[Patch net] llc: set SOCK_RCU_FREE in llc_sap_add_socket()

[Patch net] llc: set SOCK_RCU_FREE in llc_sap_add_socket()

[Cong Wang]

WHen an llc sock is added into the sk_laddr_hash of an llc_sap,
it is not marked with SOCK_RCU_FREE.

This causes that the sock could be freed while it is still being
read by __llc_lookup_established() with RCU read lock. sock is
refcounted, but with RCU read lock, nothing prevents the readers
getting a zero refcnt.

Fix it by setting SOCK_RCU_FREE in llc_sap_add_socket().

Reported-by: syzbot+11e05f04c15e03be5254@syzkaller.appspotmail.com
Signed-off-by: Cong Wang <xiyou.wangcong@gmail.com>
---
 net/llc/llc_conn.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/net/llc/llc_conn.c b/net/llc/llc_conn.c
index c0ac522b48a1..4ff89cb7c86f 100644
--- a/net/llc/llc_conn.c
+++ b/net/llc/llc_conn.c
@@ -734,6 +734,7 @@ void llc_sap_add_socket(struct llc_sap *sap, struct sock *sk)
 	llc_sk(sk)->sap = sap;
 
 	spin_lock_bh(&sap->sk_lock);
+	sock_set_flag(sk, SOCK_RCU_FREE);
 	sap->sk_count++;
 	sk_nulls_add_node_rcu(sk, laddr_hb);
 	hlist_add_head(&llc->dev_hash_node, dev_hb);
-- 
2.14.4

Re: [Patch net] llc: set SOCK_RCU_FREE in llc_sap_add_socket()

[David Miller]

From: Cong Wang <xiyou.wangcong@gmail.com>
Date: Thu, 11 Oct 2018 11:15:13 -0700

> WHen an llc sock is added into the sk_laddr_hash of an llc_sap,
> it is not marked with SOCK_RCU_FREE.
> 
> This causes that the sock could be freed while it is still being
> read by __llc_lookup_established() with RCU read lock. sock is
> refcounted, but with RCU read lock, nothing prevents the readers
> getting a zero refcnt.
> 
> Fix it by setting SOCK_RCU_FREE in llc_sap_add_socket().
> 
> Reported-by: syzbot+11e05f04c15e03be5254@syzkaller.appspotmail.com
> Signed-off-by: Cong Wang <xiyou.wangcong@gmail.com>

Applied and queued up for -stable.