net/packet: annotate accesses to po->ifindex

Like prior patch, we need to annotate lockless accesses to po->ifindex
For instance, packet_getname() is reading po->ifindex (twice) while
another thread is able to change po->ifindex.

KCSAN reported:

BUG: KCSAN: data-race in packet_do_bind / packet_getname

write to 0xffff888143ce3cbc of 4 bytes by task 25573 on cpu 1:
 packet_do_bind+0x420/0x7e0 net/packet/af_packet.c:3191
 packet_bind+0xc3/0xd0 net/packet/af_packet.c:3255
 __sys_bind+0x200/0x290 net/socket.c:1637
 __do_sys_bind net/socket.c:1648 [inline]
 __se_sys_bind net/socket.c:1646 [inline]
 __x64_sys_bind+0x3d/0x50 net/socket.c:1646
 do_syscall_64+0x4a/0x90 arch/x86/entry/common.c:47
 entry_SYSCALL_64_after_hwframe+0x44/0xae

read to 0xffff888143ce3cbc of 4 bytes by task 25578 on cpu 0:
 packet_getname+0x5b/0x1a0 net/packet/af_packet.c:3525
 __sys_getsockname+0x10e/0x1a0 net/socket.c:1887
 __do_sys_getsockname net/socket.c:1902 [inline]
 __se_sys_getsockname net/socket.c:1899 [inline]
 __x64_sys_getsockname+0x3e/0x50 net/socket.c:1899
 do_syscall_64+0x4a/0x90 arch/x86/entry/common.c:47
 entry_SYSCALL_64_after_hwframe+0x44/0xae

value changed: 0x00000000 -> 0x00000001

Reported by Kernel Concurrency Sanitizer on:
CPU: 0 PID: 25578 Comm: syz-executor.5 Not tainted 5.13.0-rc6-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011

Signed-off-by: Eric Dumazet <edumazet@google.com>
Reported-by: syzbot <syzkaller@googlegroups.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/packet/af_packet.c

@@ -3187,11 +3187,11 @@ static int packet_do_bind(struct sock *sk, const char *name, int ifindex,
 		if (unlikely(unlisted)) {
 			dev_put(dev);
 			po->prot_hook.dev = NULL;
-			po->ifindex = -1;
+			WRITE_ONCE(po->ifindex, -1);
 			packet_cached_dev_reset(po);
 		} else {
 			po->prot_hook.dev = dev;
-			po->ifindex = dev ? dev->ifindex : 0;
+			WRITE_ONCE(po->ifindex, dev ? dev->ifindex : 0);
 			packet_cached_dev_assign(po, dev);
 		}
 	}
@@ -3505,7 +3505,7 @@ static int packet_getname_spkt(struct socket *sock, struct sockaddr *uaddr,
 	uaddr->sa_family = AF_PACKET;
 	memset(uaddr->sa_data, 0, sizeof(uaddr->sa_data));
 	rcu_read_lock();
-	dev = dev_get_by_index_rcu(sock_net(sk), pkt_sk(sk)->ifindex);
+	dev = dev_get_by_index_rcu(sock_net(sk), READ_ONCE(pkt_sk(sk)->ifindex));
 	if (dev)
 		strlcpy(uaddr->sa_data, dev->name, sizeof(uaddr->sa_data));
 	rcu_read_unlock();
@@ -3520,16 +3520,18 @@ static int packet_getname(struct socket *sock, struct sockaddr *uaddr,
 	struct sock *sk = sock->sk;
 	struct packet_sock *po = pkt_sk(sk);
 	DECLARE_SOCKADDR(struct sockaddr_ll *, sll, uaddr);
+	int ifindex;
 
 	if (peer)
 		return -EOPNOTSUPP;
 
+	ifindex = READ_ONCE(po->ifindex);
 	sll->sll_family = AF_PACKET;
-	sll->sll_ifindex = po->ifindex;
+	sll->sll_ifindex = ifindex;
 	sll->sll_protocol = READ_ONCE(po->num);
 	sll->sll_pkttype = 0;
 	rcu_read_lock();
-	dev = dev_get_by_index_rcu(sock_net(sk), po->ifindex);
+	dev = dev_get_by_index_rcu(sock_net(sk), ifindex);
 	if (dev) {
 		sll->sll_hatype = dev->type;
 		sll->sll_halen = dev->addr_len;
@@ -4108,7 +4110,7 @@ static int packet_notifier(struct notifier_block *this,
 				}
 				if (msg == NETDEV_UNREGISTER) {
 					packet_cached_dev_reset(po);
-					po->ifindex = -1;
+					WRITE_ONCE(po->ifindex, -1);
 					if (po->prot_hook.dev)
 						dev_put(po->prot_hook.dev);
 					po->prot_hook.dev = NULL;
@@ -4620,7 +4622,7 @@ static int packet_seq_show(struct seq_file *seq, void *v)
 			   refcount_read(&s->sk_refcnt),
 			   s->sk_type,
 			   ntohs(READ_ONCE(po->num)),
-			   po->ifindex,
+			   READ_ONCE(po->ifindex),
 			   po->running,
 			   atomic_read(&s->sk_rmem_alloc),
 			   from_kuid_munged(seq_user_ns(seq), sock_i_uid(s)),