[PATCH net-next] can: j1939: fix uaf in j1939_session_destroy

[Edward Adam Davis]

The root cause of this problem is when both of the following conditions
are met simultaneously:
[1] Introduced commit c9c0ee5f20c5, There are following rules:
In debug builds (CONFIG_DEBUG_NET set), the reference count is always
decremented, even when it's 1.

[2] When executing sendmsg, the newly created session did not increase the
skb reference count, only added skb to the session's skb_queue.

The solution is:
When creating a new session, do not add the skb to the skb_queue.
Instead, when using skb, uniformly use j1939_session_skb_queue to add
the skb to the queue and increase the skb reference count through it.

Fixes: c9c0ee5f20c5 ("net: skbuff: Skip early return in skb_unref when debugging")
Reported-and-tested-by: syzbot+ad601904231505ad6617@xxxxxxxxxxxxxxxxxxxxxxxxx
Closes: https://syzkaller.appspot.com/bug?extid=ad601904231505ad6617
Signed-off-by: Edward Adam Davis <eadavis@xxxxxx>
---
 net/can/j1939/socket.c    | 7 ++++---
 net/can/j1939/transport.c | 2 +-
 2 files changed, 5 insertions(+), 4 deletions(-)

diff --git a/net/can/j1939/socket.c b/net/can/j1939/socket.c
index 305dd72c844c..ec78bee1bfa6 100644
--- a/net/can/j1939/socket.c
+++ b/net/can/j1939/socket.c
@@ -1170,10 +1170,11 @@ static int j1939_sk_send_loop(struct j1939_priv *priv,  struct sock *sk,
 					break;
 				}
 			}
-		} else {
-			skcb->offset = session->total_queued_size;
-			j1939_session_skb_queue(session, skb);
 		}
+		/* Session is ready, add it to skb queue and increase ref count.
+		 */
+		skcb->offset = session->total_queued_size;
+		j1939_session_skb_queue(session, skb);
 
 		todo_size -= segment_size;
 		session->total_queued_size += segment_size;
diff --git a/net/can/j1939/transport.c b/net/can/j1939/transport.c
index 4be73de5033c..dd503bc3adb5 100644
--- a/net/can/j1939/transport.c
+++ b/net/can/j1939/transport.c
@@ -1505,7 +1505,6 @@ static struct j1939_session *j1939_session_new(struct j1939_priv *priv,
 	session->state = J1939_SESSION_NEW;
 
 	skb_queue_head_init(&session->skb_queue);
-	skb_queue_tail(&session->skb_queue, skb);
 
 	skcb = j1939_skb_to_cb(skb);
 	memcpy(&session->skcb, skcb, sizeof(session->skcb));
@@ -1548,6 +1547,7 @@ j1939_session *j1939_session_fresh_new(struct j1939_priv *priv,
 		kfree_skb(skb);
 		return NULL;
 	}
+	j1939_session_skb_queue(session, skb);
 
 	/* alloc data area */
 	skb_put(skb, size);
-- 
2.43.0