Affected: Linux

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 en…