[PATCH nf] netfilter: nfnetlink_log: cope with concurrent instance destruction

Instances are refcounted. However, only memory release happens on the
1 -> 0 transition; the unlink from hashes can occur with any refcount.

Uncooperative userspace can force a situation where a queue is pending
for destruction from netlink event while a different socket with same
portid processes an UNBIND request.

With right timing, this will unhash the instance again:

Oops: general protection fault, [..]
Call Trace:
 <TASK>
 nfulnl_recv_config+0x31a/0xd50
 nfnetlink_rcv_msg+0x7c2/0xeb0

Fixes: 0597f2680d66 ("[NETFILTER]: Add new "nfnetlink_log" userspace packet logging facility")
Reported-by: Eulgyu Kim <[email protected]>
Reported-by: Jaeyoung Chung <[email protected]>
Signed-off-by: Florian Westphal <[email protected]>
---
 net/netfilter/nfnetlink_log.c | 13 +++++++++----
 1 file changed, 9 insertions(+), 4 deletions(-)

diff --git a/net/netfilter/nfnetlink_log.c b/net/netfilter/nfnetlink_log.c
index 6c7fa2ed34f5..8fc002ae08bc 100644
--- a/net/netfilter/nfnetlink_log.c
+++ b/net/netfilter/nfnetlink_log.c
@@ -228,13 +228,18 @@ static void __nfulnl_flush(struct nfulnl_instance *inst);
 static void
 __instance_destroy(struct nfulnl_instance *inst)
 {
+	spin_lock(&inst->lock);
+	if (inst->copy_mode == NFULNL_COPY_DISABLED) {
+		/* attempt to UNBIND a queue already pending
+		 * destruction via netlink close event. Ignore.
+		 */
+		spin_unlock(&inst->lock);
+		return;
+	}
+
 	/* first pull it out of the global list */
 	hlist_del_rcu(&inst->hlist);
 
-	/* then flush all pending packets from skb */
-
-	spin_lock(&inst->lock);
-
 	/* lockless readers wont be able to use us */
 	inst->copy_mode = NFULNL_COPY_DISABLED;
 
-- 
2.54.0