ipv4: update fib_info_cnt under spinlock protection

In the past, free_fib_info() was supposed to be called
under RTNL protection.

This eventually was no longer the case.

Instead of enforcing RTNL it seems we simply can
move fib_info_cnt changes to occur when fib_info_lock
is held.

v2: David Laight suggested to update fib_info_cnt
only when an entry is added/deleted to/from the hash table,
as fib_info_cnt is used to make sure hash table size
is optimal.

BUG: KCSAN: data-race in fib_create_info / free_fib_info

write to 0xffffffff86e243a0 of 4 bytes by task 26429 on cpu 0:
 fib_create_info+0xe78/0x3440 net/ipv4/fib_semantics.c:1428
 fib_table_insert+0x148/0x10c0 net/ipv4/fib_trie.c:1224
 fib_magic+0x195/0x1e0 net/ipv4/fib_frontend.c:1087
 fib_add_ifaddr+0xd0/0x2e0 net/ipv4/fib_frontend.c:1109
 fib_netdev_event+0x178/0x510 net/ipv4/fib_frontend.c:1466
 notifier_call_chain kernel/notifier.c:83 [inline]
 raw_notifier_call_chain+0x53/0xb0 kernel/notifier.c:391
 __dev_notify_flags+0x1d3/0x3b0
 dev_change_flags+0xa2/0xc0 net/core/dev.c:8872
 do_setlink+0x810/0x2410 net/core/rtnetlink.c:2719
 rtnl_group_changelink net/core/rtnetlink.c:3242 [inline]
 __rtnl_newlink net/core/rtnetlink.c:3396 [inline]
 rtnl_newlink+0xb10/0x13b0 net/core/rtnetlink.c:3506
 rtnetlink_rcv_msg+0x745/0x7e0 net/core/rtnetlink.c:5571
 netlink_rcv_skb+0x14e/0x250 net/netlink/af_netlink.c:2496
 rtnetlink_rcv+0x18/0x20 net/core/rtnetlink.c:5589
 netlink_unicast_kernel net/netlink/af_netlink.c:1319 [inline]
 netlink_unicast+0x5fc/0x6c0 net/netlink/af_netlink.c:1345
 netlink_sendmsg+0x726/0x840 net/netlink/af_netlink.c:1921
 sock_sendmsg_nosec net/socket.c:704 [inline]
 sock_sendmsg net/socket.c:724 [inline]
 ____sys_sendmsg+0x39a/0x510 net/socket.c:2409
 ___sys_sendmsg net/socket.c:2463 [inline]
 __sys_sendmsg+0x195/0x230 net/socket.c:2492
 __do_sys_sendmsg net/socket.c:2501 [inline]
 __se_sys_sendmsg net/socket.c:2499 [inline]
 __x64_sys_sendmsg+0x42/0x50 net/socket.c:2499
 do_syscall_x64 arch/x86/entry/common.c:50 [inline]
 do_syscall_64+0x44/0xd0 arch/x86/entry/common.c:80
 entry_SYSCALL_64_after_hwframe+0x44/0xae

read to 0xffffffff86e243a0 of 4 bytes by task 31505 on cpu 1:
 free_fib_info+0x35/0x80 net/ipv4/fib_semantics.c:252
 fib_info_put include/net/ip_fib.h:575 [inline]
 nsim_fib4_rt_destroy drivers/net/netdevsim/fib.c:294 [inline]
 nsim_fib4_rt_replace drivers/net/netdevsim/fib.c:403 [inline]
 nsim_fib4_rt_insert drivers/net/netdevsim/fib.c:431 [inline]
 nsim_fib4_event drivers/net/netdevsim/fib.c:461 [inline]
 nsim_fib_event drivers/net/netdevsim/fib.c:881 [inline]
 nsim_fib_event_work+0x15ca/0x2cf0 drivers/net/netdevsim/fib.c:1477
 process_one_work+0x3fc/0x980 kernel/workqueue.c:2298
 process_scheduled_works kernel/workqueue.c:2361 [inline]
 worker_thread+0x7df/0xa70 kernel/workqueue.c:2447
 kthread+0x2c7/0x2e0 kernel/kthread.c:327
 ret_from_fork+0x1f/0x30

value changed: 0x00000d2d -> 0x00000d2e

Reported by Kernel Concurrency Sanitizer on:
CPU: 1 PID: 31505 Comm: kworker/1:21 Not tainted 5.16.0-rc6-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
Workqueue: events nsim_fib_event_work

Fixes: 48bb9eb4 ("netdevsim: fib: Add dummy implementation for FIB offload")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reported-by: syzbot <syzkaller@googlegroups.com>
Cc: David Laight <David.Laight@ACULAB.COM>
Cc: Ido Schimmel <idosch@mellanox.com>
Cc: Jiri Pirko <jiri@mellanox.com>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/fib_semantics.c

@@ -249,7 +249,6 @@ void free_fib_info(struct fib_info *fi)
 		pr_warn("Freeing alive fib_info %p\n", fi);
 		return;
 	}
-	fib_info_cnt--;
 
 	call_rcu(&fi->rcu, free_fib_info_rcu);
 }
@@ -260,6 +259,10 @@ void fib_release_info(struct fib_info *fi)
 	spin_lock_bh(&fib_info_lock);
 	if (fi && refcount_dec_and_test(&fi->fib_treeref)) {
 		hlist_del(&fi->fib_hash);
+
+		/* Paired with READ_ONCE() in fib_create_info(). */
+		WRITE_ONCE(fib_info_cnt, fib_info_cnt - 1);
+
 		if (fi->fib_prefsrc)
 			hlist_del(&fi->fib_lhash);
 		if (fi->nh) {
@@ -1430,7 +1433,9 @@ struct fib_info *fib_create_info(struct fib_config *cfg,
 #endif
 
 	err = -ENOBUFS;
-	if (fib_info_cnt >= fib_info_hash_size) {
+
+	/* Paired with WRITE_ONCE() in fib_release_info() */
+	if (READ_ONCE(fib_info_cnt) >= fib_info_hash_size) {
 		unsigned int new_size = fib_info_hash_size << 1;
 		struct hlist_head *new_info_hash;
 		struct hlist_head *new_laddrhash;
@@ -1462,7 +1467,6 @@ struct fib_info *fib_create_info(struct fib_config *cfg,
 		return ERR_PTR(err);
 	}
 
-	fib_info_cnt++;
 	fi->fib_net = net;
 	fi->fib_protocol = cfg->fc_protocol;
 	fi->fib_scope = cfg->fc_scope;
@@ -1591,6 +1595,7 @@ struct fib_info *fib_create_info(struct fib_config *cfg,
 	refcount_set(&fi->fib_treeref, 1);
 	refcount_set(&fi->fib_clntref, 1);
 	spin_lock_bh(&fib_info_lock);
+	fib_info_cnt++;
 	hlist_add_head(&fi->fib_hash,
 		       &fib_info_hash[fib_info_hashfn(fi)]);
 	if (fi->fib_prefsrc) {