inet: do not call sublist_rcv on empty list

mainline inclusion
from mainline-v5.5-rc1
commit 51210ad5
category: bugfix
bugzilla: NA
CVE: CVE-2021-20177

--------------------------------

syzbot triggered struct net NULL deref in NF_HOOK_LIST:
RIP: 0010:NF_HOOK_LIST include/linux/netfilter.h:331 [inline]
RIP: 0010:ip6_sublist_rcv+0x5c9/0x930 net/ipv6/ip6_input.c:292
 ipv6_list_rcv+0x373/0x4b0 net/ipv6/ip6_input.c:328
 __netif_receive_skb_list_ptype net/core/dev.c:5274 [inline]

Reason:
void ipv6_list_rcv(struct list_head *head, struct packet_type *pt,
                   struct net_device *orig_dev)
[..]
        list_for_each_entry_safe(skb, next, head, list) {
		/* iterates list */
                skb = ip6_rcv_core(skb, dev, net);
		/* ip6_rcv_core drops skb -> NULL is returned */
                if (skb == NULL)
                        continue;
	[..]
	}
	/* sublist is empty -> curr_net is NULL */
        ip6_sublist_rcv(&sublist, curr_dev, curr_net);

Before the recent change NF_HOOK_LIST did a list iteration before
struct net deref, i.e. it was a no-op in the empty list case.

List iteration now happens after *net deref, causing crash.

Follow the same pattern as the ip(v6)_list_rcv loop and add a list_empty
test for the final sublist dispatch too.

Cc: Edward Cree <ecree@solarflare.com>
Reported-by: syzbot+c54f457cad330e57e967@syzkaller.appspotmail.com
Fixes: ca58fbe0 ("netfilter: add and use nf_hook_slow_list()")
Signed-off-by: NFlorian Westphal <fw@strlen.de>
Tested-by: NLeon Romanovsky <leonro@mellanox.com>
Tested-by: NNikolay Aleksandrov <nikolay@cumulusnetworks.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>
Signed-off-by: NYang Yingliang <yangyingliang@huawei.com>
Reviewed-by: NYue Haibing <yuehaibing@huawei.com>
Reviewed-by: NJason Yan <yanaijie@huawei.com>
Signed-off-by: NYang Yingliang <yangyingliang@huawei.com>

net/ipv4/ip_input.c

@@ -616,5 +616,6 @@ void ip_list_rcv(struct list_head *head, struct packet_type *pt,
 		list_add_tail(&skb->list, &sublist);
 	}
 	/* dispatch final sublist */
-	ip_sublist_rcv(&sublist, curr_dev, curr_net);
+	if (!list_empty(&sublist))
+		ip_sublist_rcv(&sublist, curr_dev, curr_net);
 }

net/ipv6/ip6_input.c

@@ -324,7 +324,8 @@ void ipv6_list_rcv(struct list_head *head, struct packet_type *pt,
 		list_add_tail(&skb->list, &sublist);
 	}
 	/* dispatch final sublist */
-	ip6_sublist_rcv(&sublist, curr_dev, curr_net);
+	if (!list_empty(&sublist))
+		ip6_sublist_rcv(&sublist, curr_dev, curr_net);
 }
 
 /*