net: Fix an unsafe loop on the list

The kernel may crash when deleting a genetlink family if there are still
listeners for that family:

Oops: Kernel access of bad area, sig: 11 [#1]
  ...
  NIP [c000000000c080bc] netlink_update_socket_mc+0x3c/0xc0
  LR [c000000000c0f764] __netlink_clear_multicast_users+0x74/0xc0
  Call Trace:
__netlink_clear_multicast_users+0x74/0xc0
genl_unregister_family+0xd4/0x2d0

Change the unsafe loop on the list to a safe one, because inside the
loop there is an element removal from this list.

Fixes: b827357 ("genetlink: fix netns vs. netlink table locking (2)")
Cc: [email protected]
Signed-off-by: Anastasia Kovaleva <[email protected]>
Reviewed-by: Dmitry Bogdanov <[email protected]>
Reviewed-by: Kuniyuki Iwashima <[email protected]>
Link: https://patch.msgid.link/[email protected]
Signed-off-by: Jakub Kicinski <[email protected]>

include/net/sock.h

@@ -894,6 +894,8 @@ static inline void sk_add_bind_node(struct sock *sk,
 	hlist_for_each_entry_safe(__sk, tmp, list, sk_node)
 #define sk_for_each_bound(__sk, list) \
 	hlist_for_each_entry(__sk, list, sk_bind_node)
+#define sk_for_each_bound_safe(__sk, tmp, list) \
+	hlist_for_each_entry_safe(__sk, tmp, list, sk_bind_node)
 
 /**
  * sk_for_each_entry_offset_rcu - iterate over a list at a given struct offset

net/netlink/af_netlink.c

@@ -2136,8 +2136,9 @@ void __netlink_clear_multicast_users(struct sock *ksk, unsigned int group)
 {
 	struct sock *sk;
 	struct netlink_table *tbl = &nl_table[ksk->sk_protocol];
+	struct hlist_node *tmp;
 
-	sk_for_each_bound(sk, &tbl->mc_list)
+	sk_for_each_bound_safe(sk, tmp, &tbl->mc_list)
 		netlink_update_socket_mc(nlk_sk(sk), group, 0);
 }