__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 Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2") Cc: stable@vger.kernel.org Signed-off-by: Baul Lee --- 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