[PATCH net v3] sctp: keep chunk->transport in step with the list it is queued on

[Baul Lee <baul.lee@xbow.com>]

__sctp_outq_flush_rtx() moves a gap-acked chunk onto another transport's
transmitted list without updating chunk->transport:

	if (chunk->tsn_gap_acked) {
		list_move_tail(&chunk->transmitted_list,
			       &transport->transmitted);
		continue;
	}

The chunk then sits on a live transport's list while chunk->transport still
names a different one.  If that transport is removed - sctp_assoc_rm_peer()
from an ASCONF Delete-IP - sctp_transport_free() RCU-frees it and the chunk
is left with a dangling pointer.  sctp_assoc_rm_peer() scrubs
peer->transmitted and asoc->outqueue.out_chunk_list, but the chunk is on
neither.

The pointer is not followed while tsn_gap_acked is set.  A SACK that
reneges on the TSN clears the flag, and the next SACK reaches

	tchunk->transport->flight_size -= sctp_data_size(tchunk);

inside the freed transport.  KASAN reports a slab-use-after-free read in
sctp_check_transmitted(), freed from sctp_assoc_rm_peer().  Both the
removal and the SACKs come from the association peer.

Set chunk->transport at the move.  The ordinary resend path needs nothing:
it reaches its list_move_tail() only after sctp_packet_append_chunk()
returned SCTP_XMIT_OK, and __sctp_packet_append_chunk() has rebound the
chunk by then.

Discovered by XBOW, triaged by Baul Lee <baul.lee@xbow.com>

Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Cc: stable@vger.kernel.org
Signed-off-by: Baul Lee <baul.lee@xbow.com>
---
v3:
 - drop the redundant assignment on the ordinary resend path (Xin Long)
 - trim the commit message

v2:
 - fix the root cause in __sctp_outq_flush_rtx() rather than scrubbing the
   retransmit queue in sctp_assoc_rm_peer(), which cannot see chunks already
   migrated onto another transport's list
 - correct the Fixes tag

Link to v1: https://lore.kernel.org/netdev/20260726060453.42730-1-baul.lee@xbow.com/
Link to v2: https://lore.kernel.org/netdev/20260729124929.5885-1-baul.lee@xbow.com/

 net/sctp/outqueue.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/net/sctp/outqueue.c b/net/sctp/outqueue.c
index f6b8c13dafa4..e92fb9da4647 100644
--- a/net/sctp/outqueue.c
+++ b/net/sctp/outqueue.c
@@ -650,6 +650,7 @@ static int __sctp_outq_flush_rtx(struct sctp_outq *q, struct sctp_packet *pkt,
 		if (chunk->tsn_gap_acked) {
 			list_move_tail(&chunk->transmitted_list,
 				       &transport->transmitted);
+			chunk->transport = transport;
 			continue;
 		}
 
-- 
2.53.0