skbuff: fix a data race in skb_queue_len()

sk_buff.qlen can be accessed concurrently as noticed by KCSAN,

 BUG: KCSAN: data-race in __skb_try_recv_from_queue / unix_dgram_sendmsg

 read to 0xffff8a1b1d8a81c0 of 4 bytes by task 5371 on cpu 96:
  unix_dgram_sendmsg+0x9a9/0xb70 include/linux/skbuff.h:1821
				 net/unix/af_unix.c:1761
  ____sys_sendmsg+0x33e/0x370
  ___sys_sendmsg+0xa6/0xf0
  __sys_sendmsg+0x69/0xf0
  __x64_sys_sendmsg+0x51/0x70
  do_syscall_64+0x91/0xb47
  entry_SYSCALL_64_after_hwframe+0x49/0xbe

 write to 0xffff8a1b1d8a81c0 of 4 bytes by task 1 on cpu 99:
  __skb_try_recv_from_queue+0x327/0x410 include/linux/skbuff.h:2029
  __skb_try_recv_datagram+0xbe/0x220
  unix_dgram_recvmsg+0xee/0x850
  ____sys_recvmsg+0x1fb/0x210
  ___sys_recvmsg+0xa2/0xf0
  __sys_recvmsg+0x66/0xf0
  __x64_sys_recvmsg+0x51/0x70
  do_syscall_64+0x91/0xb47
  entry_SYSCALL_64_after_hwframe+0x49/0xbe

Since only the read is operating as lockless, it could introduce a logic
bug in unix_recvq_full() due to the load tearing. Fix it by adding
a lockless variant of skb_queue_len() and unix_recvq_full() where
READ_ONCE() is on the read while WRITE_ONCE() is on the write similar to
the commit d7d16a89 ("net: add skb_queue_empty_lockless()").
Signed-off-by: Qian Cai <cai@lca.pw>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/skbuff.h

@@ -1821,6 +1821,18 @@ static inline __u32 skb_queue_len(const struct sk_buff_head *list_)
 	return list_->qlen;
 }
 
+/**
+ *	skb_queue_len_lockless	- get queue length
+ *	@list_: list to measure
+ *
+ *	Return the length of an &sk_buff queue.
+ *	This variant can be used in lockless contexts.
+ */
+static inline __u32 skb_queue_len_lockless(const struct sk_buff_head *list_)
+{
+	return READ_ONCE(list_->qlen);
+}
+
 /**
  *	__skb_queue_head_init - initialize non-spinlock portions of sk_buff_head
  *	@list: queue to initialize
@@ -2026,7 +2038,7 @@ static inline void __skb_unlink(struct sk_buff *skb, struct sk_buff_head *list)
 {
 	struct sk_buff *next, *prev;
 
-	list->qlen--;
+	WRITE_ONCE(list->qlen, list->qlen - 1);
 	next	   = skb->next;
 	prev	   = skb->prev;
 	skb->next  = skb->prev = NULL;

net/unix/af_unix.c

@@ -189,11 +189,17 @@ static inline int unix_may_send(struct sock *sk, struct sock *osk)
 	return unix_peer(osk) == NULL || unix_our_peer(sk, osk);
 }
 
-static inline int unix_recvq_full(struct sock const *sk)
+static inline int unix_recvq_full(const struct sock *sk)
 {
 	return skb_queue_len(&sk->sk_receive_queue) > sk->sk_max_ack_backlog;
 }
 
+static inline int unix_recvq_full_lockless(const struct sock *sk)
+{
+	return skb_queue_len_lockless(&sk->sk_receive_queue) >
+		READ_ONCE(sk->sk_max_ack_backlog);
+}
+
 struct sock *unix_peer_get(struct sock *s)
 {
 	struct sock *peer;
@@ -1758,7 +1764,8 @@ static int unix_dgram_sendmsg(struct socket *sock, struct msghdr *msg,
 	 * - unix_peer(sk) == sk by time of get but disconnected before lock
 	 */
 	if (other != sk &&
-	    unlikely(unix_peer(other) != sk && unix_recvq_full(other))) {
+	    unlikely(unix_peer(other) != sk &&
+	    unix_recvq_full_lockless(other))) {
 		if (timeo) {
 			timeo = unix_wait_for_peer(other, timeo);