[PATCH net 3/4] af_unix: Fix scm_stat.nr_fds leak in unix_dgram_disconnected().

[Siddharth Chintamaneni <sidchintamaneni@gmail.com>]

When a connected SOCK_DGRAM socket disconnects (e.g. via connect() with
AF_UNSPEC), reconnects to a different peer, or observes a dead peer in
unix_dgram_sendmsg(), unix_dgram_disconnected() purges all unread
datagrams from sk->sk_receive_queue via skb_queue_purge_reason().

Because skb_queue_purge_reason() frees each skb without calling
scm_stat_del(sk, skb), any SCM_RIGHTS file descriptors attached to the
purged skbs remain permanently accounted in u->scm_stat.nr_fds (reported
as scm_fds in /proc/<pid>/fdinfo/<fd>) for the remaining lifetime of the
socket.

Move scm_stat_add() and scm_stat_del() above unix_dgram_disconnected()
and dequeue each skb in unix_dgram_disconnected() so scm_stat_del() is
called before kfree_skb_reason().

Fixes: 3c32da19a858 ("unix: Show number of pending scm files of receive queue in fdinfo")
Signed-off-by: Siddharth Chintamaneni <sidchintamaneni@gmail.com>
---
 net/unix/af_unix.c | 52 +++++++++++++++++++++++++---------------------
 1 file changed, 28 insertions(+), 24 deletions(-)

diff --git a/net/unix/af_unix.c b/net/unix/af_unix.c
index 44c0a81dd423..47120d81aca2 100644
--- a/net/unix/af_unix.c
+++ b/net/unix/af_unix.c
@@ -609,6 +609,28 @@ static void unix_write_space(struct sock *sk)
 	rcu_read_unlock();
 }
 
+static void scm_stat_add(struct sock *sk, struct sk_buff *skb)
+{
+	struct scm_fp_list *fp = UNIXCB(skb).fp;
+	struct unix_sock *u = unix_sk(sk);
+
+	if (unlikely(fp && fp->count)) {
+		atomic_add(fp->count, &u->scm_stat.nr_fds);
+		unix_add_edges(fp, u);
+	}
+}
+
+static void scm_stat_del(struct sock *sk, struct sk_buff *skb)
+{
+	struct scm_fp_list *fp = UNIXCB(skb).fp;
+	struct unix_sock *u = unix_sk(sk);
+
+	if (unlikely(fp && fp->count)) {
+		atomic_sub(fp->count, &u->scm_stat.nr_fds);
+		unix_del_edges(fp);
+	}
+}
+
 /* When dgram socket disconnects (or changes its peer), we clear its receive
  * queue of packets arrived from previous peer. First, it allows to do
  * flow control based only on wmem_alloc; second, sk connected to peer
@@ -616,8 +638,12 @@ static void unix_write_space(struct sock *sk)
 static void unix_dgram_disconnected(struct sock *sk, struct sock *other)
 {
 	if (!skb_queue_empty(&sk->sk_receive_queue)) {
-		skb_queue_purge_reason(&sk->sk_receive_queue,
-				       SKB_DROP_REASON_UNIX_DISCONNECT);
+		struct sk_buff *skb;
+
+		while ((skb = skb_dequeue(&sk->sk_receive_queue)) != NULL) {
+			scm_stat_del(sk, skb);
+			kfree_skb_reason(skb, SKB_DROP_REASON_UNIX_DISCONNECT);
+		}
 
 		wake_up_interruptible_all(&unix_sk(sk)->peer_wait);
 
@@ -2058,28 +2084,6 @@ static bool unix_skb_scm_eq(const struct sock *sk, struct sk_buff *skb,
 	       unix_secdata_eq(scm, skb);
 }
 
-static void scm_stat_add(struct sock *sk, struct sk_buff *skb)
-{
-	struct scm_fp_list *fp = UNIXCB(skb).fp;
-	struct unix_sock *u = unix_sk(sk);
-
-	if (unlikely(fp && fp->count)) {
-		atomic_add(fp->count, &u->scm_stat.nr_fds);
-		unix_add_edges(fp, u);
-	}
-}
-
-static void scm_stat_del(struct sock *sk, struct sk_buff *skb)
-{
-	struct scm_fp_list *fp = UNIXCB(skb).fp;
-	struct unix_sock *u = unix_sk(sk);
-
-	if (unlikely(fp && fp->count)) {
-		atomic_sub(fp->count, &u->scm_stat.nr_fds);
-		unix_del_edges(fp);
-	}
-}
-
 static void unix_orphan_scm(struct sock *sk, struct sk_buff *skb)
 {
 	scm_stat_del(sk, skb);
-- 
2.43.0