phonet: do not call synchronize_rcu() from phonet_route_del()

Calling synchronize_rcu() while holding rcu_read_lock() is not
permitted [1]

Move the synchronize_rcu() + dev_put() to route_doit().

Alternative would be to not use rcu_read_lock() in route_doit().

[1]
WARNING: suspicious RCU usage
6.12.0-rc5-syzkaller-01056-gf07a6e6ceb05 #0 Not tainted
-----------------------------
kernel/rcu/tree.c:4092 Illegal synchronize_rcu() in RCU read-side critical section!

other info that might help us debug this:

rcu_scheduler_active = 2, debug_locks = 1
1 lock held by syz-executor427/5840:
  #0: ffffffff8e937da0 (rcu_read_lock){....}-{1:2}, at: rcu_lock_acquire include/linux/rcupdate.h:337 [inline]
  #0: ffffffff8e937da0 (rcu_read_lock){....}-{1:2}, at: rcu_read_lock include/linux/rcupdate.h:849 [inline]
  #0: ffffffff8e937da0 (rcu_read_lock){....}-{1:2}, at: route_doit+0x3d6/0x640 net/phonet/pn_netlink.c:264

stack backtrace:
CPU: 1 UID: 0 PID: 5840 Comm: syz-executor427 Not tainted 6.12.0-rc5-syzkaller-01056-gf07a6e6ceb05 #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 09/13/2024
Call Trace:
 <TASK>
  __dump_stack lib/dump_stack.c:94 [inline]
  dump_stack_lvl+0x241/0x360 lib/dump_stack.c:120
  lockdep_rcu_suspicious+0x226/0x340 kernel/locking/lockdep.c:6821
  synchronize_rcu+0xea/0x360 kernel/rcu/tree.c:4089
  phonet_route_del+0xc6/0x140 net/phonet/pn_dev.c:409
  route_doit+0x514/0x640 net/phonet/pn_netlink.c:275
  rtnetlink_rcv_msg+0x791/0xcf0 net/core/rtnetlink.c:6790
  netlink_rcv_skb+0x1e3/0x430 net/netlink/af_netlink.c:2551
  netlink_unicast_kernel net/netlink/af_netlink.c:1331 [inline]
  netlink_unicast+0x7f6/0x990 net/netlink/af_netlink.c:1357
  netlink_sendmsg+0x8e4/0xcb0 net/netlink/af_netlink.c:1901
  sock_sendmsg_nosec net/socket.c:729 [inline]
  __sock_sendmsg+0x221/0x270 net/socket.c:744
  sock_write_iter+0x2d7/0x3f0 net/socket.c:1165
  new_sync_write fs/read_write.c:590 [inline]
  vfs_write+0xaeb/0xd30 fs/read_write.c:683
  ksys_write+0x183/0x2b0 fs/read_write.c:736
  do_syscall_x64 arch/x86/entry/common.c:52 [inline]
  do_syscall_64+0xf3/0x230 arch/x86/entry/common.c:83
 entry_SYSCALL_64_after_hwframe+0x77/0x7f

Fixes: 17a1ac0 ("phonet: Don't hold RTNL for route_doit().")
Reported-by: syzbot <[email protected]>
Signed-off-by: Eric Dumazet <[email protected]>
Reviewed-by: Kuniyuki Iwashima <[email protected]>
Cc: Remi Denis-Courmont <[email protected]>
Reviewed-by: Jiri Pirko <[email protected]>
Link: https://patch.msgid.link/[email protected]
Signed-off-by: Jakub Kicinski <[email protected]>

Author: edumazet

net/phonet/pn_dev.c

@@ -406,8 +406,9 @@ int phonet_route_del(struct net_device *dev, u8 daddr)
 
 	if (!dev)
 		return -ENOENT;
-	synchronize_rcu();
-	dev_put(dev);
+
+	/* Note : our caller must call synchronize_rcu() and dev_put(dev) */
+
 	return 0;
 }
 

net/phonet/pn_netlink.c

@@ -233,6 +233,7 @@ static int route_doit(struct sk_buff *skb, struct nlmsghdr *nlh,
 {
 	struct net *net = sock_net(skb->sk);
 	struct nlattr *tb[RTA_MAX+1];
+	bool sync_needed = false;
 	struct net_device *dev;
 	struct rtmsg *rtm;
 	u32 ifindex;
@@ -269,13 +270,20 @@ static int route_doit(struct sk_buff *skb, struct nlmsghdr *nlh,
 		return -ENODEV;
 	}
 
-	if (nlh->nlmsg_type == RTM_NEWROUTE)
+	if (nlh->nlmsg_type == RTM_NEWROUTE) {
 		err = phonet_route_add(dev, dst);
-	else
+	} else {
 		err = phonet_route_del(dev, dst);
+		if (!err)
+			sync_needed = true;
+	}
 
 	rcu_read_unlock();
 
+	if (sync_needed) {
+		synchronize_rcu();
+		dev_put(dev);
+	}
 	if (!err)
 		rtm_phonet_notify(net, nlh->nlmsg_type, ifindex, dst);