[PATCH v1] net: can: j1939: Fix out-of-bounds memory access in j1939_tp_tx_dat_new

[Oleksij Rempel]

In the j1939_tp_tx_dat_new function, an out-of-bounds memory access
could occur during the memcpy operation if the size of skb->cb is
larger than the size of struct j1939_sk_buff_cb. This is because the
memcpy operation uses the size of skb->cb, leading to a read beyond
the struct j1939_sk_buff_cb.

To address this issue, we have updated the memcpy operation to use the
size of struct j1939_sk_buff_cb instead of the size of skb->cb. This
ensures that the memcpy operation only reads the memory within the
bounds of struct j1939_sk_buff_cb, preventing out-of-bounds memory
access.

Additionally, a static_assert has been added to check that the size of
skb->cb is greater than or equal to the size of struct j1939_sk_buff_cb.
This ensures that the skb->cb buffer is large enough to hold the
j1939_sk_buff_cb structure.

Fixes: 9d71dd0c7009 ("can: add support of SAE J1939 protocol")
Reported-by: Shuangpeng Bai <sjb7183@xxxxxxx>
Tested-by: Shuangpeng Bai <sjb7183@xxxxxxx>
Signed-off-by: Oleksij Rempel <o.rempel@xxxxxxxxxxxxxx>
---
 net/can/j1939/transport.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/net/can/j1939/transport.c b/net/can/j1939/transport.c
index fb92c3609e17..fe3df23a2595 100644
--- a/net/can/j1939/transport.c
+++ b/net/can/j1939/transport.c
@@ -604,7 +604,10 @@ sk_buff *j1939_tp_tx_dat_new(struct j1939_priv *priv,
 	/* reserve CAN header */
 	skb_reserve(skb, offsetof(struct can_frame, data));
 
-	memcpy(skb->cb, re_skcb, sizeof(skb->cb));
+	/* skb->cb must be large enough to hold a j1939_sk_buff_cb structure */
+	BUILD_BUG_ON(sizeof(skb->cb) < sizeof(*re_skcb));
+
+	memcpy(skb->cb, re_skcb, sizeof(*re_skcb));
 	skcb = j1939_skb_to_cb(skb);
 	if (swap_src_dst)
 		j1939_skbcb_swap(skcb);
-- 
2.39.2