From: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
To: stable@vger.kernel.org
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	patches@lists.linux.dev,
	Baul Lee <baul.lee@xbow.com>,
	Xin Long <lucien.xin@gmail.com>,
	Jakub Kicinski <kuba@kernel.org>
Subject: [PATCH 6.18 244/250] sctp: keep chunk->transport in step with the list it is queued on
Date: Mon, 17 Aug 2026 15:33:25 +0200
Message-ID: <20260817132546.463214717@linuxfoundation.org>
In-Reply-To: <20260817132536.466235697@linuxfoundation.org>
References: <20260817132536.466235697@linuxfoundation.org>
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6.18-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Baul Lee <baul.lee@xbow.com>

commit 9f2cf069a9a72a2d6b97ca8b4c70e714aac99749 upstream.

__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>
Acked-by: Xin Long <lucien.xin@gmail.com>
Link: https://patch.msgid.link/20260729160028.54546-1-baul.lee@xbow.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
 net/sctp/outqueue.c |    1 +
 1 file changed, 1 insertion(+)

--- a/net/sctp/outqueue.c
+++ b/net/sctp/outqueue.c
@@ -650,6 +650,7 @@ static int __sctp_outq_flush_rtx(struct
 		if (chunk->tsn_gap_acked) {
 			list_move_tail(&chunk->transmitted_list,
 				       &transport->transmitted);
+			chunk->transport = transport;
 			continue;
 		}