[PATCH net 2/2] can: j1939: validate the DPO packet number

[Xue Boyang <m18335910246@163.com>]

j1939_xtp_rx_dpo_one() copies the 18-bit packet number of the ETP.CM_DPO
command into session->pkt.dpo without any sanity check.  The value is
later used as a byte offset (pkt.dpo * 7) by
j1939_session_skb_get_by_offset(), both for TP.DT placement (combined
with dat[0] - 1) and for the final message distribution in
j1939_session_completed().

A DPO pointing past the end of the transfer has no valid meaning:
every legal window rebase satisfies pkt.dpo <= pkt.total.  Reject
out-of-range values and abort the session with J1939_XTP_ABORT_FAULT
instead of carrying them around, mirroring the error handling of
j1939_xtp_rx_dat_one().

Note that pkt.dpo == pkt.total combined with a DT frame of
dat[0] == 0 still encodes the final packet (dat[0] - 1 + pkt.dpo ==
pkt.total - 1) and therefore stays accepted by design; the NULL
handling in j1939_session_completed() from the previous patch covers
this case.  This patch only discards the clearly bogus values, so the
receive path no longer operates on offsets far outside the buffer.

Fixes: 9d71dd0c7009 ("can: add support of SAE J1939 protocol")
Signed-off-by: Xue Boyang <m18335910246@163.com>
---
 net/can/j1939/transport.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/net/can/j1939/transport.c b/net/can/j1939/transport.c
index ecf7f245f837..b09983d9c19c 100644
--- a/net/can/j1939/transport.c
+++ b/net/can/j1939/transport.c
@@ -1833,6 +1833,11 @@ static void j1939_xtp_rx_dpo_one(struct j1939_session *session,
 
 	/* transmitted without problems */
 	session->pkt.dpo = j1939_etp_ctl_to_packet(skb->data);
+	if (session->pkt.dpo > session->pkt.total) {
+		j1939_session_timers_cancel(session);
+		j1939_session_cancel(session, J1939_XTP_ABORT_FAULT);
+		return;
+	}
 	session->last_cmd = dat[0];
 	j1939_tp_set_rxtimeout(session, 750);
 
-- 
2.53.0