Re: [PATCH net v2] sctp: keep chunk->transport in step with the list it is queued on

__sctp_outq_flush_rtx() moves a chunk onto transport->transmitted in two
places but only one of them leaves chunk->transport describing where the
chunk actually is.  The ordinary resend path is consistent because
sctp_packet_append_chunk() rebinds the chunk in
__sctp_packet_append_chunk():

        list_add_tail(&chunk->list, &packet->chunk_list);
        packet->size += chunk_len;
        chunk->transport = packet->transport;

The gap-acked path has no such rebind.  It parks the chunk on another
transport's transmitted list and leaves the stale back-pointer alone:

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

So a chunk can sit on a live transport's transmitted list while
chunk->transport still names a different transport.  If that transport is
then removed - sctp_assoc_rm_peer() from an ASCONF Delete-IP - the last
reference goes away and sctp_transport_free() RCU-frees it, leaving the
chunk with a dangling pointer.  sctp_assoc_rm_peer() scrubs the
back-pointer on peer->transmitted and on asoc->outqueue.out_chunk_list,
but this chunk is on neither.

While tsn_gap_acked stays set the dangling pointer is not followed, since
sctp_check_transmitted() skips the flight accounting for gap-acked chunks.
A later SACK that reneges on the TSN clears the flag, and the next SACK
then reaches

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

a read-modify-write inside the freed sctp_transport.  KASAN reports a
slab-use-after-free read of 4 bytes at offset 216 of a freed kmalloc-1k
sctp_transport in sctp_check_transmitted(), freed from
sctp_assoc_rm_peer() via sctp_process_asconf().  The removal and the
faulting SACKs all come from the association peer, so an application that
speaks SCTP and accepts multihoming is enough to reach it.

Assign chunk->transport after both list_move_tail() calls, so the
back-pointer always matches the list the chunk is queued on.  Doing it on
the ordinary path too keeps the invariant local to the move instead of
resting on a rebind that happens later and only if the chunk is appended to
a packet.  Fixing it here rather than by scrubbing more lists in
sctp_assoc_rm_peer() covers the chunks that were already migrated before
the transport went away, which a scrub at removal time cannot see.

Discovered by XBOW, triaged by Baul Lee [off-list ref]
[Reported privately to the maintainers on 2026-07-10 with root-cause
analysis, a PoC, a KASAN log and a fix; posting to the list was requested
as the follow-up.]

Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Cc: stable@vger.kernel.org
Signed-off-by: Baul Lee <redacted>
---
v2:
 - fix the root cause in __sctp_outq_flush_rtx() instead of clearing the
   back-pointer for the retransmit queue in sctp_assoc_rm_peer().  The v1
   scrub could not see chunks that had already been migrated onto another
   transport's transmitted list before the removal, which is the case the
   reviews on v1 asked about
 - correct the Fixes tag: the inconsistency is as old as the code, not
   introduced by df132eff4638

Link to v1: https://lore.kernel.org/netdev/20260726060453.42730-1-baul.lee@xbow.com/ (local)

 net/sctp/outqueue.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/net/sctp/outqueue.c b/net/sctp/outqueue.c
index f6b8c13da..1b2d08f2e 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;
                }

@@ -715,6 +716,7 @@ static int __sctp_outq_flush_rtx(struct sctp_outq *q, struct sctp_packet *pkt,
                         */
                        list_move_tail(&chunk->transmitted_list,
                                       &transport->transmitted);
+                       chunk->transport = transport;