mpls: Use rcu_dereference_rtnl() in mpls_route_input_rcu().

From: Kuniyuki Iwashima <[email protected]>

As syzbot reported [0], mpls_route_input_rcu() can be called from mpls_getroute(), where is under RTNL.

net->mpls.platform_label is only updated under RTNL.

Let's use rcu_dereference_rtnl() in mpls_route_input_rcu() to silence the splat.

[0]:
WARNING: suspicious RCU usage 6.15.0-rc7-syzkaller-00082-g5cdb2c77c4c3 #0 Not tainted
----------------------------
net/mpls/af_mpls.c:84 suspicious rcu_dereference_check() usage!

other info that might help us debug this:

rcu_scheduler_active = 2, debug_locks = 1
1 lock held by syz.2.4451/17730:
#0: ffffffff9012a3e8 (rtnl_mutex){+.+.}-{4:4}, at: rtnl_lock net/core/rtnetlink.c:80 [inline] #0: ffffffff9012a3e8 (rtnl_mutex){+.+.}-{4:4}, at: rtnetlink_rcv_msg+0x371/0xe90 net/core/rtnetlink.c:6961

stack backtrace:
CPU: 1 UID: 0 PID: 17730 Comm: syz.2.4451 Not tainted 6.15.0-rc7-syzkaller-00082-g5cdb2c77c4c3 #0 PREEMPT(full)
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 05/07/2025
Call Trace:
 <TASK>
 __dump_stack lib/dump_stack.c:94 [inline]
 dump_stack_lvl+0x16c/0x1f0 lib/dump_stack.c:120
 lockdep_rcu_suspicious+0x166/0x260 kernel/locking/lockdep.c:6865
 mpls_route_input_rcu+0x1d4/0x200 net/mpls/af_mpls.c:84
 mpls_getroute+0x621/0x1ea0 net/mpls/af_mpls.c:2381
 rtnetlink_rcv_msg+0x3c9/0xe90 net/core/rtnetlink.c:6964
 netlink_rcv_skb+0x16d/0x440 net/netlink/af_netlink.c:2534
 netlink_unicast_kernel net/netlink/af_netlink.c:1313 [inline]
 netlink_unicast+0x53a/0x7f0 net/netlink/af_netlink.c:1339
 netlink_sendmsg+0x8d1/0xdd0 net/netlink/af_netlink.c:1883
 sock_sendmsg_nosec net/socket.c:712 [inline]
 __sock_sendmsg net/socket.c:727 [inline]
 ____sys_sendmsg+0xa98/0xc70 net/socket.c:2566
 ___sys_sendmsg+0x134/0x1d0 net/socket.c:2620
 __sys_sendmmsg+0x200/0x420 net/socket.c:2709
 __do_sys_sendmmsg net/socket.c:2736 [inline]
 __se_sys_sendmmsg net/socket.c:2733 [inline]
 __x64_sys_sendmmsg+0x9c/0x100 net/socket.c:2733
 do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
 do_syscall_64+0xcd/0x230 arch/x86/entry/syscall_64.c:94
 entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7f0a2818e969
Code: ff ff c3 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 40 00 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 c7 c1 a8 ff ff ff f7 d8 64 89 01 48
RSP: 002b:00007f0a28f52038 EFLAGS: 00000246 ORIG_RAX: 0000000000000133
RAX: ffffffffffffffda RBX: 00007f0a283b5fa0 RCX: 00007f0a2818e969
RDX: 0000000000000003 RSI: 0000200000000080 RDI: 0000000000000003
RBP: 00007f0a28210ab1 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
R13: 0000000000000000 R14: 00007f0a283b5fa0 R15: 00007ffce5e9f268
 </TASK>

---
 net/mpls/af_mpls.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

index d536c97144e9..47d7dfd9ad09 100644
@@ -81,8 +81,8 @@ static struct mpls_route *mpls_route_input_rcu(struct net *net, unsigned index)
 
 	if (index < net->mpls.platform_labels) {
 		struct mpls_route __rcu **platform_label =
-			rcu_dereference(net->mpls.platform_label);
-		rt = rcu_dereference(platform_label[index]);
+			rcu_dereference_rtnl(net->mpls.platform_label);
+		rt = rcu_dereference_rtnl(platform_label[index]);
 	}
 	return rt;
 }

2.49.0