[PATCH net] net/llc: avoid BUG_ON() in skb_orphan()

From: Eric Dumazet <edumazet@google.com>

It seems nobody used LLC since linux-3.12.

Fortunately fuzzers like syzkaller still know how to run this code,
otherwise it would be no fun.

Setting skb->sk without skb->destructor leads to all kinds of
bugs, we now prefer to be very strict about it.

Ideally here we would use skb_set_owner() but this helper does not exist yet,
only CAN seems to have a private helper for that.

Fixes: 376c7311bdb6 ("net: add a temporary sanity check in skb_orphan()")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reported-by: Andrey Konovalov <redacted>
---
 net/llc/llc_conn.c |    3 +++
 net/llc/llc_sap.c  |    3 +++
 2 files changed, 6 insertions(+)

diff --git a/net/llc/llc_conn.c b/net/llc/llc_conn.c
index 3e821daf9dd4a2fbf00550591e92b153efd4a73a..8bc5a1bd2d453542df31506f543feb64b64cdd96 100644
--- a/net/llc/llc_conn.c
+++ b/net/llc/llc_conn.c

@@ -821,7 +821,10 @@ void llc_conn_handler(struct llc_sap *sap, struct sk_buff *skb)
 		 * another trick required to cope with how the PROCOM state
 		 * machine works. -acme
 		 */
+		skb_orphan(skb);
+		sock_hold(sk);
 		skb->sk = sk;
+		skb->destructor = sock_efree;
 	}
 	if (!sock_owned_by_user(sk))
 		llc_conn_rcv(sk, skb);

diff --git a/net/llc/llc_sap.c b/net/llc/llc_sap.c
index d0e1e804ebd73dcebcf2f930b921233a49b0f454..5404d0d195cc581613e356b75bd70321e617673e 100644
--- a/net/llc/llc_sap.c
+++ b/net/llc/llc_sap.c

@@ -290,7 +290,10 @@ static void llc_sap_rcv(struct llc_sap *sap, struct sk_buff *skb,
 
 	ev->type   = LLC_SAP_EV_TYPE_PDU;
 	ev->reason = 0;
+	skb_orphan(skb);
+	sock_hold(sk);
 	skb->sk = sk;
+	skb->destructor = sock_efree;
 	llc_sap_state_process(sap, skb);
 }