Re: [PATCH net] sctp: clear new_transport when removing a peer

[Xin Long]

On Tue, Aug 11, 2026 at 11:28 AM Qing Ming <a0yami@xxxxxxxxxxx> wrote:
>
> sctp_process_asconf_param() stores a newly added peer transport in
> asoc->new_transport. After all parameters in the ASCONF chunk have been
> processed, sctp_sf_do_asconf() uses this pointer to send a HEARTBEAT to the
> new transport.
>
> An authenticated ASCONF from a remote SCTP peer can add a transport and
> remove it again with a wildcard DEL-IP parameter in the same chunk. The
> wildcard deletion preserves the transport on which the ASCONF arrived, but
> removes the newly added transport through
> sctp_assoc_del_nonprimary_peers(). The removal does not clear
> asoc->new_transport, leaving it pointing to the removed transport.
>
> sctp_sf_do_asconf() then creates a HEARTBEAT whose chunk->transport points
> to the removed transport without holding a transport reference. During
> local address replacement, src_out_of_asoc_ok keeps this HEARTBEAT on
> control_chunk_list. After the transport is freed by RCU, a successful
> ASCONF_ACK for the replacement address releases the queued HEARTBEAT and
> sctp_outq_select_transport() reads the freed transport's state.
>
> The issue was found during a static audit of SCTP objects. With an
> authenticated peer, the reproducer triggered the same KASAN report in 2
> of 2 unpatched runs on a KASAN-enabled netdev/main kernel:
>
>   BUG: KASAN: slab-use-after-free in sctp_outq_select_transport
>   Read of size 4 at addr ffff88800b9bd95c by task python3/197
>
>   Call Trace:
>    sctp_outq_select_transport+0x549/0x8b0 [sctp]
>    sctp_outq_flush+0x306/0x2c60 [sctp]
>    sctp_transport_immediate_rtx+0xaf/0x260 [sctp]
>    sctp_process_asconf_ack+0xa48/0xf70 [sctp]
>
>   Allocated by task 197:
>    sctp_transport_new+0x68/0x650 [sctp]
>    sctp_assoc_add_peer+0x258/0x12a0 [sctp]
>    sctp_process_asconf+0x5e9/0x1090 [sctp]
>
>   Last potentially related work creation:
>    __call_rcu_common.constprop.0+0x77/0xb70
>    sctp_assoc_del_nonprimary_peers+0x7c/0xd0 [sctp]
>    sctp_process_asconf+0xd9c/0x1090 [sctp]
>
> The first invalid access was a four-byte read of transport->state at
> net/sctp/outqueue.c:833. The same reproducer completed the full
> authenticated ASCONF and local-address replacement sequence with this
> change without a KASAN report or oops.
>
> Clear new_transport when its peer is removed, before it can be used to
> create the HEARTBEAT.
>
> Fixes: 6af29ccc223b ("sctp: Bundle HEAERTBEAT into ASCONF_ACK")
> Cc: stable@xxxxxxxxxxxxxxx
> Assisted-by: Codex:gpt-5
> Signed-off-by: Qing Ming <a0yami@xxxxxxxxxxx>

Acked-by: Xin Long <lucien.xin@xxxxxxxxx>