1. a conntrack entry is about to be confirmed (added to hash table).
2. a clash with existing entry is detected.
3. nf_ct_resolve_clash() puts skb->nfct (the "losing" entry).
4. this entry now has a refcount of 0 and is freed to SLAB_TYPESAFE_BY_RCU
kmem cache.
skb->nfct has been replaced by the one found in the hash.
Problem is that nf_conntrack_confirm() uses the old ct:
static inline int nf_conntrack_confirm(struct sk_buff *skb)
{
struct nf_conn *ct = (struct nf_conn *)skb_nfct(skb);
int ret = NF_ACCEPT;
if (ct) {
if (!nf_ct_is_confirmed(ct))
ret = __nf_conntrack_confirm(skb);
if (likely(ret == NF_ACCEPT))
nf_ct_deliver_cached_events(ct); /* This ct has refcount 0! */
}
return ret;
}
As of "netfilter: conntrack: free extension area immediately", we can't
access conntrack extensions in this case.
To fix this, make sure we check the dying bit presence before attempting
to get the eache extension.
Reported-by: syzbot+c7aabc9fe93e7f3637ba@syzkaller.appspotmail.com Fixes: 2ad9d7747c10d1 ("netfilter: conntrack: free extension area immediately") Signed-off-by: Florian Westphal <fw@strlen.de> Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>