llc: use refcount_inc_not_zero() for llc_sap_find()

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: <>
Signed-off-by: Cong Wang <>
Signed-off-by: David S. Miller <>
diff --git a/include/net/llc.h b/include/net/llc.h
index dc35f25..890a873 100644
--- a/include/net/llc.h
+++ b/include/net/llc.h
@@ -116,6 +116,11 @@
+static inline bool llc_sap_hold_safe(struct llc_sap *sap)
+	return refcount_inc_not_zero(&sap->refcnt);
 void llc_sap_close(struct llc_sap *sap);
 static inline void llc_sap_put(struct llc_sap *sap)
diff --git a/net/llc/llc_core.c b/net/llc/llc_core.c
index 8904126..260b3dc 100644
--- a/net/llc/llc_core.c
+++ b/net/llc/llc_core.c
@@ -73,8 +73,8 @@
 	sap = __llc_sap_find(sap_value);
-	if (sap)
-		llc_sap_hold(sap);
+	if (!sap || !llc_sap_hold_safe(sap))
+		sap = NULL;
 	return sap;