inet: frags: drop fraglist conntrack references

In the Linux kernel, the following vulnerability has been resolved:

inet: frags: drop fraglist conntrack references

Jakub added a warning in nf_conntrack_cleanup_net_list() to make debugging

leaked skbs/conntrack references more obvious.

syzbot reports this as triggering, and I can also reproduce this via

ip_defrag.sh selftest:

conntrack cleanup blocked for 60s

WARNING: net/netfilter/nf_conntrack_core.c:2512

[..]

conntrack clenups gets stuck because there are skbs with still hold nf_conn

references via their frag_list.

net.core.skb_defer_max=0 makes the hang disappear.

Eric Dumazet points out that skb_release_head_state() doesn't follow the

fraglist.

ip_defrag.sh can only reproduce this problem since

commit 6471658dc66c ("udp: use skb_attempt_defer_free()"), but AFAICS this

problem could happen with TCP as well if pmtu discovery is off.

The relevant problem path for udp is:

1. netns emits fragmented packets

2. nf_defrag_v6_hook reassembles them (in output hook)

3. reassembled skb is tracked (skb owns nf_conn reference)

4. ip6_output refragments

5. refragmented packets also own nf_conn reference (ip6_fragment

calls ip6_copy_metadata())

6. on input path, nf_defrag_v6_hook skips defragmentation: the

fragments already have skb->nf_conn attached

7. skbs are reassembled via ipv6_frag_rcv()

8. skb_consume_udp -> skb_attempt_defer_free() -> skb ends up

in pcpu freelist, but still has nf_conn reference.

Possible solutions:

1 let defrag engine drop nf_conn entry, OR

2 export kick_defer_list_purge() and call it from the conntrack

netns exit callback, OR

3 add skb_has_frag_list() check to skb_attempt_defer_free()

2 & 3 also solve ip_defrag.sh hang but share same drawback:

Such reassembled skbs, queued to socket, can prevent conntrack module

removal until userspace has consumed the packet. While both tcp and udp

stack do call nf_reset_ct() before placing skb on socket queue, that

function doesn't iterate frag_list skbs.

Therefore drop nf_conn entries when they are placed in defrag queue.

Keep the nf_conn entry of the first (offset 0) skb so that reassembled

skb retains nf_conn entry for sake of TX path.

Note that fixes tag is incorrect; it points to the commit introducing the

'ip_defrag.sh reproducible problem': no need to backport this patch to

every stable kernel.