Re: [PATCH net-next] inet: do not call sublist_rcv on empty list

[Leon Romanovsky <leon@kernel.org>]

On Tue, Oct 29, 2019 at 01:44:04AM +0100, Florian Westphal wrote:
> 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: ca58fbe06c54 ("netfilter: add and use nf_hook_slow_list()")
> Signed-off-by: Florian Westphal <fw@strlen.de>
> ---
>  net/ipv4/ip_input.c  | 3 ++-
>  net/ipv6/ip6_input.c | 3 ++-
>  2 files changed, 4 insertions(+), 2 deletions(-)
>

It fixed my crash on boot.

Thanks,
Tested-by: Leon Romanovsky <leonro@mellanox.com>