg_to_asoc
So, sctp_wfree was not called here)

then migrate happened

        sctp_for_each_tx_datachunk(
        sctp_clear_owner_w);
        sctp_assoc_migrate();
        sctp_for_each_tx_datachunk(
        sctp_set_owner_w);
SKB was not in the outq, and was not changed to newsk

finally

__sctp_outq_teardown
        sctp_chunk_put (for another skb)
                sctp_datamsg_put
                        __kfree_skb(msg->frag_list)
                                sctp_wfree (for SKB)
	SKB->sk was still oldsk (skb->sk != asoc->base.sk).

Reported-and-tested-by: syzbot+cea71eec5d6de256d54d@xxxxxxxxxxxxxxxxxxxxxxxxx
Signed-off-by: Qiujun Huang <hqjagain@xxxxxxxxx>
---
 net/sctp/socket.c | 31 +++++++++++++++++++++++--------
 1 file changed, 23 insertions(+), 8 deletions(-)

diff --git a/net/sctp/socket.c b/net/sctp/socket.c
index 1b56fc440606..f68076713162 100644
--- a/net/sctp/socket.c
+++ b/net/sctp/socket.c
@@ -147,29 +147,44 @@ static void sctp_clear_owner_w(struct sctp_chunk *chunk)
 	skb_orphan(chunk->skb);
 }
 
+#define traverse_and_process()	\
+do {				\
+	msg = chunk->msg;	\
+	if (msg == prev_msg)	\
+		continue;	\
+	list_for_each_entry(c, &msg->chunks, frag_list) {	\
+		if ((clear && asoc->base.sk == c->skb->sk) ||	\
+		    (!clear && asoc->base.sk != c->skb->sk))	\
+			cb(c);	\
+	}			\
+	prev_msg = msg;		\
+} while (0)
+
 static void sctp_for_each_tx_datachunk(struct sctp_association *asoc,
+				       bool clear,
 				       void (*cb)(struct sctp_chunk *))
 
 {
+	struct sctp_datamsg *msg, *prev_msg = NULL;
 	struct sctp_outq *q = &asoc->outqueue;
+	struct sctp_chunk *chunk, *c;
 	struct sctp_transport *t;
-	struct sctp_chunk *chunk;
 
 	list_for_each_entry(t, &asoc->peer.transport_addr_list, transports)
 		list_for_each_entry(chunk, &t->transmitted, transmitted_list)
-			cb(chunk);
+			traverse_and_process();
 
 	list_for_each_entry(chunk, &q->retransmit, transmitted_list)
-		cb(chunk);
+		traverse_and_process();
 
 	list_for_each_entry(chunk, &q->sacked, transmitted_list)
-		cb(chunk);
+		traverse_and_process();
 
 	list_for_each_entry(chunk, &q->abandoned, transmitted_list)
-		cb(chunk);
+		traverse_and_process();
 
 	list_for_each_entry(chunk, &q->out_chunk_list, list)
-		cb(chunk);
+		traverse_and_process();
 }
 
 static void sctp_for_each_rx_skb(struct sctp_association *asoc, struct sock *sk,
@@ -9574,9 +9589,9 @@ static int sctp_sock_migrate(struct sock *oldsk, struct sock *newsk,
 	 * paths won't try to lock it and then oldsk.
 	 */
 	lock_sock_nested(newsk, SINGLE_DEPTH_NESTING);
-	sctp_for_each_tx_datachunk(assoc, sctp_clear_owner_w);
+	sctp_for_each_tx_datachunk(assoc, true, sctp_clear_owner_w);
 	sctp_assoc_migrate(assoc, newsk);
-	sctp_for_each_tx_datachunk(assoc, sctp_set_owner_w);
+	sctp_for_each_tx_datachunk(assoc, false, sctp_set_owner_w);
 
 	/* If the association on the newsk is already closed before accept()
 	 * is called, set RCV_SHUTDOWN flag.
-- 
2.17.1