kcm: annotate data-races around kcm->rx_psock

kcm->rx_psock can be read locklessly in kcm_rfree().
Annotate the read and writes accordingly.

We do the same for kcm->rx_wait in the following patch.

syzbot reported:
BUG: KCSAN: data-race in kcm_rfree / unreserve_rx_kcm

write to 0xffff888123d827b8 of 8 bytes by task 2758 on cpu 1:
unreserve_rx_kcm+0x72/0x1f0 net/kcm/kcmsock.c:313
kcm_rcv_strparser+0x2b5/0x3a0 net/kcm/kcmsock.c:373
__strp_recv+0x64c/0xd20 net/strparser/strparser.c:301
strp_recv+0x6d/0x80 net/strparser/strparser.c:335
tcp_read_sock+0x13e/0x5a0 net/ipv4/tcp.c:1703
strp_read_sock net/strparser/strparser.c:358 [inline]
do_strp_work net/strparser/strparser.c:406 [inline]
strp_work+0xe8/0x180 net/strparser/strparser.c:415
process_one_work+0x3d3/0x720 kernel/workqueue.c:2289
worker_thread+0x618/0xa70 kernel/workqueue.c:2436
kthread+0x1a9/0x1e0 kernel/kthread.c:376
ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:306

read to 0xffff888123d827b8 of 8 bytes by task 5859 on cpu 0:
kcm_rfree+0x14c/0x220 net/kcm/kcmsock.c:181
skb_release_head_state+0x8e/0x160 net/core/skbuff.c:841
skb_release_all net/core/skbuff.c:852 [inline]
__kfree_skb net/core/skbuff.c:868 [inline]
kfree_skb_reason+0x5c/0x260 net/core/skbuff.c:891
kfree_skb include/linux/skbuff.h:1216 [inline]
kcm_recvmsg+0x226/0x2b0 net/kcm/kcmsock.c:1161
____sys_recvmsg+0x16c/0x2e0
___sys_recvmsg net/socket.c:2743 [inline]
do_recvmmsg+0x2f1/0x710 net/socket.c:2837
__sys_recvmmsg net/socket.c:2916 [inline]
__do_sys_recvmmsg net/socket.c:2939 [inline]
__se_sys_recvmmsg net/socket.c:2932 [inline]
__x64_sys_recvmmsg+0xde/0x160 net/socket.c:2932
do_syscall_x64 arch/x86/entry/common.c:50 [inline]
do_syscall_64+0x2b/0x70 arch/x86/entry/common.c:80
entry_SYSCALL_64_after_hwframe+0x63/0xcd

value changed: 0xffff88812971ce00 -> 0x0000000000000000

Reported by Kernel Concurrency Sanitizer on:
CPU: 0 PID: 5859 Comm: syz-executor.3 Not tainted 6.0.0-syzkaller-12189-g19d17ab7-dirty #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 09/22/2022

Fixes: ab7ac4eb ("kcm: Kernel Connection Multiplexor module")
Reported-by: syzbot <syzkaller@googlegroups.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/kcm/kcmsock.c

@@ -178,7 +178,7 @@ static void kcm_rfree(struct sk_buff *skb)
 	/* For reading rx_wait and rx_psock without holding lock */
 	smp_mb__after_atomic();
 
-	if (!kcm->rx_wait && !kcm->rx_psock &&
+	if (!kcm->rx_wait && !READ_ONCE(kcm->rx_psock) &&
 	    sk_rmem_alloc_get(sk) < sk->sk_rcvlowat) {
 		spin_lock_bh(&mux->rx_lock);
 		kcm_rcv_ready(kcm);
@@ -283,7 +283,8 @@ static struct kcm_sock *reserve_rx_kcm(struct kcm_psock *psock,
 	kcm->rx_wait = false;
 
 	psock->rx_kcm = kcm;
-	kcm->rx_psock = psock;
+	/* paired with lockless reads in kcm_rfree() */
+	WRITE_ONCE(kcm->rx_psock, psock);
 
 	spin_unlock_bh(&mux->rx_lock);
 
@@ -310,7 +311,8 @@ static void unreserve_rx_kcm(struct kcm_psock *psock,
 	spin_lock_bh(&mux->rx_lock);
 
 	psock->rx_kcm = NULL;
-	kcm->rx_psock = NULL;
+	/* paired with lockless reads in kcm_rfree() */
+	WRITE_ONCE(kcm->rx_psock, NULL);
 
 	/* Commit kcm->rx_psock before sk_rmem_alloc_get to sync with
 	 * kcm_rfree