[Patch net] llc: fix NULL pointer deref for SOCK_ZAPPED

[Cong Wang]

For SOCK_ZAPPED socket, we don't need to care about llc->sap,
so we should just skip these refcount functions in this case.

Fixes: f7e43672683b ("llc: hold llc_sap before release_sock()")
Reported-by: kernel test robot <l...@intel.com>
Signed-off-by: Cong Wang <xiyou.wangc...@gmail.com>
---
 net/llc/af_llc.c | 21 ++++++++++++---------
 1 file changed, 12 insertions(+), 9 deletions(-)

diff --git a/net/llc/af_llc.c b/net/llc/af_llc.c
index 6d29b2b94e84..cb80ebb38311 100644
--- a/net/llc/af_llc.c
+++ b/net/llc/af_llc.c
@@ -189,7 +189,6 @@ static int llc_ui_release(struct socket *sock)
 {
        struct sock *sk = sock->sk;
        struct llc_sock *llc;
-       struct llc_sap *sap;
 
        if (unlikely(sk == NULL))
                goto out;
@@ -200,15 +199,19 @@ static int llc_ui_release(struct socket *sock)
                llc->laddr.lsap, llc->daddr.lsap);
        if (!llc_send_disc(sk))
                llc_ui_wait_for_disc(sk, sk->sk_rcvtimeo);
-       sap = llc->sap;
-       /* Hold this for release_sock(), so that llc_backlog_rcv() could still
-        * use it.
-        */
-       llc_sap_hold(sap);
-       if (!sock_flag(sk, SOCK_ZAPPED))
+       if (!sock_flag(sk, SOCK_ZAPPED)) {
+               struct llc_sap *sap = llc->sap;
+
+               /* Hold this for release_sock(), so that llc_backlog_rcv()
+                * could still use it.
+                */
+               llc_sap_hold(sap);
                llc_sap_remove_socket(llc->sap, sk);
-       release_sock(sk);
-       llc_sap_put(sap);
+               release_sock(sk);
+               llc_sap_put(sap);
+       } else {
+               release_sock(sk);
+       }
        if (llc->dev)
                dev_put(llc->dev);
        sock_put(sk);
-- 
2.13.0