[PATCH 4.9 03/25] llc: use refcount_inc_not_zero() for llc_sap_find()

[Greg Kroah-Hartman]

4.9-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Cong Wang <xiyou.wangcong@xxxxxxxxx>

[ Upstream commit 0dcb82254d65f72333aa50ad626d1e9665ad093b ]

llc_sap_put() decreases the refcnt before deleting sap
from the global list. Therefore, there is a chance
llc_sap_find() could find a sap with zero refcnt
in this global list.

Close this race condition by checking if refcnt is zero
or not in llc_sap_find(), if it is zero then it is being
removed so we can just treat it as gone.

Reported-by: <syzbot+278893f3f7803871f7ce@xxxxxxxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Cong Wang <xiyou.wangcong@xxxxxxxxx>
Signed-off-by: David S. Miller <davem@xxxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
 include/net/llc.h  |    5 +++++
 net/llc/llc_core.c |    4 ++--
 2 files changed, 7 insertions(+), 2 deletions(-)

--- a/include/net/llc.h
+++ b/include/net/llc.h
@@ -116,6 +116,11 @@ static inline void llc_sap_hold(struct l
 	atomic_inc(&sap->refcnt);
 }
 
+static inline bool llc_sap_hold_safe(struct llc_sap *sap)
+{
+	return atomic_inc_not_zero(&sap->refcnt);
+}
+
 void llc_sap_close(struct llc_sap *sap);
 
 static inline void llc_sap_put(struct llc_sap *sap)
--- a/net/llc/llc_core.c
+++ b/net/llc/llc_core.c
@@ -73,8 +73,8 @@ struct llc_sap *llc_sap_find(unsigned ch
 
 	rcu_read_lock_bh();
 	sap = __llc_sap_find(sap_value);
-	if (sap)
-		llc_sap_hold(sap);
+	if (!sap || !llc_sap_hold_safe(sap))
+		sap = NULL;
 	rcu_read_unlock_bh();
 	return sap;
 }