ipv4: ip_tunnel: Fix suspicious RCU usage warning in ip_tunnel_init_f…

…low()

There are code paths from which the function is called without holding
the RCU read lock, resulting in a suspicious RCU usage warning [1].

Fix by using l3mdev_master_upper_ifindex_by_index() which will acquire
the RCU read lock before calling
l3mdev_master_upper_ifindex_by_index_rcu().

[1]
WARNING: suspicious RCU usage
6.12.0-rc3-custom-gac8f72681cf2 #141 Not tainted
-----------------------------
net/core/dev.c:876 RCU-list traversed in non-reader section!!

other info that might help us debug this:

rcu_scheduler_active = 2, debug_locks = 1
1 lock held by ip/361:
 #0: ffffffff86fc7cb0 (rtnl_mutex){+.+.}-{3:3}, at: rtnetlink_rcv_msg+0x377/0xf60

stack backtrace:
CPU: 3 UID: 0 PID: 361 Comm: ip Not tainted 6.12.0-rc3-custom-gac8f72681cf2 #141
Hardware name: Bochs Bochs, BIOS Bochs 01/01/2011
Call Trace:
 <TASK>
 dump_stack_lvl+0xba/0x110
 lockdep_rcu_suspicious.cold+0x4f/0xd6
 dev_get_by_index_rcu+0x1d3/0x210
 l3mdev_master_upper_ifindex_by_index_rcu+0x2b/0xf0
 ip_tunnel_bind_dev+0x72f/0xa00
 ip_tunnel_newlink+0x368/0x7a0
 ipgre_newlink+0x14c/0x170
 __rtnl_newlink+0x1173/0x19c0
 rtnl_newlink+0x6c/0xa0
 rtnetlink_rcv_msg+0x3cc/0xf60
 netlink_rcv_skb+0x171/0x450
 netlink_unicast+0x539/0x7f0
 netlink_sendmsg+0x8c1/0xd80
 ____sys_sendmsg+0x8f9/0xc20
 ___sys_sendmsg+0x197/0x1e0
 __sys_sendmsg+0x122/0x1f0
 do_syscall_64+0xbb/0x1d0
 entry_SYSCALL_64_after_hwframe+0x77/0x7f

Fixes: db53cd3 ("net: Handle l3mdev in ip_tunnel_init_flow")
Signed-off-by: Ido Schimmel <[email protected]>
Reviewed-by: David Ahern <[email protected]>
Link: https://patch.msgid.link/[email protected]
Signed-off-by: Jakub Kicinski <[email protected]>

include/net/ip_tunnels.h

@@ -354,7 +354,7 @@ static inline void ip_tunnel_init_flow(struct flowi4 *fl4,
 	memset(fl4, 0, sizeof(*fl4));
 
 	if (oif) {
-		fl4->flowi4_l3mdev = l3mdev_master_upper_ifindex_by_index_rcu(net, oif);
+		fl4->flowi4_l3mdev = l3mdev_master_upper_ifindex_by_index(net, oif);
 		/* Legacy VRF/l3mdev use case */
 		fl4->flowi4_oif = fl4->flowi4_l3mdev ? 0 : oif;
 	}