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
