[PATCH net] netfilter: ctnetlink: fix suspicious RCU usage in expect_iter_name

expect_iter_name() is invoked by nf_ct_expect_iterate_net() under
spin_lock_bh(&nf_conntrack_expect_lock). It does not hold
rcu_read_lock().

When accessing exp->helper with rcu_dereference() in syzbot's report,
lockdep warns:

  =============================
  WARNING: suspicious RCU usage
  syzkaller #0 Not tainted
  -----------------------------
  net/netfilter/nf_conntrack_netlink.c:3393 suspicious rcu_dereference_check() usage!

  locks held by syz-executor381/5628: 2, last CPU#1:
   #0: ffffffff9aee42a0 (nfnl_subsys_ctnetlink_exp){+.+.}-{4:4},
       at: nfnetlink_rcv_msg+0xa69/0x12b0
   #1: ffffffff8ea74d58 (nf_conntrack_expect_lock){+...}-{3:3},
       at: nf_ct_expect_iterate_net+0x38/0x180

  Call Trace:
   <TASK>
   dump_stack_lvl+0xe8/0x150
   lockdep_rcu_suspicious+0x140/0x1d0
   expect_iter_name+0xfb/0x100
   nf_ct_expect_iterate_net+0xf2/0x180
   ctnetlink_del_expect+0x45d/0x640
   nfnetlink_rcv_msg+0xcc2/0x12b0
   netlink_rcv_skb+0x226/0x4a0
   nfnetlink_rcv+0x2b9/0x28c0
   netlink_unicast+0x7bd/0x940
   netlink_sendmsg+0x813/0xb40
   ____sys_sendmsg+0x54e/0x850
   ___sys_sendmsg+0x2a5/0x360
   __sys_sendmsg+0x2a5/0x360
   do_syscall_64+0x166/0x520
   entry_SYSCALL_64_after_hwframe+0x77/0x7f

Use rcu_dereference_protected() with lockdep_is_held() on
nf_conntrack_expect_lock instead, similar to expect_iter_me() in
nf_conntrack_helper.c.

Fixes: f01794106042 ("netfilter: nf_conntrack_expect: use expect->helper")
Reported-by: syzbot+4bd730aede2791e40bdf@syzkaller.appspotmail.com
Closes: https://lore.kernel.org/netdev/6aa4a377.f81106d8.2ab401.0024.GAE@google.com/T/#u (local)
Signed-off-by: Naman Gulati <redacted>
---
 net/netfilter/nf_conntrack_netlink.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/net/netfilter/nf_conntrack_netlink.c b/net/netfilter/nf_conntrack_netlink.c
index 579ada063b1b..4e5d7c701436 100644
--- a/net/netfilter/nf_conntrack_netlink.c
+++ b/net/netfilter/nf_conntrack_netlink.c

@@ -3392,7 +3392,8 @@ static bool expect_iter_name(struct nf_conntrack_expect *exp, void *data)
 	struct nf_conntrack_helper *helper;
 	const char *name = data;
 
-	helper = rcu_dereference(exp->helper);
+	helper = rcu_dereference_protected(exp->helper,
+					   lockdep_is_held(&nf_conntrack_expect_lock));
 	if (!helper)
 		return false;
 

-- 
2.55.0.1007.g17ff1f9808-goog